Map State To Props Readme

When we last left off, we successfully used our createStore() method, and integrated the method into our React application to update our state. Unfortunately, our React application did not re-render in response to changes in the state. In this lesson, we'll fix that.

Use the Provider component from React Redux

The reason why the application did not re-render previously is because our React and Redux libraries could not properly communicate to each other correctly to specify that a change in the store's state occurred. Luckily, we can use the React Redux library to get React and Redux talking to one another. Run npm install react-redux --save to install it and add to our package.json.

The React Redux library gives access to a component called the Provider. The Provider is a component that comes from our React Redux library. It wraps around our App component. It does two things for us. The first is that it will alert our Redux app when there has been a change in state, and this will re-render our React app. Let's give it a shot.

Let's add the following code to our src/index.js file:

// ./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';
import App from './App';
import './index.css';

const store = createStore(
  shoppingListItemReducer,
  window.__REDUX_DEVTOOLS_EXTENSION__ && window.__REDUX_DEVTOOLS_EXTENSION__()
);

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

We just did a few things here:

  • We imported Provider from React Redux
  • We used Provider to wrap our React application
  • We passed our store instance into Provider as a prop, making it available to all of our other components.

Step 2: Connecting The Container Component to Store

Using the <Provider> component provided by the React Redux library, we gave our components the ability to be connected to the store. However, we don't want every component re-rendering in response to every change in the state. So the React Redux library requires us to specify which changes to the store's state should prompt a re-render of the application. We will specify this with the connect() function.

Using the connect() function

For a component to be connected to the store, i.e. to be able to get data from the store's internal state and to be told to re-render and get new data when that state changes, we will use the connect() function made available to us by React Redux.

Here's how it works:

Open up ./src/App.js and add the following:

// ./src/App.js

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

class App extends Component {

  handleOnClick() {
    this.props.dispatch({
      type: 'INCREASE_COUNT',
    });
  }

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

// start of code change
const mapStateToProps = (state) => {
  return { items: state.items };
};

export default connect(mapStateToProps)(App);
// end of code change

Holy cow those last few lines are confusing. Let's see if we can understand them. Remember, that we have two goals here: (a) to only re-render our App component when specific changes to the state occur, and (b) to only provide the slice of the state that we need to our App component. So we will need (1) a function that listens to every change in the store and then (2) filters out the changes relevant to a particular component to (3) provide to that component. That's exactly what's happening here. In the next paragraph, let's go through what is doing what.

export default connect(mapStateToProps)(App);

The connect function is taking care of task 1, it is synced up to our store, listening to each change in the state that occurs. When a change occurs, it calls a function that we write called mapStateToProps(), and in mapStateToProps() we specify exactly which slice of the state we want to provide to our component. Here, we want to provide state.items, and allow our component to have access to them through a prop called items. So that completes task 2. Then we have to say which component in our application we are providing this data to: you can see that we write connect(mapStateToProps)(App) to specify that we are connecting this state to the App component. Finally this entire connect() method returns a new component, it looks like the App component we wrote, but now it also receives the correct data. This is the component we wish to export. So at the bottom of the file, you see:

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

export default connect(mapStateToProps)(App);

Note: We didn't have to import anything to define a mapStateToProps() function! We wrote that function ourselves.

Finally, in our mapStateToProps() function we are saying that we are providing a new prop called items, so in our App component, that is the prop we want to reference.

Ok, mapStateToProps() and connect() is very confusing, so we'll go dig through it some more. But for now, let's boot up our application, click the button, and see if we can finally get our application to render. Ok, it works - our component now properly re-renders!

A Note on dispatch

In the example code for App, you may have noticed something odd:

  handleOnClick() {
    this.props.dispatch({
      type: 'INCREASE_COUNT',
    });
  }

We have a prop named dispatch! But where did it come from if it's a prop? We will go into greater detail later, but dispatch is automatically provided by connect if it is missing a second argument. That second argument is reserved for mapDispatchToProps, which allows us to customize how we send actions to our reducer. Without the second argument we will still be able to use dispatch on any component wrapped with connect.

Conclusion

We learned of two new pieces of React Redux middleware: connect() and Provider. The two pieces work hand in hand. Provider ensures that our entire React application can potentially access data from the store. Then connect(), allows us to specify which data we are listening to (through mapStateToProps), and which component we are providing the data. So when you see lines like this:

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

connect(mapStateToProps)(App);

That is saying connect the data in mapStateToProps() (the items portion of the state) to the App component. And the App component can access that state with this.props.items. Don't fret if you still feel hazy on connect() and mapStateToProps(). This is a new middleware api that takes time to learn. We won't introduce any new material in the next code along, we'll just try to deepen our understanding of the material covered in this section. First, please take at least a 15 minute break before moving on.

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