Map State to Props

Summary

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 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. The redux and react-redux packages 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.

The React Redux library gives us access to a component called the Provider. This component does two things for us. First, it will make the store available to nested components once they have been configured using a second method provided by the React Redux library, connect() — more on that later. The second thing it does for us is to alert our Redux app when there has been a change in state, which will then re-render our React app.

The first step in getting it working is to import Provider from React Redux then wrap the Provider component around our App component.

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 counterReducer from "./reducers/counterReducer.js";
import App from "./App";
import "./index.css";

const store = createStore(
  counterReducer,
  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
Instead of passing our store instance directly into the App component, we are now passing it in to Provider as a prop, which will make it available to other components (after some additional configuration).

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

Connecting a component to the store means that it will be able to get data from the store's internal state. It also means that it will be told to re-render and get new data when that state changes.

Let's open up ./src/App.js so we can get the App componenet connected. First, we need to import the connect() function from React Redux:

// ./src/App.js

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

...

Next, we need to modify our render and handleOnClick methods to reflect the fact that the store is no longer being passed directly to App from index.js:

...

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.clicks}</p> {/* code change */}
			</div>
		);
	}
}

...

Finally, let's create a new method, mapStateToProps, and modify our export statement to use connect to wire everything together:

...

const mapStateToProps = (state) => {
	return { clicks: state.clicks };
};
  
export default connect(mapStateToProps)(App);

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 needed slice of the state to our App component. So we will need (1) a function that listens to every change in the store and then (2) accesses the changes relevant to a particular component to (3) provide to that component. That's exactly what's happening here. 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.clicks, and allow our component to have access to them through a prop called clicks. We are then able to render the number of clicks in our render method using this.props.clicks. So that completes task 2.

Next 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. In the end, the connect() method returns a new component which looks like the App component we wrote, but is connected up to receive the correct data. This new component is the component we wish to export.

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

Once we've made all the changes, our final code should look like this:

// ./src/App.js

import React, { Component } from "react";
import { connect } from "react-redux"; 

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.clicks}</p>
			</div>
		);
	}
}


const mapStateToProps = (state) => {
	return { clicks: state.clicks };
};
  
export default connect(mapStateToProps)(App);

There's a lot to absorb here about using mapStateToProps() and connect(); we'll be digging through it more in upcoming lessons. But for now, let's boot up our application, click the button, and verify that we can finally get our application to re-render.

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 connect is missing its second argument. That second argument is reserved for mapDispatchToProps, which allows us to customize how we send actions to our reducer. But even 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 to. So when you see lines like this:

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

connect(mapStateToProps)(App);

This is saying connect the data in mapStateToProps() (the clicks portion of the state) to the App component. And the App component can access that state with this.props.clicks.

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.

Alex-Harron/map-state-to-props-readme