订阅ref.on时出现Firebase错误('value',回调); | 属性'subscribe'在类型'上不存在(a:DataSnapshot,b？:string)=> any'

Question

我在角度使用firebase实时数据库.我试图从firebase服务器实时获取一些数据:(来自服务的代码)

getData(child){
        return firebase.database().ref(child).on('value', (snapshot) => {
            console.log(snapshot.val())
        })
    }

并订阅我的组件中的上述功能:

this.examinerService.getData('batches/names').subscribe(
      (batches) => {
        this.batches = batches.val();
      }
    )

这给了我错误:

Property 'subscribe' does not exist on type '(a: DataSnapshot, b?: string) => any'

我尝试使用ref().once()哪个工作正常,但我想要实时行为.

更新:目前我database().ref().on('value', (snapshots) => { console.log(snapshots.val()); });在我的组件中使用它工作正常,但我想在我的服务中做它并在我的组件中订阅它.有人告诉我,它不是一个可观察的,所以你不能订阅它.我是角度新手,所以我不知道如何制作一个可观察的并用它绑定快照.

Answer 1

该函数getData返回传递的回调而不是Observable代码似乎期望的回调.您可以修改此功能,以便返回Observable您可以.subscribe()使用的功能.

import { Observable } from 'rxjs/Observable';

getData(child) {

  return Observable.create(subscriber => {
    const ref = firebase.database().ref(child);

    const callbackFn = ref.on('value',
      // emit a value from the Observable when firebase data changes
      (snapshot) => subscriber.next(snapshot.val()),

      // error out the Observable if there is an error
      // such as permission denied
      error => subscriber.error(error)
    );

    // The function passed to Observable.create can return a callback function
    // which will be called when the observable we created is unsubscribed from.
    // Just as we used `ref.on()` previously our callback function calls `ref.off`
    // to tell firebase that we are no longer interested in the changes
    return () => ref.off('value', callbackFn);
  });
}