Intro to Redux: Reading Data from State

Objectives

Use the createStore() method provided by the redux library.

Introduction

In the previous section, we have been building using a createStore() method 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(), 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 shopping list application that will allow a user to view an existing shopping list.

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 method 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 shoppingListItemReducer from './reducers/shoppingListItemReducer.js';
import App from './App';
import './index.css';

const store = createStore(shoppingListItemReducer); /* 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 shoppingListItemReducer from './reducers/shoppingListItemReducer.js';
import App from './App';
import './index.css';

const store = createStore(shoppingListItemReducer);

// 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() 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/shoppingListItemReducer.js:

// ./src/reducers/shoppingListItemReducer.js

export default function shoppingListItemReducer(
  state = {
    items: []
  },
  action
) {
  switch (action.type) {
    case 'INCREASE_COUNT':
      return {
        ...state,
        items: state.items.concat(state.items.length + 1)
      }

    default:
      return state;
  }
}

Ok so effectively, 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, shoppingListItemReducer, 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.

To gain access to the store somewhere in our app, we use a second function provided by react-redux, connect. By modifying a component's export statement and included connect, we are able to take data from our Redux store and map them to a component's props. Similarly, we can also take actions, and by wrapping them in a dispatch and an anonymous function, be able pass them as props as well:

// ./src/App.js

import React, { Component } from 'react';
import { connect } from 'react-redux';
import './App.css';

class App extends Component {
  handleOnClick = event => {
    this.props.increaseCount();
  };

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

const mapStateToProps = state => {
  return {
    items: state.items
  };
};

const mapDispatchToProps = dispatch => {
  return {
    increaseCount: () => dispatch({ type: 'INCREASE_COUNT' })
  };
};

export default connect(
  mapStateToProps,
  mapDispatchToProps
)(App);

Ok, so this code places a button on the page with an onClick event listener pointed to this.handleOnClick. When this.handleOnClick is invoked, it calls a function, this.props.increaseCount. Well.. increaseCount is actually being provided by the new function below our App component: mapDispatchToProps.

Meanwhile, we've also got this.props.items.length, which is also a prop created from our Redux store. As the store's items property increases, App will display a different number!

If you boot up the app, you should see a button on the page, followed by a zero, using the core above for index.js and App.js, we can see Redux in action. Every button click dispatches an action to our store, causing it to change. Since data (items) from that store is being accessed in App, App 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 were 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/reducers/shoppingListItemReducer

export default function shoppingListItemReducer(
  state = {
    items: []
  },
  action
) {
  console.log(action);
  switch (action.type) {
    case 'INCREASE_COUNT':
      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;
  }
}

Ok, so 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: 'INCREASE_COUNT' }) 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 (press command+shift+c to pull it up), 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 method to the following:

// ./src/index.js

import React from 'react';
import ReactDOM from 'react-dom';
import { createStore } from 'redux';
import { Provider } from 'react-redux';
import shoppingListItemReducer from './reducers/shoppingListItemReducer';
import App from './App';
import './index.css';

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

ReactDOM.render(
  <Provider store={store}>
    <App />
  </Provider>,
  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__. If that method is there, the method 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 devtool. 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!

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 will 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.

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