当商店状态改变时，React 组件不会更新

Question

Below is my component class. The component never seems to execute componentWillUpdate(), even when I can see the state updating by logging before the return in mapStateToProps. The state is 100% changing, however the component doesn't refresh.

import React, { Component } from 'react'
import { connect } from 'react-redux'
import { search } from './mapActions'
import L from 'leaflet'


class Map extends Component {
  componentDidMount() {
    L.Icon.Default.imagePath = './images'
    this.map = new L.Map('map', {
      center: new L.LatLng(this.props.lat, this.props.lng),
      zoom: this.props.zoom,
      layers: L.tileLayer('http://{s}.tile.osm.org/{z}/{x}/{y}.png', {
        attribution: '<a href="http://osm.org/copyright">OpenStreetMap</a> contributors'
      })
    })
  }
  componentWillUpdate() {
    console.log('UPDATE MAP')
    L.geoJson(this.props.data).addTo(this.map)
  }
  render() {
    return <div id="map"></div>
  }
}

const mapStateToProps = (state) => {
  return {
    isFetching: state.isFetching,
    data: state.data
  }
}

const mapDispatchToProps = (dispatch) => {
  return {
    search: (name) => {
      dispatch(search(name))
    }
  }
}

export default connect(
  mapStateToProps,
  mapDispatchToProps
)(Map)

And here is the map reducer:

const initialState = {
  isFetching: false,
  data: {}
}

export const map = (state = initialState, action) => {
  switch(action.type) {
    case 'REQUEST_SEARCH_RESULTS':
      return Object.assign({}, state, {
        isFetching: true
      })
    case 'RECEIVE_SEARCH_RESULTS':
      return Object.assign({}, state, {
        isFetching: false,
        data: action.data
      })
    default:
      return state
  }
}

After some more testing and logging it seems that when it goes to map state to props the state object it uses to map to props contains the correct data, so state.map.data is correct and I can see the return from the fetch. However when I then log this.props in componentWillUpdate(), the data object is there but empty.

Answer 1

I had a similar problem and I found out the answer after read this:

Data gets set/updated/deleted in the store via the results of handling actions in reducers. Reducers receive the current state of a slice of your app, and expect to get new state back. One of the most common reasons that your components might not be re-rendering is that you're modifying the existing state in your reducer instead of returning a new copy of state with the necessary changes (check out the Troubleshooting section). When you mutate the existing state directly, Redux doesn't detect a difference in state and won't notify your components that the store has changed. So I'd definitely check out your reducers and make sure you're not mutating existing state. Hope that helps! ( https://github.com/reactjs/redux/issues/585 )

When I tried use Object.assign({}, object) as you, it didn't work anyway. So was when I found this:

Object.assign only makes shallow copies. ( https://scotch.io/bar-talk/copying-objects-in-javascript )

Then I understood that I had to do this: JSON.parse(JSON.stringify(object))

or just this: {...object}

For Arrays: [...theArray]

I hope that this will help you

Answer 2

Because you're not changing the reference, so React's shallow compare doesn't detect the update.

I'm going to use a simple example with blog posts. In your reducer, you're probably doing something as follows:

case FETCH_NEW_POSTS
    let posts = state.posts;
    posts.push(action.payload.posts);
    return {
        ...state, 
        posts
    };

Instead of that, you must do something like the following:

case FETCH_NEW_POSTS
    let posts = [...state.posts]; // we're destructuring `state.posts` inside of array, essentially assigning the elements to a new array.
    posts.push(action.payload.posts);
    return {
        ...state, 
        posts
    };

Depending on your use case Object.assign() or lodash's clone/deepclone may be more idiomatic.

Answer 3

componentWillUpdate receives the incoming props as an argument . At this time, this.props is still the old props. Try changing your method like so:

void componentWillUpdate(nextProps, nextState) {
    L.geoJson(nextProps.data).addTo(this.map);
}

Answer 4

Also, be careful when using the constructor of the component. That was my problem.

eg

class extends Component {
   private elementState: ElementState // BAD idea!!

   constructor(props: any) {
      super(props)
      elementState = stateFromReduxStore // BAD idea!!!
   }

   render() {
      return <SomeElement state={elementState} /> // will not be updated!!!
   }
}

Answer 5

Make sure you are listening to that store in your maps file if you are passing props to your component through maps file.

export const listenToStores = [CommonStore, store];
@connection(maps.listenToStores, maps.getStateFromStores)

Answer 6

Building on what Marina answered going from

var tempArray = imageArrayState
tempArray.push(5)
setImageArray(tempArray)

to

var tempArray = []
imageArrayState.foreach(value => {tempArray.push(value)})
tempArray.push(5)
setImageArray(tempArray)

made my app refresh