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Intro to Redux: Reading Data from State

Objectives

• Use the createStore() method provided by the Redux library.

Introduction

In the previous section, we used a createStore() method that we wrote, and passed a reducer to it. We used 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(), method 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() method 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 && npm install 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.

In this code along, we'll be building a simple counter application that displays the value of the counter along with a button to increment it.

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. We want to import createStore() in our src/index.js file, where ReactDOM renders our application, and then use that function to create the store.

// ./src/index.js

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

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

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

Now, with the above set up, let's pass store down to App as a prop so it can access the Redux store.

// ./src/index.js

import React from "react";
import ReactDOM from "react-dom";
import { createStore } from "redux";
import counterReducer from "./reducers/counterReducer.js";
import App from "./App";
import "./index.css";

const store = createStore(counterReducer);


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

So, to recap, just like we did previously, we call our createStore() method 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/reducers/counterReducer.js:

// ./src/reducers/counterReducer.js

export default function counterReducer(
	state = {
		clicks: 0
	},
	action
) {
	switch (action.type) {
		case "INCREASE_COUNT":
			return {
				clicks: state.clicks + 1
			}
		default:
			return state;
	}
}

Each time an action with type 'INCREASE_COUNT' is dispatched to the reducer, the value of the counter is incremented.

Instead of having all of our functions encapsulated in a closure within index.js as we did while building our own Redux setup, 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.

Once we've created the store and passed it to the App component as a prop, we can access it using this.props.store:

// ./src/App.js

import React, { Component } from "react";
import "./App.css";

class App extends Component {
	handleOnClick = () => {
		this.props.store.dispatch({
		  type: 'INCREASE_COUNT',
		});
	  }

	render() {
		return (
			<div className="App">
				<button onClick={this.handleOnClick}>Click</button>
				<p>{this.props.store.getState().clicks}</p>
			</div>
		);
	}
}

export default App;

As you recall, the store contains two methods: dispatch and getState. We use the getState method in our render method to get the current state so we can display it on the page. We also have an event handler that calls the dispatch method, passing in our action, when the button is clicked.

Now, with this code in place, if you boot up the app you should see a button on the page, followed by a zero. Then, if you click on the button... nothing happens. So what's gone wrong here? Well, we've done the work necessary to create the store and made it accessible to our app so the state is updated when we click the button, but we haven't yet done all the work necessary to get our React and Redux libraries communicating with each other properly so the page re-renders once the state is updated. We'll tackle that in the next lesson. In the meantime, how do we know our state is getting updated? Let's get some feedback so we can find out.

Add Logging to Our Reducer

First, let's log our action and the new state. So we'll change the reducer as follows:

// ./src/reducers/counterReducer

export default function counterReducer(
  state = {
    clicks: 0
  },
  action
) {
  console.log(action);
  switch (action.type) {
    case "INCREASE_COUNT":
      console.log("Current state.clicks %s", state.clicks);
      console.log("Updating state.clicks to %s", state.clicks + 1);
      return {
        clicks: state.clicks + 1
      };

    default:
      console.log("Initial state.clicks: %s", state.clicks);
      return state;
  }
}

Ok, so this may look like a lot, but really all we're doing is adding some logging behavior. At the top of the function, we are logging the action. After the case statement, we are logging our current state first, followed by the value the state is being updated to. Under the default case statement, we can just log the previous state because this state will be unchanged.

Now, refresh your app, and give it a shot. When the page loads, you should see the initial action being logged, along with the initial value of the counter, 0. This is coming from our default case. Then, when you click the button, you should see the INCREASE_COUNT action logged along with the current state and the state we are updating to. We know that we are dispatching actions because each time we click the button, we can see that the call to this.props.store.dispatch({ type: 'INCREASE_COUNT' }) is hitting our reducer, and we can also see the value of the current state increasing each time. 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. (In actuality, the software can do a lot more for us than that; you can read up on it here: redux-devtools-extension.) Ok, so let's get to incorporating the devtools. 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. You should find the Chrome extension for Redux. Please download it, and refresh Chrome. To verify that you have successfully installed the extension, go to your developer console in Google Chrome (press command+shift+c to pull it up). In the top bar you will see a couple of arrows. Click those arrows, and if you see Redux in the dropdown, you have 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. Let's change the arguments to our createStore method to the following:

// ./src/index.js

import React from "react";
import ReactDOM from "react-dom";
import { createStore } from "redux";
import counterReducer from "./reducers/counterReducer";
import App from "./App";
import "./index.css";

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

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

Ok, notice that we are still passing through our reducer to the createStore method. The second argument is accessing our browser to find a method called __REDUX_DEVTOOLS_EXTENSION__. Now let's open the Redux Devtools (press command+shift+c, click on the arrows at the top right, and select the extension in the dropdown). You should see the initial action, @@INIT in the inspector. Now click on the tab that says state. You should see { clicks: 0 }. If you do, it means that your app is now communicating with the devtool. Each time you click on the button in your application, you should see the action name (INCREASE_COUNT) and the updated state show up in the devtools.

Whew!

Summary

In this lesson, we saw how to use the createStore() method. We saw that we can rely on the Redux library to provide this method, and that we still need to write our own reducer to tell the store what the new state should be given a particular action. We saw that when using the createStore() method 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.