React component not updating when store state has

问题:

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.

回答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

回答2:

I had a similar problem to Marina, which I solved in a similar fashion. Hopefully this explanation helps someone.

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 the following:

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

In the first example, you're not actually changing the object reference. Your new posts variable is a reference to your old posts variable.

回答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);
}

回答4:

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

e.g.

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!!!
   }
}

回答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)