About connecting react-redux in React and Redux (detailed tutorial)

This article mainly introduces react-redux, the connection between React and Redux. The editor thinks it is quite good, so I will share it with you now and give it as a reference. Let’s follow the editor to take a look

I have been exploring React-related things before, and I have a SPA project in hand, so I am ready to try Redux. Redux itself has no connection with React. It is a general Javscript App module used for App State management. To use Redux in a React project, a better way is to use the react-redux library to connect. What this means is that without react-redux, these two libraries will not be used together. react-redux provides some encapsulation, a more scientific way of organizing code, allowing us to use Redux in React code more comfortably.

I only learned about react-redux through the Redux documentation before. After a period of practice, I am going to look through the source code and make some relevant summaries. The npm version of the code I'm looking at is v4.0.0, which means the React version used is 0.14.x.

react-redux provides two key modules: Provider and connect.

Provider

Provider module is used as the container of the entire App. It is wrapped with another layer based on your original App Container. Its work is very simple. , which means accepting the Redux store as props and declaring it as one of the attributes of context. Subcomponents can conveniently access the store through this.context.store after declaring contextTypes. However, our components usually do not need to do this. The store is placed in the context for the following connect.

This is an example of using Provider:

// config app root
const history = createHistory()
const root = (
 <Provider store={store} key="provider">
  <Router history={history} routes={routes} />
 </Provider>
)

// render
ReactDOM.render(
 root,
 document.getElementById(&#39;root&#39;)
)

connect

This module is a real connection between Redux and React, just like its name Also called connect.

First consider how Redux works: First, a state is maintained in the store, we dispatch an action, and then the reducer updates the state according to this action.

Mapped to our React application, the state maintained in the store is our app state. A React component serves as the View layer and does two things: render and respond to user operations. So connect is to pass the necessary data in the store as props to the React component for rendering, and wrap the action creator to dispatch an action in response to user operations.

Okay, let’s take a detailed look at what the connect module does. Let’s start with its use. Its API is as follows:

connect([mapStateToProps], [mapDispatchToProps], [mergeProps], [options])

mapStateToProps is a function, and the return value represents the state that needs to be merged into props. The default value is () => ({}), which means nothing is passed.

(state, props) => ({ }) // 通常会省略第二个参数

mapDispatchToProps can be a function. The return value represents the actionCreators that need to merge only props. The actionCreator here should have been packaged with dispatch. It is recommended to use the bindActionCreators function of redux.

(dispatch, props) => ({ // 通常会省略第二个参数
 ...bindActionCreators({
  ...ResourceActions
 }, dispatch)
})

It is more convenient to accept an object directly. At this time, the connect function will convert it into a function. This function is exactly the same as the above example.

mergeProps is used to customize the merge process. The following is the default process. The value of parentProps is the props of the component itself. You can find that if the same name appears on the props of the component, it will be overwritten.

(stateProps, dispatchProps, parentProps) => ({
 ...parentProps,
 ...stateProps,
 ...dispatchProps
})

options have two switches: pure represents whether to turn on optimization. The details will be mentioned below. The default is true. withRef is used to give a ref to the component packaged inside. This ref can be obtained through the getWrappedInstance method. The default is false.

connect returns a function that accepts the constructor of a React component as the connection object, and ultimately returns the connected component constructor.

Then a few questions:

1. How do React components respond to store changes?

2. Why does connect selectively merge some props instead of directly passing in the entire state?

3. What does pure optimize?

We call the function returned by connect Connector. It returns an internal component called Connect. On the basis of packaging the original component, it also listens to Redux internally. Store changes, in order to allow the components wrapped by it to respond to store changes:

trySubscribe() {
 if (shouldSubscribe && !this.unsubscribe) {
  this.unsubscribe = this.store.subscribe(::this.handleChange)
  this.handleChange()
 }
}

handleChange () {
 this.setState({
  storeState: this.store.getState()
 })
}

But usually, what we connect is a Container component, which does not carry all App state, but our handler is responsive All state changes, so what we need to optimize is: when storeState changes, only when we really depend on that part of the state change, will we re-render the corresponding React component. So what is the part we really depend on? It is obtained through mapStateToProps and mapDispatchToProps.

The specific optimization method is to check in shouldComponentUpdate. If shouldComponentUpdate will return true only when the component's own props change, or the result of mapStateToProps changes, or the result of mapDispatchToProps changes, the checking method is Comparison of shallowEqual.

So for a certain reducer:

export default (state = {}, action) => {
 return { ...state } // 返回的是一个新的对象，可能会使组件reRender
 // return state // 可能不会使得组件reRender
}

In addition, when connecting, be careful to map the really needed state or actionCreators to props to avoid unnecessary performance loss.

Finally, according to the connect API, we found that we can use the ES7 decorator function to match the writing method of React ES6:

@connect(
 state => ({
  user: state.user,
  resource: state.resource
 }),
 dispatch => ({
  ...bindActionCreators({
   loadResource: ResourceActions.load
  }, dispatch)
 })
)
export default class Main extends Component {

}

The above is what I compiled for everyone. I hope it will be helpful to everyone in the future.

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