Question

我目前正在实施一个依赖Firebase数据的应用程序，然后才能继续使用。但是，我经常（除非我故意等待）得到结果Cannot read property [property] of null。我非常怀疑这是因为在我调用此对象之前无法进行firebase请求。

现在我正在寻找实现rendez-vous或屏障的方法，实际上是在继续之前实现接收所有firebase数据的检查点的任何方法。 Javascript中有什么可以帮助我这样做的，或者任何库，或任何反应原生的库可以帮助我实现这个目标吗？

我的代码如下所示：（Fb是firebase接口）

@action
  bookInterestedBike() {
    this.bookedBikeNo = this.interestBikeNo;
    this.downloadBikeObj();
    this.updateBikeDataStartRide();
    this.updateUserDataStartRide();
    //
    //This is where the firebase checkpoint / barrier should happen, and no progress should be made unless this data was fetched!
    //
    this.startTimer();
  }

@action
  downloadBikeObj() {
    Fb.staticBikes.child(String(this.bookedBikeNo)).once('value', (bikeObj) => {
      this.bikeObj = bikeObj.val();
      console.log("Save object is: ");
      console.log(this.bikeObj);
    });
  }

  updateBikeDataStartRide() {
    var updateVals = {}
    updateVals[this.interestBikeNo] = {
      bike_no: this.interestBikeNo,
      current_user: "self",
      positionLat: this.usrLat,
      positionLng: this.usrLng
    };
    Fb.bikes.update(updateVals);
    return false;
  };

  updateUserDataStartRide() {
    var updateVals = {}
    updateVals[this.uuid] = {
      bike_no: this.bookedBikeNo,
      uuid: this.uuid //TODO: remove this in deployment
    };
    Fb.users.update(updateVals);
    return false;
  };

在另一个组件上，这是被调用的函数（在调用updateBookedBikeBegin()之前收到navigateToSuccessBookedBike()的所有数据至关重要

  updateBookedBikeBegin() {
    this.userStore.bookInterestedBike();
    this.navigateToSuccessBookedBike();
  }

Answer 1

为方便起见，对于这种情况，首选使用回调承诺。您需要在downloadBikeObj中返回承诺。

downloadBikeObj() {
    return Fb.staticBikes
      .child(String(this.bookedBikeNo))
      .once('value')
      .then(bikeObj => {
        this.bikeObj = bikeObj.val;
        console.log("Save object is: ");
        console.log(this.bikeObj);
      }); // return promise
}

并在bookInterestedBike中撰写已退回的承诺。

bookInterestedBike() {
    this.bookedBikeNo = this.interestBikeNo;

    this.downloadBikeObj()
        .then(() => this.updateBikeDataStartRide())
        .then(() => this.updateUserDataStartRide())
        .then(() => this.startTimer());    
}

参考：

https://firebase.googleblog.com/2016/01/keeping-our-promises-and-callbacks_76.html

React-native和Firebase：等待Firebase数据

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