类型“Observable”上不存在属性“unsubscribe”

Typescript(atom编辑器中的tslint)给了我一个Typescript错误,但我不知道如何设置正确的类型。

错误消息:

聊天组件:

  private _chatObserver: Observable<firebase.database.DataSnapshot>

  otherMethod () {
        this._chatObserver = this._chat.observe(alarmId)
        this._chatObserver.subscribe(
          (messageSnap: firebase.database.DataSnapshot) => {
            this.messages.push(messageSnap.val())
          },
          error => {throw error})
    }

    ionViewDidLeave() {
       this._chatObserver.unsubscribe() 
    }

_聊天提供商:

  public observe (alarmId){
    let messagesRef = this._ref.child(`alarms/${alarmId}/messages`)

    const observable = Observable.create(observer => {
      messagesRef.on('child_added',(messageSnap) => {
            observer.next(messageSnap)
        },
        (error) => observer.error(error)
      )
      return () => {
        messagesRef.off('value')
      };
    });

    return observable
  }