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Redux – Lecture 9 – CS50’s Mobile App Development with React Native

Redux – Lecture 9 – CS50’s Mobile App Development with React Native

welcome back for lecture 9. This week we’ll be talking about Redux. So in the last couple of lectures,
we talked about a few things. We talked about APIs, which
is the way that you interface with external-facing services. We talked about making
network requests, so how do you actually access these APIs if
they’re not on your own computer– maybe out in the cloud somewhere. And we talked about
promises and async/await, which are basically a couple ways to
handle asynchronous actions, namely making network requests. We talked about how to transform data
after you get it back from an API. It might not be exactly in
the shape that you want. And so we talked about a
few different strategies by which we could get it
from the shape that it’s in from the API to exactly what
we want in our application. We then talked about some
simple authentication, meaning how do you ensure that
users are who they say they are? And how do you let them
into your app after that? We talked about some HTTP
methods and response codes, which are basically a specification that
lets you take a response from a network request and know exactly if the
request succeeded or if it failed. And if it failed, then why did it fail? And then lastly, last week, we
had Charlie Cheever as our guest to talk about expo components. So our applications, as we
move throughout the class, are getting more and more complex. And now we’re starting to
run into some problems. Our apps thus far have
been pretty simple. But we’re already starting to see some
bugs related to complexity, namely maybe we forget to pass a prop
or have some troubles directly managing some state that’s
possibly deeply nested. With more complicated
state, we might be starting to duplicate some information
in that state object. Or maybe with that
duplicated information, when we update one of
the props, maybe we forget to update another prop or a
bunch of the other dependent props. Or maybe we have components
with the very large number of props that don’t necessarily need
to have that large number of props. Or maybe we see a prop
that’s not behaving how we think it should be
behaving, and we’re not really sure where exactly it’s going wrong and where
that props should be being managed. And so this lecture, we’re going
to take a step back and rethink the architecture by which
we’ve been managing our data. So one company that’s run into
scaling issues is Facebook. So they found that the MVC
architecture is a little bit too complex for their scale. And so those of you who took CS50, we
talked about this architecture called model view controller. And what exactly is that? So in the MVC architecture,
we have models. We have views. And we have controllers. And so views are basically
what a user sees. And the data by which– are loaded into the views
come from these models. And how does a user
interface with these views? Well, they do it through this
thing called a controller. And then maybe the controller
will then dispatch an action when a user clicks a
button or something. And maybe that goes and updates a model. But also some views might
directly change models. And so you start to have this two-way
connection between models and views. And when you’re operating
at Facebook size, you don’t just have
one model and one view. Facebook actually has something
like 30,000 different React components that they use. And so there are a large
number of views here. And with that larger
number of views, they’re also a very large number of models. And maybe this view talks to this
model, and back and forth, and also talks to that model. And maybe that view talks to this model. And now with all of
these connections, you see how it’s starting to
get very, very complicated. And when you scale up
to a size like Facebook, you might start to run into some bugs. And so one thing that
happened with Facebook is that the complexity manifested
itself into some bugs, and namely a pretty famous one that has to
do with unread chat notifications. And so what basically
happened was Facebook started to add more and more different
views for how you access their chat. And so at first it was a little pop-up
box that happened in the bottom. But then they added a
separate page for chats, and then they added something in the
nav bar that drops down with some chats. And then they started adding other
applications that handle chats. And all of a sudden, there’s
a lot of different places where a chat could be
updated or vise versa. And so they had a big bug
where users would log in, and it would have a notification
that says you have an unread message. But that wasn’t necessarily
always the case. And so this bugged a lot
of users, and Facebook had to take a step back
and figure out a better way to handle this large
complexity that they’re starting to have in their application. And so what they did is they
re-architected everything into a single-direction data flow. And so data only comes
in from one direction, and there’s only one
way to update that data. And how did they do that? Well, they created–
well, if you want to see additional background
into this problem, you can watch this YouTube
video by Jing Chen. And they talk about
exactly what the bug was and why they created
this new architecture. And so this new architecture
was called Flux. And what is Flux. Well, it’s a new application
architecture for React that utilizes a
unidirectional data flow. What the heck does that mean? Well, views react in changes
to some number of stores. And what it stores, it stores the data. And the only thing that can update this
data is this thing called a dispatcher, and we’ll talk about it
a little more in a sec. And how do we trigger this dispatcher? Well, you invoke an action. And what is the way
you invoke an action? Well, actions are triggered from views. And so let’s draw this picture out. So you have a central store that
stores all of your information, or maybe some number of stores. And these stores will
store all of the data that you need for that application. And how do you update that store? Well, it’s through this
thing called a dispatcher. And so this dispatcher will actually
send information to these stores. And these stores will update. And that’s the only possible
way to update these stores. And how do you trigger the dispatcher? Well, it’s through this
thing called an action. And what happens when you have all
this information in your store. You use that store to
render a bunch of views. And so maybe this view listens
from this store and that store. And maybe a view here
listens to all three stores. And the view here only
listens to that store. And so we have a large number
of arrows here, potentially. But one thing to note
is that the arrows all go this direction, which means that
there is no bi-directional flow. And so multiple views
don’t necessarily update, or they can’t update, multiple stores. Which is a good thing
because a view can just say, hey, give me the information
from here and here. But I’m not going to worry about
having to go update those back. What I do is I dispatch an action. And so these views are the
ways to trigger an action, and the only ways to trigger an action. And so you see what is looking
like a large number of arrows, but you see that they’re all
going in a singular direction. So you have this
unidirectional data flow. So to repeat myself,
the only way to update the information, the data
that’s stored in an application, is through this thing
called a dispatcher. And we don’t really
control this dispatcher. We only tell the dispatcher to
start by dispatching an action. And how do we trigger an action? Well, we interact with the views. And by we, I mean our users. And how do these views know
what they should render? Well, they get that
information from stores. And as stores are updated,
the views are automatically notified that they should re-render. And so there’s no arrows that
are going back and forth here, so the complexity is
an order of magnitude smaller, because the views
only care about the information that they’re getting from the stores. And they’re automatically notified
when that information changes. So Flux is a general architecture. And there are actually
many implementations. Facebook has written one. There’s also this thing called Redux. So whether or not Redux is
actually an implementation of Flux is an opinion that can
be argued either way. But what’s unarguable is that
Redux was inspired by Flux. So Redux is a data management library,
and it takes a lot of the paradigms that Flux created and
implements them for itself. And so there are three
big pillars of Redux. One is that there is a single
source of truth for all of the data. So in the Flux architecture,
there might be multiple stores. But in Redux, there’s
just one, one big object that keeps track of all
the application state. The only way that you
can update the state is by an action that triggers
a re-computation of the state. And so it’s very similar to the Flux in
that the only way that you can update a store is by dispatching an action. And Redux is very similar
in that the only way to update the centralized data– so you can think of the store
as one big thing in Redux. And the only way to update that
is by dispatching an action. And lastly, updates are
made using a pure function. And so by pure function, I mean a
function that takes in arguments and will always return the
same thing for those arguments. So it’s fully deterministic,
meaning given the same arguments, it will always return the same answer. And also, it doesn’t
look anywhere else other than those arguments for the
computation that it gives. So you’re not using things
like the current time or anything in the environment
to calculate things, only a function of the input. And lastly, there are no side effects. So I take input, and I give output. And I don’t do anything else. I don’t print. I don’t change variables outside
my scope, nothing like that. And so we have this thing. We have an action, goes to a reducer,
which then updates the store. And so to draw that– if you remember this from
Flux, we have actions go through a dispatcher, which
updates some number of stores, which then update some number of views. And in Reduxland, we have
an action, gets dispatched, and updates some central store. And that will update any of its views. And those views, with user
interaction, can trigger more actions. And so it’s a very
similar diagram to Flux. And it’s the exact same in that
it’s a unidirectional flow of data. But the main difference
is here where you have a singular store, rather than
a potentially large number of stores like in Flux. And if you want to read about
Redux and its background, you can go to this
URL, which is ITS Docs. So let’s do something fun this
lecture and actually implement something like Redux ourselves. And so where do we start? Let’s go ahead and
just create a directory into which we’ll start
creating these files. So what’s first? So there’s this thing
called a reducer, which is what calculates the next state. And so it takes a previous state,
and it takes some information on how it should update. And it goes ahead and applies the
update and returns some new state. So some constraints, it
should be a pure function. So in other words, the
result is deterministic. So given the same input, it’ll
always give the same output. And it’s determined
exclusively by those arguments. And there are no side effects, so
no logging, no changing variables, no doing anything like that– no changing variables outside
of its own scope that should. Should be immutable. And so we’ve talked about
immutability in previous lectures. So it should always return a new object. So the keys in the
object might not change, but the object reference
itself should change. And let’s actually go ahead and
start implementing a reducer. And so we said a reducer takes the
previous state and some sort of update and returns some new state. And let’s go ahead and implement that. So some function that takes a
previous state and an update and it returns some sort of new state. And so how it calculates that
state is completely up to us. And so let’s make the
simplest reducer possible. Well, the simplest reducer
possible would actually be this, where we take the previous
date, we take an update, and we just completely ignore the
update and return the previous state. But let’s actually do something. So rather than just returning the
state blindly, let’s apply some update. And so let’s assume the
state isn’t an object, and let’s assume the
update is also an object. And let’s just merge those objects. And so we could do something like this. So this would be a very,
very simple reducer. So given a state and an update,
where they’re both objects, just merge those objects. And so take all of the
keys and states of– you don’t remember what the syntax is. It’s object spread by which you
take all of the key and value pairs of an object, and you add them
to this new object that we’re creating. And then do the same thing with update. Take all of its keys and value pairs,
and then apply them to the same object. And so if there are any
duplicate keys, they’ll get overwritten by this second update. And so this is an example of
an extremely simple reducer. And so let’s actually play
with our reducer a little bit. We can do something like let
state start as an empty object. We can do state is now– invoke the reducer on the old state,
and that it’s passing something like foo gets foo. And then now let’s
update the state again. And so that’s passed into our reducer– the old states, and maybe bar gets bar. And lastly, let’s update the state
again, passes into the reducer, and give it now foo gets baz. And so what do we
expect to see back here? So we have state
initialises an empty object. And then first we invoke it
and passing this object that has a key value pair of foo and foo. And so when we invoke that, we have
an empty object here and foo here. And so after line 7, our
state is just foo, foo. How about line 8? Now we pass foo foo into the reducer,
and also pass an update called bar bar. And so state is foo foo. Update is bar bar. And so we now are ended up with
an object with a key of foo and a value of foo, a key
of bar and a value of bar. And then lastly, we pass that
object into the reducer again, this time with a key
value pair of foo and baz. And so what we expect to see out of
that is an object with a key of foo, value of baz, and a key
bar with a value of bar. So let’s go ahead and run this. But first, since object spread
is not yet supported by node, let’s just replace it with something
like a helper function called merge, where it’ll take a bunch of
arguments– some number of arguments. Let’s actually just to prev and next. And it will do this thing
called object, dot, assign, which is the way that we
used to merge objects before. This object spread became a syntax. And so let’s just assign a new object,
the key value pairs of previous, and then the key value pairs of next. And then rather than
doing this manually here, let’s just merge state and update. So let’s run this and see what we get. I guess nothing will happen
when you run that, so let’s just console log the state at the end. And so we get what we expected. So we expect it to get this
thing with foo baz and bar bar. And that’s exactly what we got. And so to recap, all a
reducer is is something that takes some previous state and
something that we want to update it with and then returns a new state. So in our particular reducer, we chose
to just merge the update into states. But what happens here
doesn’t matter, as long as it’s returning a new state object. And so like I alluded to earlier,
we can make our reducer even simpler by just doing this. So if we did this, and
ran it, we just get an empty object, which is as expected. Because all we’re doing is we’re
just passing the old state backward and completely ignoring the
update and passing an old state. So this is a completely valid reducer. It just happened to be that
it doesn’t do anything. And so instead, we made it do
something by merging these objects. Cool. So let’s make our Redux model
a little bit more advanced. So how was it responsible
for invoking the reducer? And how do we actually manage the state? Because currently, it’s
not very good that the way that we’re managing the state is just
this state object that we’re mutating. Why not try to wrap this up in
something self-contained, like a class? And that’s exactly what Redux
does with this concept of a store. So what is a store? So as we talked about
earlier, a store is responsible for maintaining
some sort of state. It does a bunch of other stuff. But at its core, it’s just responsible
for remembering what the state is. The way that you access that state
is through this [INAUDIBLE] method called getState. And so you can invoke this
method called getState, and it’ll return the current state. And how do you update that state? Well, as we talked about
earlier, the only way to do it is by using this thing called
dispatch– by dispatching an action to update the state. And lastly, you can add listeners that
get invoked when the state changes. And so it wouldn’t be super
great of a reactive model if the store just updated
itself and didn’t tell anybody, because the whole point of
React is that the UI will react to updates in the application state. And so what Redux does is when the
application state gets updated, it’ll run some callbacks. But let’s not include that in
our implementation for now. So currently, we have
this very simple reducer. And now let’s turn it into a store. And so what does a store need? Well, it should maintain
the state within itself. It should expose some sort of
getter method called getState. And it should expose this
method called dispatch, which is the way that
you update these actions. And so let’s start to do that. And so let’s create
this class called Store. And how do you create a store? Well, let’s just say you
pass it, maybe a reducer. So how does a store know
how to update itself? And you also pass it,
maybe some initial state. And what should it do
in the constructor? Well, it should remember these things. And so maybe it’ll say this.reducer– is that reducer that you passed it. And maybe the initial state, or
the state, is that initial state. Cool. So now we have a store. It’s kind of useless because all it
does is create this class instance, and you can’t actually
interface with it at all. And so let’s start to add these methods
by which you can interface with it. So first, we said you should be
able to get the state by invoking this thing called getState. And so let’s create this method called
getState, which is pretty simple. All it does is return this.state. And so now we’re starting
to make our simple store. We can test it a little bit. Let’s just pass our simple
reducer, and then you’ll see the initial state of empty. And then let’s see what
the initial state is. And so we can do store,
dot, get the State. So if we run that, we just get an empty
object back, which is as expected. All store is currently
a constructor that takes the reducer in an
initial state, initializes the state to the initial state. And then we have no way
of updating the state. And so when you call this.state,
it just returns an empty object. And if we add instead past its arbitrary
object like foo foo, and then ran it, we’ll get that same object back. Cool. So not very useful yet, and so let’s
start to add some features to it. So first, and most importantly,
we should have some sort of way to update that state. And the way that we do this in
Redux is by dispatching an action or by just invoking this method called
dispatch with some way of updating. And so the way that we’ve
been doing this thus far is by just passing an object
that we want merged into a state. And so let’s do that. So let’s have this
thing called dispatch. We pass it. Well, we pass it some sort of update. And what does it do? Well, it should update the
current state to be what? We want to run this reducer function. And which reducer function? Well, whichever one that we
passed in the original constructor and stored as this.reducer. So let’s invoke that with the
current state, and then any update that we want to pass it. And that’s pretty much it. And so now down here, rather than
having to handle this ourselves– and by this I mean replacing
the old state with the reducer, with value after passing
it through the reducer– that’s now abstracted away from us
using this thing called a store. And so now we can just do store,
dot, dispatch this object. And the same goes to here. And so now, we’re doing things
a little bit differently. So rather than creating
the state ourselves and manually going through and replacing
state with the output of the reducer and passing in the old state, now
that’s handled for us by this thing called a store. And how is it handled for us? Well, the store is actually storing
the state as part of its class. And so the state gets initialized
to some initial state. And every time we dispatch an
action or dispatch an update, it handles that for us. It remembers what the reducer is. It knows how it should update
its own state by just doing this, dot, state, equals the old state,
applying an update through the reducer. But we don’t care about that. We don’t need to know the
implementation details. We don’t really care. All we care is that we just pass
this store.dispatch function, some update that we want to apply. And it goes ahead and
does the rest of it. We don’t even care that
state is stored as an object. That’s all just abstracted away. All that we care about
is we update the state by passing something into dispatch. And we get the state back by
invoking this thing called getState. We don’t care that the state is
just stored on the constructor. For all we know, maybe
it’s a tree, or maybe it’s some weird data structure
for performance. It doesn’t matter. All that we care about is
the API for this store. And this API is just getting the
state and dispatching state updates. And so let’s make sure it works. So we create a new store down here
by saying, give me a new store. I’m going to pass it, the reducer
function that we defined up here. So let’s actually just move this down. And maybe this as well. So now, let’s just not care about
the implementation details of store. All that we care about is we
define this thing called merge, which merges objects. We define a reducer which takes
some states and some update, and applies the update by merging it in. And then we create that new store with
that reducer and some initial value called foo foo. And let’s, for now, actually just
not pass an initial value at all. And let’s see what happens. And so we dispatch a few
updates, and then now let’s log some initials, or the
state after those dispatches. And so we get back what we wanted. We get back foo baz, bar bar,
which is the same thing as up here. And what’s the main difference? Well, the main difference
is that in reducer, we had to handle the updates ourselves. We had to handle passing in to reducer
the old state and the new updates. And in this new file called Store,
we no longer care about those things. We just care that we have a new store,
and we’re passing a new reducer. And we’re just dispatching
some additional changes. Cool. And so this is not really
super helpful for us right now. All it does is merge some objects. But let’s actually modify this
such that it will help us. And so in previous weeks, what we’ve
been working on is this contacts app. And so I changed a few things from last
week, or couple weeks ago to this week, namely that the login screen is
gone and the asynchronous request for fetching the contacts is gone. And so I just reverted
the app to where it was before we add asynchronous actions. And so what do we care about
in this simple application? Well, we care about
the context, obviously. We care about maybe adding new contacts
by using this Add button in the top. And we kind of care about the user. And so before we had this login
page where a user would log in– and presumably we should be
storing some sort of token or store the fact that
the users logged in– and so we need to care
about the contacts, and we need to care about some
concept of having a user-user app, so we can tell if
they’re logged in or not. And so let’s use what we’ve
implemented in Redux thus far to actually keep track
of that information in Redux, rather than having to do it
ourself scattered throughout our app. So currently our store takes
a reducer in an initial state. It stores those things. It gives us this thing called
getState, which returns our state, and gives us a method called dispatch
which applies any updates that we pass it to our state. And so we don’t have to
change anything there for the context of our contacts app. Though we do have to
change this reducer. So presumably, if we want to update– maybe we want to add a contact. The way that we would do that would
be appending that to an array. And so there’s not
really a convenient way to do this with our current reducers,
since it’s just merging objects. So let’s actually implement
a reducer that will. So say we– let’s call
it the contacts reducer. And the contacts reducer, or
contact reducer, takes the state and presumably the state
is all the old contacts, and rather than passing an update,
let’s just pass it a new contact. And what should this do? So state, presumably, is
just a list of old contacts. And so let’s just spread
those old contacts, and then add a new content at the end. And there’s our reducer
for our contacts. How about four-hour user? So maybe we should have– how should we keep track of our user? Or more importantly, what is
important to know about our user? Maybe something about their username,
maybe whether they are logged in or not, maybe some other
keys which have values. And so what’s the way that we generally
store key value pairs in JavaScript? It’s usually just a simple array. I mean a simple object. So let’s have this thing called
a userReducer, which takes an old state and maybe an update again. And how are we going to handle this? Well, we’ll just apply
the update to this state. So it’s basically the same
as our reducer from before. We’ll just merge into the
old state that update. Cool. So now we have two separate reducers. One handles any updates for contacts. One handles any updates for a user. And so how are we going to apply
that for our store as implemented? Because the store right now
only takes a single reducer. It doesn’t expect a reducer for
contacts, a reducer for users, a reducer for metadata, a reducer for
any other stuff that we want to store. It only expects one. And so we actually have to combine our
reducers into just a singular reducer. And so if you remember, when we were
talking about the pillars of Redux, everything is stored in one
singular store, so one big object. But right now, we have
two separate things that track our application state,
one for contacts and one for users. And so how might we go about
combining those reducers such that we have just one reducer or
one source of truth for our app? Well, the reducer is going to
just take a state and some update, just like all of our other reducers. And it’s going to have
to return some new state. And so we have to somehow tell, via
this update, what we should be changing. So we’re starting to run into
a little bit of trouble here. In our last iteration, we were just
passing an object to this dispatch. So its object just gets blindly
merged into our state in our store. But now our store is a
little bit complicated. As soon as we’re hitting
a real world use case, now we have multiple
different keys in our store. And it’s nontrivial how we’re going
to know exactly what we should update. If we just pass something like
foo foo into our new store, how do we know whether that
foo foo is an update for user, or foo foo is a new contact? We’re starting to run into
the limitations of how we’ve implemented our current reducer. And so how does Redux? Actually solve this? Well, through these
things called actions. So an action is basically
just a piece of data that contains all of the information
required to make a state update. And so in our previous
store implementation, this is true about our current update. Everything that we needed to update
our state is passed as an argument to dispatch. So this could be considered an action,
because it has everything that we need to update our state appropriately. But now we’re starting
to hit a limitation when we have a more complex store like this. We can’t just pass an
object like foo foo, because there’s no
longer enough information to make that full-state update. And so how are we going to do that? Well, generally actions are
objects with the type key. That didn’t necessarily have to be true
in our naive implementation of store. But now it makes more
sense to have an object, because we need more key value pairs. Usually, this object
has a key called type. Or in other words,
what are we doing here? Like, what update are we making? What should we be updating? And so there actually
is a loose spec for what is considered an action in Flux. And so let’s go ahead and open that up. And we see here a general guideline to
this thing called a standard action, or an action that we should
be using universally for all of our Flux or Redux applications. And so the motivation
behind that is that it’s a lot easier to work with actions if we
can make certain assumptions about how they are shaped. Because it wouldn’t really make
sense if in one application we have our actions as objects,
and maybe in a separate application we have a raise. In a separate application,
it’s a number. And maybe in another
application it’s a string. And so as we scale up, we want
to have some sort of uniformity across our applications. And so no matter what application
we go to maintain, we know, hey, our application actions
should all look the same. They should have some sort of key
that tells me the type of update that we’re doing here. And so the goal of these actions is
that they should be human-friendly, so Flux standard actions should
be easy to read, easy to write. They should be useful. Presumably, we’re not just
creating these arbitrary things for the sake of it. We want them to actually have a use
case, to be useful for application. And lastly, we want them to be simple. There’s no reason to add additional
complexity when we don’t need to. And so a basic action
might look like this. So we have a type that designates what
exactly should this action be doing. We have a payload, or in
other words, like, OK, I know I should be updating this. What should I actually update it with? And so, generally, actions
are objects with the type key, and maybe they have other things like
an air property, payload, or meta. And for our particular
implementation, we’ll be creating objects with
a type and a payload. And so let’s go ahead and do that now. All right. So now we know that when we pass
in this argument to dispatch, we need to include a little
bit more information. It’s not enough just to
have an object with foo foo. So let’s actually pass
a more complex object where the payload ends up being
foo foo, but that’s passing a type, so that we know exactly what
we should be doing here. And maybe the type is
something like update the user. And maybe down here,
we do the same thing. We want to update that user
with a payload of that bar bar, and the same thing done here. So now it looks a little
bit more complicated. Our actions are no
longer simple objects. Now they are a nested
object where they have a type that is letting us know that
we should be updating the user. And it has a payload, which
is just letting us know how we should be updating that user. And there’s a syntax error here. And so now let’s actually look
for that type in our reducer. And so now, rather than passing
a state and update to a user, now we’re passing a state and an action. And so now we have some sort of
signal about what part of the state we should be updating. And so let’s do if action,
dot, type is this thing called Update User, what should we do here? Well, we know exactly how
we want to update the user. It’s dictated by this userReducer. And we know how we want
to update the contact. We don’t. And so let’s actually return here an
object that maintains the old shape. So the old state is intact. And now we want to update the user key. And the user key, how do we update it? Well, we pass it into the userReducer. And what do we pass
into the userReducer? So right now, the userReducer
expects a state in update, and it’s going to merge the
update into the old state. And so what states are we
passing onto user-reducer here? Is it the state here? Not really, right? Because the state here is
responding to what state? It’s responding to
the entire application state, that includes all of the
contacts that we have and the user. And the userReducer really
doesn’t care about any contacts. It only cares about the user itself. And so rather than passing the
entire application state there, we only care about the user state. And so we can just do
state, dot, user here. And how do you want to update it? Well, it’s just that actions payload. And then if the action type isn’t
update user, just return the old state. So there’s going to be
invalid syntax here. So let’s just use our
merge helper function here, where we’re merging this
new object into state. So it’s basically the same thing
that we had written before, just in different syntax. So can we all follow what happened here? So we create a store,
and we pass in a reducer. What is the reducer? Well, the reducer, it takes a
state, and it takes an action. And what’s the logic behind the
way that we update the state? Well, if the action has a
type called Update User, we’re going to update the user. And we don’t care about any other types. If it passes any other type, then we’re
just going to return the state as is. But what exactly happens
when you passed in something where the type is update user? Well, we call that
userReducer with the payload. What is the payload? Well, it’s just any
update that we want to do. Why are we calling it payload? Well, that’s just the convention
as dictated by Flux [INAUDIBLE] And so what happens in this userReducer? Well, we pass it the
old state of the user. And we pass it the update
that we want to make. And we don’t really
care what happens there. It just happens to be
the case that it merges. And so what do we expect to happen
after we dispatch these three actions? We update the user with foo foo. We update the user with bar bar. We update the user again with foo baz. And we expect to see
exactly what we saw before. That is an object with a
key foo with the value baz, and a key bar with the value bar. Is that what we actually get back,
when we call store, dot, getState? I don’t think that’s going to be the
case, because there’s a syntax error. This error here is because
state, right now, is undefined. And so there’s no such
thing as state, dot, user. And so we should probably pass in
some sort of default user or default application state. And so let’s just say
the default state is– let’s create a const
called Default State. And let’s say the user
is just an empty object, and the contacts are an empty array. And let’s pass that in when
we create our new store. So now we’ve passed in
some default states. So when we do state, dot, user,
we’re not getting an error there. And what do we get back? We don’t get back what we
were getting back earlier. And what we’re getting
back earlier was just this, an object with a couple of key
values where it’s foo baz and bar bar. Now we’re getting back this thing. We see that object within our state,
but that’s not our entire state. That object is contained within
a part of our state called User. And there’s also a separate
part of our state called Contacts, which is an empty array– which makes sense when you
think about what we just did. Our application is no
longer just a single object. It’s now an object that looks like this. We have a key that corresponds
to our user information. You have a key that corresponds to
all of the contacts that we have. And so when we’re dispatching
actions that update the user, they’re updating the user. But there’s still a whole separate
part of our application state that corresponds to those contacts. And so now let’s strengthen our
store by allowing us to add contacts. How might we want to do that? Maybe we should add a new type. So remember, at the core
of Redux is the reducer, which handles the logic
between receiving the action and updating the state
of our application. And so in our reducer here,
we’re going to add more logic for when we want to add a contact. And how do we know when we’re
trying to add a contact? Well, it’s dictated by the type
of action that we’re receiving. So if we get a new action type,
and let’s call this Update Contact, now we want to do something different. Now, we don’t really care
about any update with the user. Now, we only care about
updating our contacts. And so what might we want to do? Well, let’s return a new state. And what does that new state look like? Well, let’s update the old
state, so merge in old state this new thing where
contacts are updated. And so how are you going
to update contacts? What logic dictates how we
want to update our contact? We could just do, give me the old
contacts and add a new contacts. And where is the new contact
action, dot, payload? But this logic is already
written somewhere in our app. We already declared this thing
called a contactReducer, which does this exact thing. What it does is it expects the state
of the old contacts and a new contact. And so we abstracted that
information away into this thing called the contactReducer, which
expects the old state of contacts. And so we don’t want to pass just
the entire application state. We only care about the
contacts portion of it. And then we pass the payload. So very similar to what
we did up here, up here when we receive an action
called update our user, we go ahead and invoke the
userReducer on that user. And when we receive an
action to update the contact, now we just do the same thing
but for the contacts instead. So now let’s test this out. So now let’s call store,
dot, dispatch, this thing with a type of Update Contact,
and a payload of name– my name– and number of
some arbitrary string. So now we just sent a separate action. And so now, if you notice, this action
has the same shape as the action up here. It’s a type and a payload. Now the type is Update
Contact, and the payload– rather than being the update for a
user– is actually the contact itself. And then our reducer knows exactly
what to do with all of these things. And so now let’s just run
this and see what happens. So now our state got updated. So our user still has the information
that we expect the user to have. But now we have some contacts. Now it’s an array that includes
me and my phone number. Don’t call it, because it’s
not actually my phone number. And so what happens when you
try to add a separate user? Maybe our contact has two friends of
the exact same name and the same phone number. So now our application has
two contacts in the contacts. And they just happen to be the same
people because I cut and paste. Cool. So now we have some sort
of store that’s maintaining all of our application information. But admittedly, it’s not super clean. So let’s clean it up a little bit. So right now, we have our
action types hardcoded. We can keep them hardcoded,
but let’s actually create a variable for our actions. That way we can easily keep
track of our available actions. So let’s just create this thing called
Update User and call it Update User. And same thing, let’s create a const
for our action called Update Contact. And now, rather than hardcoding this
string here, we can use that constant. And then same here. So what this does is it
protects us against typos. So say I had accidentally typed
this in as Update Users plural, now it’s never going to match. And it might be difficult
when we’re trying to debug. Like, hey, what’s going wrong? I’m passing this action called Update
User, and it’s not updating our user. Why? Because we’re actually looking for an
action type called Update Users plural. And so to save us from that, we just
create a constant called Update User. And the way that we enforce that
our action types are the same is by using JavaScript itself. If we made a typo here called Update
Users, what’s going to happen? We get an error, because this constant
called Update Users is not defined. And so by storing our
actions as constants, we can ensure that we don’t have
any weird bugs due to typos. So let’s actually move this to the top
of the file, that way we can use it in our reducers. And we’ll leave a little
comment for ourself, and let’s call these Action Types. Cool. So a little bit cleaner now. We no longer have those
hardcoded strings. But now, every single time we
want to dispatch an action, we have to type that entire action out. And so maybe it might be better
to create a function that creates our actions for us. And so let’s just create a
function called Update User. And what does it do? Well, it takes some new user– let’s just update. So it takes some sort of
update, and what does it do? Well, it returns a new object,
because, as we remember, actions are just objects
of a certain type. And the reason that we create
those is because of the standard called Flux standard action. And so each of them have a type key. And what is the type every time
you want to update the user? Well, it’s just going to be Update User. And what is the payload here? Well, it depends what we’re
invoking Update User on. And so let’s just pass the updated in. So now, rather than typing this whole
thing out, we can just do Update User and pass the update
that we want to make. And what happens here? Well, Update User up here gets invoked. We pass the update that we want
to make, and it returns an action. And the action gets passed to
dispatch as if we had done this. It’s just a little bit
cleaner because we don’t have to type this whole string out. So now let’s do the same
thing here, and here. Cool. Looking cleaner already. This also looks a little bit gross,
so let’s create a creator for that as well. And let’s call this Add Contact. And we’re going to
pass it to newContact. And how is this going to look? Well, just like our
other action creator, it’s going to return an action. What does an action look like? Well, it has a type attribute. And what type is it? It’s going to be Update Contact. And it’s also going to have a payload. And what is the payload here? Well, it’s going to be that NewContact. And so now we can do the same thing by
calling Add Contact here and on here as well. Cool. And now it’s looking
a lot more readable. So let’s just label these as are
action creators, because they’re creating actions. We have our store– our reducer. We have our specific
reducers for each key. And then we pass those to our store, and
we go ahead and dispatch foo actions. And what actions are we dispatching? Well, it seems a lot easier to read now. First we say, hey store, go
ahead and update the user. And what are we updating it with? Well, we’re just passing foo foo. Go ahead and update the
user again with this. Update user again with this. Now add a contact that looks like this. Add a contact that looks like this. And now, all of a sudden,
it’s very readable. We create a new store. We pass it in the reducer
in the default state. We dispatch a few actions. And then we go ahead and
get the store, get the state and see what it looks like. And if I didn’t make any typos– oh, I made some typos. I never fixed the typo that I made
intentionally, so let’s fix that. Now we get back exactly what we wanted. Great. So now our homemade simpleRedux
is looking pretty good. We’ve implemented most
of what is core to Redux. There is a single source
of truth for our data. It’s in our store. The state can only be
updated by actions. So remember, we’re dispatching
actions down there, and there’s really no
other way to update. And how are we doing
the update ourselves? Well, it’s a pure function
that takes the old state, and some sort of payload or update– or action, I should say– and then goes ahead and does it. Did it do anything else? Did it console log? Did it change any variables
outside its scope? Did it use anything outside of
the arguments that it was passed? No, it didn’t. It was just a pure function of
the old state and the updates. And it has this pattern, right? We’re sending actions that pass through
the reducer and it updates our store. And so now, we’ve pretty much
implemented a simpleRedux. There’s just one scalability
problem with our current Redux. So what happens when we
want to update a user? We also– or maybe we want to update a
contact, and maybe, in our userReducer, we want to keep track of the number
of contacts in our user metadata. We’re going to have to change a lot
of lines of code in order to do that. And so as we start to add more reducers,
start to add more things to our state, in our current implementation of our
reducer, it’s non-trivial to do that. And so let’s actually figure
out a better way to do that. Right now, what we’re doing is
our reducer is taking an action, consuming it, and sending it
to one of the smaller reducers that we’ve created. But there’s really no reason
that we need to do that. Maybe a better way would
be to take an action and pass it to every single reducer. That way, it’s up to the
smaller reducer to decide, hey, I want to listen to this action. Or maybe I’m just going to ignore
this action and return my type. So again, why are we doing that? It’s because we run
into scalability issues if we want multiple actions, or
multiple of these smaller reducers, to react to multiple of the types. So what exactly would that look like? So now, rather than our main reducer
here deciding which of these reducers to pass to, we should just
pass the action to all of them. And so let’s actually redefine
our reducer down here. So right now, our reducer
is taking a state in action. And maybe what it should
be doing is passing back a function where we know
we want users, our user, and we know we want our contacts. And what is responsible
for keeping track of our user part of our global state? Well, it’s the reducer
that’s specific to the user. And so maybe here we want to invoke
the userReducer with some values. And maybe here, we want to invoke
the contacts reducer with some value. So now it’s starting
to look a lot simpler. And what values does the
userReducer care about? Well, just the part of the state that
it cares about, so the state, dot, user. And same thing with the contactReducer. We only care about the part of state
that’s useful to the contactReducer. And how does it know what it should do? Well, we just passed the action. So that there is our new reducer. And so let’s now modify our
userReducer and contactReducer to only respond to the action
types that they care about. So let’s get rid of this, because
now we need additional logic here, and maybe some additional logic here. So currently, what
cares about Update User? Well, it’s just the userReducer. So if the action type is
Update User in the userReducer, let’s go ahead and return that update. Otherwise, let’s just return
the state as it was before. And now we can get rid of
all of that logic here. And now let’s do a similar thing
with the logic for contactReducer. So if the action type is Update Contact,
the contactReducer cares about that. And we should return just this. Else we’ll return the basic state. So let me just finish deleting
all the code that we don’t need. All right. Almost there. There are still a couple of bugs. So for the conductReducer,
what is action? Action, dot, type
doesn’t exist currently, so we’re no longer
passing it the newContact. Now, we’re passing it the action. And so we check the action, dot, type. And newContact no longer exists. Where are we storing the newContact? Well, it’s just action, dot, payload. And we do the same thing here. We’re no longer just passing the update. We’re passing the whole action, and
then we can do action, dot, payload. And now we’re done. So we can make this a
little bit more concise, if it’s easier to read this way. And now it all fits on one page. So we have our contactReducer,
which cares about the state. And when we say state, it’s local only
to the contacts part of the state. It received actions. And if the action is
that it should update the contact, returning a new state. And what is the new state? Well, it’s the old state, and we append
to the end whatever the payload is. And in this case, the action,
dot, payload is the new user. Does contactReducer
care about Update User? No, it doesn’t. So if the action, dot,
type is an Update Contact, we just return whatever the
state was before, no big deal. And the same thing is
true for userReducer. It’s taking a state. What state does it care about? Well, it’s only the user part
of our application state. It takes an action as well. And if the action type is that
it should update the user, then it goes ahead and does that. It returns the old state. And into that, we merge the
new action, dot, payload. And if the action type
is in Update User, just return whatever the
user information was before. And one key difference here is
that for every single action, every single one of
these reducers is called. This reducer is called and passes that
action along with the subset of state that each one of these
reducers cares about to them. And then these reducers
only do something if the type is something
that they care about. Otherwise, they just return
the state blindly as it was. So now everything is a lot more concise. And now if we wanted
to respond to changes in other types in the
userReducer, we can. So if we wanted to store something
in the user part of the state every single time we add a
contact, we can do that now. So maybe if we add a new
user, or a NewContact– maybe want to store that most recently
added contact in the user information– we could do that by doing state. And what are we merging into state? Well, let’s just do recently
added or prevContact is whatever that contact
is, so action, dot, payload. And now when we add new contacts, the
previous contact is saved in the user. And so now, every single reducer
can respond to every single action. And let’s just make
sure it works by adding the contact that wasn’t
the same as before, as soon as my computer unfreezes. Let’s just kill. So let’s, again, go into our
simpleRedux and ensure that we don’t pass the same user in again. So we can just sanity check to make
sure you’re saving the most recent user. We can add David here. And now we can confirm that the
previous contacts stored in the user is indeed the most
recently-added contact. And so now we have pretty much
a full implementation of Redux. So it’s a data-management
utility that we’ve created. There is a single source of
truth for all of our data. It’s stored in our store. The state can only be
updated by our actions. And updates are made using our
peer functions or our reducers. And we’ve created a reducer. We’ve created a store
that maintains the state. We can get the state using getState. And we’ve created some actions. So let’s go ahead and
take a short break. And then when we come back, we can
add Redux to our actual application. All right, hello and welcome back. Before the break, we actually
implemented our own small simpleRedux. And now, let’s actually start
moving from our simpleRedux, to the actual Redux implementation. Our Redux implementation already
has a pretty similar API. What we’re missing is just a way to
notify that the state is updated. And so let’s actually install
Redux and start to convert what we have into actual Redux. So one good place to start
would be to NPM install Redux. And so, as we talked about
in previous lectures, NPM install will
install some NPM package and add it automatically to
our package, dot, json file. And so if we now look at
our package, dot, json, we see that Redux has
been added down here. So let’s now go ahead and start
moving what we have into actual Redux. So I just copied exactly what we
had from our simple implementation into this new directory called Redux. And here, we’re going to
start actually using Redux. And so first thing we need to do is
add this thing called createReducer, or createStore rather. So if we import createStore
from Redux, now we have Redux’s implementation of a store. And so previously we
had our class store, which took a reducer, and
maybe some initial state, and implemented all of the things
that we thought we needed ourselves. But now, we’re actually going to
use this function called createStore from Redux. And so now, we can actually just
delete that implementation ourselves. And so now when we do new store down
here, now instead of doing new store, we’ll just invoke that thing
called createStore from Redux. And since I read that
documentation, I know that it takes a reducer and some default state. And then we can dispatch some actions. And other than that, I think that’s it. So that one line change
there actually took us from our simple implementation of Redux
to actual implementation of Redux. Unfortunately, if we
want to run this in node, it doesn’t support
that import statement. So I’m just going to do this real quick,
which does effectively the same thing, just so that we can test it. So let’s run this. And what do you know? We got back exactly what we had before. So it turns out the simple
implementation of Redux that we actually wrote pretty
much adheres to the same API that Redux itself has. And so it didn’t really take much for us
to move from our simple implementation to actual Redux. Redux actually, also, gives
us a few other goodies. So right now, the way that we
create a reducer that dispatches or that passes actions on
to their relevant reducers is by combining them using a
function that we defined here. But it turns out Redux also gives us a
function called combineReducer, which does effectively the same thing for us. Rather than us writing a function
that combines the reducers, we can actually invoke this
thing called createReducer, or combineReducers from Redux. That just takes an object that
maps the key in the actual store with the reducer that controls that key. And so it can do user gets userReducer
and contacts gets contactReducer. And now, we’ve basically taken
a little bit more complexity out of our application and
maintain the same functionality. So again, if we test this, we
don’t have the initial state. So one difference here is that– did we not pass the initial state in? Let’s actually– just to safeguard real
quick, that if this is null, or empty, we’ll go ahead and
just create in Default State for each individual reducer. So for the user, let’s
have an empty object. And for the contacts, let’s have an
empty array like that, and now run it. Now we get back exactly what
we had before, as expected. The one minor difference is that rather
than having a default state app-wide, we can actually just define the Default
State in the reducers themselves. And so the Default State for the
contacts in our contactReducer is just an empty array. And the Default State for
our user, if the userReducer is invoked with no current state,
we’ll go ahead and initialize it to some empty object. And so now, I see even
more complexity gone that just goes into the
Redux library, rather than our own implementation in this file. Great. So now we have a working store. It’s just a lot of
stuff in a single file. There’s no reason that
our store, dot, js file should have our store,
but also our action creators, also our reducers, and
also all of our actions. And so let’s start to split
those out into separate files so that they’re easier
to maintain and find. So let’s copy the store in to read
this thing called reducer, dot, js– and also copy the store into this
thing called actions, dot, js. So in reducer– actually,
let’s do actions first. In actions, what do we
care about in actions? We really only care about our action
types and our action creators. CombineReducers, those
things about stores, we really don’t care about at
all, so we can just delete that. We don’t care about that. We don’t care about
any of these reducers. But we do, however, care
about the action creators. So now, in our file
called actions, dot, js, we define the things that
have to do with our actions. So there are our action types
and our action creators. Some people prefer to have action types
and action creators in separate files. But since we only have
two of each, let’s just keep them in these files called
actions, dot, js for now. And let’s go ahead and export them. So now in our actions,
dot, js file, we are just exporting the available action types. And we’re also exporting the
available action creators. And that’s all that this file
called actions cares about. Now let’s move on to reducer. So does the reducer care about
combineReducers and createStore? Well, half of it, it cares about
combineReducers but not createStore. And let’s actually move back
to our ES6 import, syntax. Do we care about action types? Yes, we do, because the reducer
is checking against action types. But there’s no reason that we
should define them in this file. We should actually just import
them from our file called actions. And now we can go ahead
and delete the lines here. And so now, what we have in this
file is basically only the things that we need to define our reducers. And so we need this helper
function called merge, which takes a couple of objects
and merges them together. We define our Default State here,
though we don’t really need to. And then we have two separate reducers
for two parts of our application. So for the part of our application
that cares about contacts, we have this reducer,
which takes state, which is initialized to an empty array–
so just an empty list of contacts. We take an action, and we match against
the action to see what we should do. We have the reducer for users here. So we say, give me a
starting state for our users. If there’s no previous state, just
initialize it to an empty object. And give me an action, and we’ll go
ahead and match against the action type to see what we should do. And then we have our
actual reducer that we’re going to pass to our store, which
just combines those two reducers. So just to stay maintainable
and scalable, what we’re doing is rather than having one reducer
check against all action types and maintain the entire state,
we have separate reducers for the part of the state that
cares about user and the part of the state that cares about contacts. And if we want to scale
up to the point where our user is starting
to be an object that is unmaintainable by a single
reducer, what we can do is we can create one reducer
for half of that user, and maybe another user for another
half, or maybe a third reducer for a third key in there– and go ahead and do something here,
where it’s, like, combine reducers. And maybe we have something
that cares about the user’s meta data that’s called userMetaReducer. And maybe we have something that
cares about the user’s logins. And so every time they
have a login, we’ll go ahead and add that to this
part of the user’s state. And so maybe that is
the userLoginReducer. And I’m just making up examples
that are somewhat arbitrary. But I’m just trying to show that
as our application scales up, and as our application state gets
larger and larger, what we can do is we can still take
bite-sized pieces of that state and manage them by one small reducer. And we can compose these reducers
into just one massive reducer that can take care of the entire state. And so this combineReducer,
what it does is it allows us to split up one object
into a bunch of separate object keys that are maintained
by a separate reducer. And so since our application
is pretty simple right now, we can do that all in one. But if we wanted to scale up, it is
possible to start nesting reducers like that. Great. So let’s actually export
default, this reducer, because that’s the only thing that
we want to expose from this file. We don’t ever want somebody to use
contactReducer or userReducer alone. We just want them to interface
with that through this big reducer that we have at the end. And if it’s easier for
you to follow like this, we can define this variable
called reducer and export default at the end, which is exactly
the same as what we did before. But now, we have a handy
variable name to let us know exactly what this combineReducer is. Great. So that’s the reducer. And now, lastly, let’s
clean out this store file. So we no longer care about
that combineReducer function. Now we only care about createStore. Do we care about action
types in our store file? No, we don’t. So let me delete that. Do we care about merge
and those two reducers? No, we don’t. We can delete that. Do we care about our main reducer? Yes, we do, but not in
the context of this file. We can just import it
from our other file. And so rather than
implementing it here, we can just import reducer from that
file and delete the declaration here. Do we care about action creators? Not in this file. Do we care about store? Yes, we do. So let’s have const
store be this thing that creates the store from the reducer. And we don’t really
need the Default State since we declared it in each
one of the reducers themselves. So the Default State is basically
this, where an empty contacts key just becomes an empty array. And an empty user
becomes an empty object. And so we’re still dispatching
a bunch of actions. Let’s comment those out for
now and export this store. Great. So now, everything is in
its own separate file. Everything is very readable. I believe every single file can
basically be read without scrolling. Oh, I lied. But very close to that, which
is pretty impressive since we’re only showing fewer than
20 lines at a time. So it’s very easy to now
navigate through our application and know exactly where we need to
update something if we need to do so. So if we need to add a new action,
where are we going to do that? Well, in our actions file, we
can add any action types up here and any action creators down here. If we want to change any logic in our
reducer, we know exactly where that is. It’s right here. If we want to change how we handle
contacts, we can do that here. If we want to do so with four
users, we can do that here. And I immediately see a way that
we can make this more readable. Right now, what we’re doing
is we’re doing if action, dot, type is Update User, do this. If action, dot, type is this, do this. If action, dot, type is this, do this. So we keep matching against this
string called action, dot, type. There’s actually something
built into JavaScript that allows us to more easily
match against a single string. And that’s the switch statement. So we could do switch based
on action, dot, type here. And if you’re new to the
syntax, basically what it does is it allows us to check
a particular variable and match it against a bunch
of different possible values. And so if the action, dot, type– it’s the case where it’s Update User,
then we’re going to do something. And that something is to do this. If it’s Update Contact,
instead we’re going to do this. Otherwise, just return the old state. And now it’s a lot easier to read. So there is no longer a
bunch of if-statements. We just know that what we care
about is action, dot, type. And if the case is a Update
User, then we return this. If the case is Update
Contact, we return this. And if it doesn’t match any
of those things, by default we should just return
to the original state. And then again, we combine the
reducers and export it at the end. And then we import it into
our file in store, dot, js, which just creates a store out
of that reducer and exports it. So again, we didn’t really
change any of the logic there. We just split things into
smaller, bite-sized pieces. And so a lot of a recurring
theme that we see in this class is take a big problem and split
it into smaller pieces that are bite-size and maybe easier to digest. So we went ahead and did
that with our store here. And so now, let’s talk about
it in the context of React. So far, we’ve only
really talked about Redux as a standalone library
that tracks some data. But what we really care about is
having that data update some views. And so we want to respond
to, or React to rather, changes in data in
our application or UI. And so now we have a
few questions to answer. How do we get the info from
our store into our components? Because right now, we’ve been using
our store as just a standalone module and accessing effectively
the command line. But now, how do you get this
to run in an application? Well, it turns out our store has a
handy method for getting the info. It’s called store, dot, getState. And so in our application,
what we can do is we can actually access our
state from our store directly. So we can do store, dot,
getState to receive that. And so now let’s start integrating
Redux into our application. So we already have this
thing called store. Let’s uncomment our
mode of adding contacts. And so right now, I can’t
run this at the command line, because we’re using this import syntax. But our application state should be some
contacts where the three contacts are myself twice and David once. And now we can go ahead and read from
our store in our React application. So if you remember what our
application looks like is we just have a bunch of contacts. And these contacts can be
toggleable, but they just really just show on this simple list page. And so to dive back into the
code from the past few lectures, where do we actually get those contacts? Is it in the contact list file? No, everything is in
our app, dot, js file. Right now, we’re just
implementing our contacts from our random-contact generator
file that we wrote a few weeks ago. But ultimately, we’re setting states in
our application class and passing that through our navigation
via screen props, and then accessing it in our relevant page by
doing, like, this, dot, props, dot, navigator, dot, getParam, dot, whatever. And so it’s a lot of work just
to get our single key there. But there’s actually more to that. We’re also passing our contacts to every
single other page in our application, even the ones that don’t care about it. And so now what’s nice
about Redux is we can only listen to that information
in the pages that we actively care about the information. And so we don’t really care about
contacts in all of our pages, right? We only care about it in
the page that displays them. And so let’s delete this code that
passes the contacts as a screen prop to every single page. So if you remember,
back a few weeks ago, Brent was saying that there’s
a better way than passing screen props to every page. And he used that word Redux,
which we didn’t know what that meant a few weeks ago, but now we do. Now there’s actually a
way where we can say, oh, this information is only relevant
to this particular component. And so now in that
particular component, we can just have it listen
directly to the state without having to pass that
state to every single component. And again, how are we going
to get that information from the store to our components? Well, we can use store, dot, getState. And so let’s do that. So our screen called
ContactListScreen is where we’re getting that information, right? Now, what we’re doing is this, dot,
props, dot, screenProps, dot, contacts. And so let’s actually
do something different. So rather than looking
at our screenProps, let’s do import store
for Redux, slash, store. So now that’s our store. How do we get our information? Well, we can just do const
contacts is store, dot, getState– which is how we get the
state from our store– dot, contacts. Because getState returns the
entire state of our application. And if you remember, what that looks
like is user with some user information and contacts with our list of contacts. We only care about
the dot contacts part. And so we can grab that and
just pass that directly to here. And so now, hopefully– so AddContact. Oh, I uncommented something
without importing what it needed. So let’s just go back to Redux. So I ended up using addContact
here without importing it. And so if I wanted to use that
addContact action creator, I need to first import
addContact from our actions file. And so now, we can see
that we’re creating our– people are showing up, however
their phone numbers are not– let’s see why that’s happening. So we’re creating people
with name and numbers. And I assume that row is
actually looking at a different– I think it’s looking
for a prop called phone, so let’s actually just
change number to phone. And so now, we should be
getting those phone numbers. So basically we had a bug where I
thought contacts were name and number, but we actually defined a
contact to be name and phone. And so by storing the correct data,
we then get it back in our application here. Cool. So this is awesome because if you
ignore the keys that we didn’t add, now we’re getting that data in only
the list that cares about it. So the separate screens that
don’t care about the contacts don’t get access to our contacts. The scope of that
information is limited only to the component that cares
about it, which is a good thing. So now, how are we going
to update that store? So if you remember back
to the drawing of Redux, what’s the only way to update a store? Well, it’s by dispatching an action. And how do we dispatch an action? Well, we do it from a view. And so we can go ahead and
add that action to our view. So we need to use store,
dot, dispatch in order to send that action to our store. We need to do store,
dot, dispatch in order to send that information to our store. And so let’s actually do that. So in our previous
iteration of the app– earlier what we were
doing is we were passing this function called Add Contact
to every single one of our screens via this thing called screen prompts. But now, we can do better by only
passing the Add Contact function to the screen that cares about it. And what screen is that? Well, it’s the one
called Add ContactScreen. And if you remember right now, the
way that we’re handling the Submit is by doing this dot props, dot,
screenProps, dot addContact, and then we pass form state. But we have a better way to do that now. We can actually just
directly dispatch an action. And so let’s do that. So first, we need to import
store from our store. And we also need our
action that we care about. So let’s import Add
Contact from Redux actions. And so now, what do we want
to do for handleSubmit? So rather than this, we can actually
just do store, dot, dispatch. And what are we dispatching? Well, it’s the same thing that
we were dispatching before. It’s an action called
addContact, or action creator I should say– called addContact. And what is our action? Well, it’s just our form state. And that’s it. Actually, our form state has more
information than we care about. So if we really want
to be succinct, we can say the name is formState, dot, name. And our phone is formState, dot, phone. And now, presumably we
did what we wanted to do. So let’s go and check. We can add a new contact. Let’s add Yowon Y, and
give a phone number. What do you know? He has the same phone number as I do. And submit– oh, wait a second. Yowon didn’t show up. But if we toggle contacts and reappear,
look, there– oh, I spelled name wrong. But there is what should be Yowon. So why didn’t he show up the first time? So there’s one thing that we’re missing
as we’ve added Redux to our app. It’s that we’re not
getting the application to update when the store changes. And this is a bug that we’ve seen
before, but this time it’s different. And so how might we get our
application to automatically re-render every single time our store changes? Well, presumably, we would want to
use the other part of Redux that calls a callback every
single time it adds, which is a lot of extra work for us. I wonder if there’s a way
that we could do that better. So we’ve talked about this
thing in a few lectures before, this thing called a HOC, or in
other words a higher-order component. What a higher-order component is it
takes a component as an argument, or it returns a component
as a result of a function. And so we could actually
create a HOC that does a lot of what we’ve
just done manually for us. What if we created a HOC
that is the following? It checks for state
updates automatically, or it subscribes to them, and then it
passes new props to when that happens. And wouldn’t it be also cool if
rather than having to go bind our action creators
ourselves, like we did here– or here, this line– what if we instead did
that automatically? What if our HOC for us automatically
bound our action creators to our dispatch function for us? That would be really cool. Then we wouldn’t need to
subscribe to store updates. Our higher-order component
can do that for us. And when I say our
higher-order component, I mean the one that React
conveniently implemented for us. And so if we check this
thing called React Redux, we see some official
React bindings for Redux, which gives us a few cool things. And by a few, I mean two. We have this thing called a provider
and this thing called connect. And if we look at what
this is, a provider– it basically handles the binding for us. It listens to a store, and
it’ll see when it updates. And then this thing called connect
is a higher-order component, so it takes a component as an argument. Or actually it takes some configuration
as an argument, returns a function that’s now expecting a
component as an argument. We’ll see exactly what
that means in a second. But basically it does
a lot of things for us. It passes only the relevant
props that we care about. So this function– the
first argument we pass it is this thing called mapStateToProps,
which is actually a function that takes our entire application state
and will return an object of props that we care about, meaning
for this particular component, it doesn’t care about our
entire application state. It only cares about a subset of it. And for a particular example,
our ContactListScreen doesn’t care about our entire
state of our component. We only care about this
one key called contacts. And so it would be great if we
had in higher-order function– which handles this for us. It listens to the store. It grabs the state, and it
only passes contacts down. And the way it does it is a prop. And so this connect function
implements that behavior for us. We pass a function that
maps our application state to own the props
that we care about, and then it passes those
down as props for us. And so let’s go ahead and first install
this library so that we can use it. And if you haven’t,
install React, Redux. And as you remember, this will also
add it to our package, dot, json. So if I look at our package, dot,
json, I can see React, Redux got added. And now, we can go ahead and
use that connect function. So in our ContactListScreen, rather
than importing the store itself, let’s actually only import connect. So we now have access to this
higher-order component called connect, which we can now use. So rather than default
exporting this entire class, let’s actually wrap it in
a higher-order component. So let’s down here do
export, default, connect. We’re going to pass it something. And then we’re going to pass
the class that we created, so this thing called ContactListScreen. And what do we pass it? Well, we pass it a function that maps
our application state to our props. And so we can do const mapStateToProps. I can really call this whatever I want. Let’s have it take our
entire application state and only return the subset that we care
about, or in other words, the contacts. And so a prop called contacts
will get mapped to sate, dot, contacts, and then of course
pass that function into connect. So again, the first argument
to connect is just a function, which I happen to call mapStateToProps. We can call it whatever you want. We can call it getState– or PropsFromState. But I’ll just follow convention
and call it mapStateToProps, which takes the entire
application state and returns a subset that we care
about, or in other words, the prop contacts, which
maps to state, dot, contacts. And then in here, we no
longer have to do that. Now we just do this, dot,
props, dot, contacts. Because what this higher-order
function does is it listens to our application state. It will automatically
update as the state updates, and it will pass down
some number of props. And the props are just contacts for now. And what is contacts? Well, it’s whatever the
state value of contacts is. And so that is done. Unfortunately, it doesn’t
quite work, because it doesn’t know what our store is. And so now we also have to use
the other part of React Redux and let our application
know what our store is. And that is through that
component called Provider. So if we import Provider from
React, Redux, what that does is it provides our app with a
concept of what our Redux store is. So we can just wrap our entire
application in this provider and let it know what our store is. And this component will provide
any of its children with our store. And so our store, we should just
import from Redux, or the reduction implementation that we wrote. And now, the store is passed to any
of the connect functions that we have. And the connect function, which
we wrote in ContactListScreen, it can now listen to the store and map
any changes in the application state to props that it passes to this
ContactListScreen, which we then listen to to fill our list. And so now we see that it’s
been sent here, which is cool. Lastly, real quick. What it also does is
we can automatically bind our action creators to dispatch. And so a quick example of that
is in our addContact screen. Currently we’re not
doing any binding at all. We’re just passing addContact,
the return value of that, straight to store, dot, dispatch. And it turns out, rather than
doing that manually, we can just import connect from React, Redux. And rather than default
exporting this entire class, we can just export a
wrapped version of this. So we’ll pass connect,
some configuration, and then pass it our addContact screen. And connect takes a couple of arguments. The first one is mapStateToProps. Our addContacts screen doesn’t care
about any of our application states. So let’s just pass
some null value there. But it does care about our actions. And so we can pass an object
into our second argument here, and it will automatically bind
our action creators with dispatch. And so if I did something
like addContact, so we’re going to get a
prop called addContact. What’s its value going to be? Well, it’s going to be addContact,
which is the function that we imported from our Redux actions. And then it’ll actually just
pass as a prop to our component. So we can do this, dot,
props, dot, addContact, which will automatically dispatch
that up for us, because it gets automatically bound by connect. And if you want to read
more about how that happens, the documentation is
linked in the slide. And now, just to ensure this works,
we can go ahead and add Yowon Y here with a phone number like so. Submit. And look at that. Our application automatically updated
due to a update in our application state. So this was a quick example
of how we use Redux and React. And next week, we’ll dive
even farther into Redux and show how we handle stuff
like asynchronous actions once we do some data-fetching. So thanks, I’ll see you next week.

7 comments on “Redux – Lecture 9 – CS50’s Mobile App Development with React Native

  1. Jordan i love your explanation but don't fool me dude that FLUX diagram is no unidirectional. Come on now.

  2. I have to watch each of these lectures 3x in order to truly get everything with the pace Jordan and Brian go at. Man, Harvard students must be smart.

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