使用React componentDidMount中的child_add通过键访问Firebase数据

Question

I'm experimenting with Firebase and I've written code like the following before but for some reason I am not getting what I expect.

When the React component below mounts I expect the this.state.clients array to be populated with the queried firebase data. The problem is that when the component loads this.state.clients is empty (and there are no errors). I've also included a snap shot image of my schema. I am using the "reatime database" and not the newer "Cloud Firestore".

Thank you!

Code

import React, {Component} from 'react';
import firebase from 'firebase/app';

class Dashboard extends Component{
    constructor(props){
        super(props)
        this.state = {
            clients: []
        }    

      this.clientsRef = firebase.database().ref('clients')

    }

    componentDidMount(){
       this.clientsRef.on('child_added', snapshot => {
       const client = snapshot.val();
       client.key = snapshot.key;
       this.setState({ clients: this.state.clients.concat( client ) })
     });
    }



    render(){
      console.log(this.state.clients);   // <---- is an empty array after component mounts but *should* contain data per the above firebase query

        return(
          <div>


           <h2>All Clients </h2>

          </div>

        )
    }
}

export default Dashboard

Answer 1

It's an empty array because you declared it an empty array in the constructor. The 'child added' event hasn't fired yet. If you go to your firebase console and manually add an item to 'clients' then this will trigger the 'child added' event and the event will show up in your UI. Replace 'child_added' with 'value' if you want the value of all existing events onload.

Answer 2

我忘记在“规则”选项卡中设置读取权限