React Redux不同步mapStateToProps中的道具

Question

I am using React redux with firebase realtime database. In App.js I am dispatching an action fetchAllPosts

App.js

class App extends Component {
  componentDidMount() {
    this.props.fetchAllPosts();
  }
  render() {
    return (
      <div className="App">
          // something ...
      </div>
    );
  }
}
const mapDispatchToProps = dispatch => {
  return {
      fetchAllPosts: () => {dispatch(allPosts())}
  }
}

My action looks like this (I am using redux-thunk):

action

export function allPosts() {
    return (dispatch) => {  
firebase.database().ref('posts/').on('value', (snapshot) => {
        dispatch({type: "ALL_POSTS", postsArray: snapshot.val(), loading: false})
    })
    }
}

Then I am combining reducers (I know in this case it is not necessary):

const rootReducer = combineReducers({
    allPosts: postsReducer
})

My reducer looks like this:

reducer

const initialState = {
    allPosts: []
}

const postsReducer = (state = initialState, action) => {
    switch(action.type) {
        case "ALL_POSTS" :
        console.log("action payload all posts", action.postsArray)
        return {
            ...state,
            loading: false,
            allPosts: action.postsArray
        }
        break;
        default:
        return state
    }
    return state
}

And finally: my SinglePostview component looks like this: SinglePostview.js

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

class SinglePostview extends Component {
    render() {
        console.log("ppp", this.props)
        return (
            <h2>{this.props.post.title}</h2>
        )
    }
}

const mapStateToProps = (state, ownprops) => {
    const postId = ownprops.match.params.postid
    return {
        post: state.allPosts.allPosts[postId]
    }
}

export default connect(mapStateToProps)(SinglePostview);

Here when the render method is executing, this.props.post is undefined and I have the error:

TypeError: Cannot read property 'title' of undefined.

The problem is: when the app loads for the first time, props.post is undefined (so I have an error) and after about 1 second it receives the value but it doesn't change anything - the error still exists and the value is not displaying. Could anyone help me?

Answer 1

Assuming your reducer is fine, you can fix this by

changing this

render() {
        return (
            <h2>{this.props.post.title}</h2>
        )
    }

To this:

render() {
        if (!this.props.post){
            return null;
        }
        return (
            <h2>{this.props.post.title}</h2>
        )
    }

or

render() {
        return (
            <h2>{this.props.post && this.props.post.title}</h2>
        )
    }

Answer 2

You are defining allPosts to be an array

    const initialState = {
    allPosts: []
    }

But you are trying to access it like an object.

state.allPosts.allPosts[postId]

Hence, if your state.allPosts.allPosts is an array , try using the ES6 find() method to get a post from the array with the postId.

Assuming

state.allPosts.allPosts = [
   {postId: 1,title:'abcd'},
   {postId:2,title:'def'}
]
 state.allPosts.allPosts.find(post => postId === post.postId)