Intro to Redux: Reading Data from State

Learning Goals

Use the createStore() function provided by the Redux library
Use the useSelector() and useDispatch() hooks provided by React-Redux to access and update the store

Introduction

In the previous section, we have been building using a createStore() function that we wrote, and passing a reducer to it. We have been using the dispatch() method from the store, to dispatch actions and update the state.

Now let's think about which part of our application would belong in the official Redux library — that is, which part of our codebase would be common to all applications. Well, probably not the reducer as our reducers seem unique to each React & Redux application. The reducers are unique because sometimes we have reducers that would add or remove items, or add or remove users, or edit users, etc. What these actions are and how the reducer manages the state is customized. Thus, the reducer would not be part of the redux library that other developers would use to build their application.

The createStore() function, however, is generic across Redux applications. It always returns a store (given a reducer) that will have a dispatch method and a getState method.

So from now on, we will import our createStore() function from the official Redux library. Normally, to install Redux into a React application, you need to install two packages, redux and react-redux by running npm install redux react-redux. These are already included in this lesson's package.json file, so all you need to do is run npm install && npm start to get started.

Step 1: Setting Up The Store

First things first, we'll use Redux to initialize our store and pass it down to our top-level container component.

Redux provides a function, createStore(), that, when invoked, returns an instance of the Redux store for us. So we can use that function to create a store. We want to import createStore() in our src/index.js file, where ReactDOM renders our application.

// ./src/index.js
import React from "react";
import ReactDOM from "react-dom";
import { createStore } from "redux"; /* code change */
import counterReducer from "./features/counter/counterSlice.js";
import App from "./App";
import "./index.css";

const store = createStore(counterReducer); /* code change */

ReactDOM.render(<App />, document.getElementById("root"));

Notice that we are importing the createStore function from Redux. Now, with the above set up, we could pass store down through App and we would be able to access the Redux store.

However, reducing the need for passing props is part of why Redux works well with React. To avoid passing store as a prop, we use the Provider component, which is imported from react-redux. The Provider component wraps the top level component, App, in this case, and is the only component where store is passed in:

// ./src/index.js
import React from "react";
import ReactDOM from "react-dom";
import { createStore } from "redux";
import { Provider } from "react-redux"; /* code change */
import counterReducer from "./features/counter/counterSlice.js";
import App from "./App";
import "./index.css";

const store = createStore(counterReducer);

// code change - added Provider to wrap around App
ReactDOM.render(
  <Provider store={store}>
    <App />
  </Provider> /* code change */,
  document.getElementById("root")
);

By including the Provider, we'll be able to access our Redux store and/or dispatch actions from any component we want, regardless of where it is on the component tree.

So, to recap, just like we did previously, we call our createStore() function in src/index.js. We pass our createStore() method a reducer, and then we pass our newly created store to our App component as a prop. You can find the reducer in ./src/features/counter/counterSlice.js:

// ./src/features/counter/counterSlice.js

const initialState = {
  items: [],
};

function counterReducer(state = initialState, action) {
  switch (action.type) {
    case "count/increment":
      return {
        ...state,
        items: state.items.concat(state.items.length + 1),
      };
    default:
      return state;
  }
}

export default counterReducer;

Our reducer is just producing a counter. It adds a new item to the list each time it is called, and that item is one more than the last item.

Instead of having all of our functions encapsulated in a closure within index.js as we did while building our own Redux set up, we've now separated out the reducer function, giving it a relevant name, counterReducer, and let the Redux library take care of our createStore function. These two pieces are both imported into src/index.js and used to create store.

This store value is then passed in as a prop to Provider.

Interacting with the Store: useDispatch and useSelector

To gain access to the store somewhere in our app, we use two hooks provided by react-redux: the useDispatch hook (for dispatching actions to the store), and the useSelector hook (for selecting parts of state to access within our components).

// ./src/features/counter/Counter.js

import React from "react";
import { useDispatch, useSelector } from "react-redux";

function Counter() {
  // read from the Redux store
  const items = useSelector((state) => state.items);

  // gives us the dispatch function to send actions to the Redux store
  const dispatch = useDispatch();

  function handleOnClick() {
    // dispatching an action on click
    dispatch({ type: "count/increment" });
  }

  return (
    <div>
      <button onClick={handleOnClick}>Click</button>
      <p>{items.length}</p>
    </div>
  );
}

export default Counter;

This code places a button on the page with an onClick event listener pointed to handleOnClick. When handleOnClick is invoked, it calls the dispatch function, provided by useDispatch, to send an action to our Redux store.

Remember from our earlier lessons that our Redux store has a special dispatch method that we must call any time we want to create a new state? The useDispatch hook gives us access to that dispatch method so we can use it from any of our components!

Similarly, in our previous Redux code, any time we wanted to access our store's internal state, we used the store's getState method. In the example above, the way we can interact with the getState method is via the useSelector hook. This hook takes a callback function that will get called with the state object from our Redux store. Whatever the callback function returns will be returned by the hook.

So in this example:

const items = useSelector((state) => state.items);

We're calling useSelector with a callback function, and returning the items key from our Redux store state.

Another effect of using the useSelector hook is that it effectively 'subscribes' our components to changes in the Redux store state. Whenever the value returned by our useSelector hook changes, the useSelector hook will cause our component to re-render. So as the store's items property increases, Counter will display a different number!

If you boot up the app, you should see a button on the page, followed by a zero. Every button click dispatches an action to our store, causing it to change. Since data (items) from that store is being accessed in Counter, Counter will re-render and display the updated counter.

Add Logging to Our Reducer

Ok, so getting our application to re-render takes a bit of work, and we're going to go into greater depth in the next sections. In the meantime, let's get some feedback. First, let's log our action and the new state. So we'll change the reducer to the following:

// ./src/features/counter/counterSlice.js

const initialState = {
  items: [],
};

function counterReducer(state = initialState, action) {
  switch (action.type) {
    case "count/increment":
      console.log("Current state.items length %s", state.items.length);
      console.log("Updating state.items length to %s", state.items.length + 1);
      return {
        ...state,
        items: state.items.concat(state.items.length + 1),
      };
    default:
      console.log("Initial state.items length %s", state.items.length);
      return state;
  }
}

export default counterReducer;

This may look like a lot, but really all were doing is adding some logging behavior. At the top of the function, we are logging the action. After the case statement, we are storing our state as current state first. Then we are logging the updating state value. Then under the default case statement, we just can log the previous state because this state is unchanged.

Now, refresh your app, and give it a shot. You should see the correct action being dispatched, as well as an update to the state. While we aren't getting our state directly from the store, we know that we are dispatching actions. We know this because each time we click a button, we call store.dispatch({ type: 'count/increment' }) and somehow this is hitting our reducer. So things are happening.

Redux DevTools

There is this amazing piece of software that allows us to nicely view the state of our store and each action that is dispatched. The software does a lot more than that. I'll let you read about it here: redux-devtools-extension. Ok, so let's get to incorporating this. In fact, every time we use the Redux library going forward, we should make sure we incorporate devtools. Otherwise, you are flying blind.

First, just Google for Redux Devtools Chrome. There you will find the Chrome extension for Redux. Please download it, and refresh Chrome. You will know that you have installed the extension if you go to your developer console in Google Chrome, and then at the top bar you will see a couple of arrows. Click those arrows, and if you see Redux as your dropdown, you properly installed the Chrome extension. Step one is done.

Second, we need to tell our application to communicate with this extension. Doing so is pretty easy. Now we change the arguments to our createStore function to the following:

// ./src/index.js
import React from "react";
import ReactDOM from "react-dom";
import { createStore } from "redux";
import { Provider } from "react-redux"; /* code change */
import counterReducer from "./features/counter/counterSlice.js";
import App from "./App";
import "./index.css";

const store = createStore(
  counterReducer,
  window.__REDUX_DEVTOOLS_EXTENSION__ && window.__REDUX_DEVTOOLS_EXTENSION__()
); /* code change */

ReactDOM.render(
  <Provider store={store}>
    <App />
  </Provider>,
  document.getElementById("root")
);

Notice that we are still passing through our reducer to the createStore function. The second argument is accessing our browser to find a function called __REDUX_DEVTOOLS_EXTENSION__. If that function is there, the function is executed. Now if you have your Chrome console opened, make sure the Redux DevTools Inspector is open (press command + shift + c, click on the arrows at the top right, and the dropdown for the extension). Now click on the tab that says state. You should see { items: [] }. If you do, it means that your app is now communicating with the DevTools. Click on the button in your application, to see if the state changes. Now for each time you click on it, you should see an action in the devtools that has the name of that action. If you are looking at the last state, you should see the changes in our state.

Whew!

Conclusion

In this lesson, we saw how to use the createStore() function. We saw that we can rely on the Redux library to provide this function, and that we still need to write our own reducer to tell the store what the new state will be given a particular action. We saw that when using the createStore() function, and passing through a reducer, we are able to change the state just as we did previously. We were able to see these changes by hooking our application up to a Chrome extension called Redux Devtools, and then providing the correct configuration.

marilynsaur / react-hooks-redux-intro-to-redux-codealong