Question

因此，我尝试从Firestore中获取数据，当我对其进行控制台日志记录时，我将集合的内容取回，但是当我将代码移至某个函数时，便无法将其返回。

此代码有效：

const db = firebase.firestore();
db.settings({ timestampsInSnapshots: true});
db.collection('story').get().then((snapshot) => {
snapshot.docs.forEach(doc => {console.log(doc.data())
    ;})
})

这不起作用。（它可以编译，但不返回任何内容）：

...
getMyStory = () => {
        const db = firebase.firestore();
        db.settings({ timestampsInSnapshots: true});
        db.collection('story').get().then((snapshot) => {
        snapshot.docs.forEach(doc => {
            let items = doc.data();
        })
        });
        return this.items;
    }


    render () {


        return (
        <p>{this.getMyStory}</p>
);
}

我在做什么错了？

Answer 1

您的呈现逻辑将需要考虑对Firebase的查询是异步的。考虑通过对代码进行以下调整来利用组件state来解决此问题：

getMyStory() { /* Remove arrow function */

    const db = firebase.firestore();
    db.settings({ timestampsInSnapshots: true});
    db.collection('story').get().then((snapshot) => {

      snapshot.docs.forEach(doc => {
          let items = doc.data();

          /* Make data suitable for rendering */
          items = JSON.stringify(items);

          /* Update the components state with query result */
          this.setState({ items : items }) 
      });

    });
}

接下来，将componentDidMount()添加到您的组件中，然后像这样将调用添加到getMyStory()：

componentDidMount() {

    /* Cause your component to request data from Firebase when
       component first mounted */
    this.getMyStory()
}

Finall，更新渲染方法以使用状态，而不是方法：

  render () {

    return (<p>{ this.state.items || 'Loading' }</p>);
 }

希望这会有所帮助！

Answer 2

Promise对象代表最终的完成（或失败）异步操作及其结果值

根据您的代码首先返回this.items; ，然后解析 db.collection（'story'）。get（），最后再也没有得到结果。

基本上此行：

db.collection('story').get()

这是一个承诺，然后您必须等待下面的代码来解决结果：

getMyStory = () => {
    const db = firebase.firestore();

    db.settings({ timestampsInSnapshots: true});

    return db.collection('story').get()
}

Read More About Map

Answer 3

这里的主要问题是您试图同步呈现异步数据，而这与react (not yet at least)无关。

呈现异步数据时，通常将利用组件state。

以下是加载和呈现异步内容时的标准使用模式。

class YourComponent extends Component {
  state = {
    items: []
  }

  // This method is called when the component mounts
  // Usually  a good place to do async requests
  componentDidMount() {

    db.collection('story').get().then(snapshot => {
      // After promise is finished set data to local state
      // When setting state the render method will be called, thus rerendering the UI
      this.setState({ items: snapshot })
    })
  }

  render() {
    // Access your items from local component state
    const { items } = this.state;

    return (
      <div>
        {items.forEach(doc => {
          // Render whatever data is in your document
          return <p key={doc.id}> { Your data goes here }</p>
        })
        }
      </div>
    )
  }
}

Answer 4

import React, { Component } from "react";
import firebase from "../config";

class App extends Component {
  constructor(props) {
    super(props);

    // Reference firebase in this.state, creating a global key-value pair
    this.state = {
      roomsRef: firebase.firestore().collection("rooms"),
      rooms: []
    };
  }

  // Retrieve data from firebase and set state
  getDb(db) {
    db.get().then(querySnapshot => {
      querySnapshot.forEach(doc => {
        let room = doc.data();
        room.key = doc.id;
        this.setState({
          rooms: this.state.rooms.concat(room)
        });
      });
    });
  }

  // 
  componentDidMount() {
    this.getDb(this.state.roomsRef);
  }

  render() {
    const rooms = this.state.rooms.map((r, i) => <li key={i}>{r.name}</li>);

    return (
      <div className="App">
        <section className="room-list">
          {this.state.rooms.length === 0 ? (
            <p>loading...</p>
          ) : (
            <ul>{rooms}</ul>
          )}
        </section>
      </div>
    );
  }
}

export default App;

Answer 5

如果有人正在将React与Redux与Firebase一起使用，并且有困难，这里是如何做的在这里，我使用redux从firestore中获得了所有出版物

// publicationsActions.js

var publicationsRef = db.collection("publications")
var publicationsList = []

// Function that returns the publications list
export const getPublications = () => async (dispatch) => {
    await publicationsRef.get().then((res) => {
        res.docs.forEach(doc => {
            publicationsList.push(doc.data())
        })
        dispatch({
            type: GET_PUBS,
            payload: publicationsList
        })
    })
}


// publicationReducer.js

export default function (state = initialState, action) {
    switch(action.type){
        case GET_PUBS:
            return {
                ...state,
                items: action.payload
            }
        case ADD_PUB:
            return {
                ...state,
                items: [action.payload, ...state.items]
            }
        default:
            return state; 
    }
}

从Firestore提取数据

5 个答案: