Android + Firebase:同步为异步功能 [英] Android + Firebase: synchronous for into an asynchronous function

问题描述

我正在制作一个函数(Java-Android)，该函数应该返回一个包含Firebase数据的对象列表.我的问题是，在遍历for中的所有项目之后，我需要将结果发送给侦听器，看看代码:

I'm making a function (Java - Android) which should return me a list of objects filled with data from Firebase. My problem is that I need to send the result to the listener just after I loop through all the items in a for, take a look at the code:

final DatabaseReference beaconMessageRef = dataSnapshot.getRef().getRoot().child("region_messages/" + regionKey);
beaconMessageRef.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {

        if(!dataSnapshot.exists())
            FirebaseHelper.this.messageByBeaconListener.onSuccess(null);

        for (DataSnapshot regionMessageDS : dataSnapshot.getChildren()) {

            RegionMessage regionMessage = regionMessageDS.getValue(RegionMessage.class);
            String msgKey = regionMessageDS.getKey();

            final DatabaseReference messageRef = regionMessageDS.getRef().getRoot().child("messages/"+msgKey);
            messageRef.addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {

                    if(dataSnapshot.exists()){
                        Message msg = dataSnapshot.getValue(Message.class);
                        FirebaseHelper.this.addMessageByBeacon(msg);
                    }
                }

                @Override
                public void onCancelled(DatabaseError databaseError) {
                    FirebaseHelper.this.messageByBeaconListener.onFail(databaseError);
                }
            });
        }

        List<Message> msgs = FirebaseHelper.this.beaconMessageList;
        FirebaseHelper.this.beaconMessageList = null;
        FirebaseHelper.this.messageByBeaconListener.onSuccess(msgs);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        FirebaseHelper.this.messageByBeaconListener.onFail(databaseError);
    }
});

我需要执行以下行: FirebaseHelper.this.messageByBeaconListener.onSuccess(msgs); 在遍历for中的所有项目之后.

I need to execute this line: FirebaseHelper.this.messageByBeaconListener.onSuccess(msgs); just after I loop through all the items in the for.

我该如何实现?

推荐答案

在Android上，无法可靠地使异步状态同步.每当您有这样做的欲望时，请深呼吸并重复第一句话.或在这里阅读我的答案:在Firebase侦听器中设置Singleton属性值

Something that is asynchronous cannot reliably be made synchronous on Android. Whenever you feel the urge to do that, take a deep breath and repeat that first sentence. Or read my answer here: Setting Singleton property value in Firebase Listener

相反，您将必须做每个人都要做的事情:将需要数据的代码移到数据可用时触发的回调中.

Instead you will have to do what everyone does: move the code that needs the data into the callback that is triggered when the data is available.

在您的情况下，这有点棘手，因为您要加载多个项目.幸运的是，尽管您知道需要加载多少个项目.因此，如果您引入一个计数器来跟踪已加载了多少物品，那么您将知道完成的时间:

In your case it is a bit trickier, since you're loading multiple items. Luckily though you know how many items you need to load. So if you introduce a counter to track how many items have loaded, you will know when you're done:

final DatabaseReference beaconMessageRef = dataSnapshot.getRef().getRoot().child("region_messages/" + regionKey);
beaconMessageRef.addListenerForSingleValueEvent(new ValueEventListener() {
  @Override
  public void onDataChange(DataSnapshot dataSnapshot) {
    if(!dataSnapshot.exists())
      FirebaseHelper.this.messageByBeaconListener.onSuccess(null);

    for (DataSnapshot regionMessageDS : dataSnapshot.getChildren()) {

      RegionMessage regionMessage = regionMessageDS.getValue(RegionMessage.class);
      String msgKey = regionMessageDS.getKey();

      final Integer[] counter = new Integer[1];
      final DatabaseReference messageRef = regionMessageDS.getRef().getRoot().child("messages/"+msgKey);
      messageRef.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot innerSnapshot) {
          if(innerSnapshot.exists()){
            Message msg = innerSnapshot.getValue(Message.class);
            FirebaseHelper.this.addMessageByBeacon(msg);
          }
          counter[0] = new Integer(counter[0].intValue()+1);
          if (counter[0].equalTo(dataSnapshot.numChildren()) {
            List<Message> msgs = FirebaseHelper.this.beaconMessageList;
            FirebaseHelper.this.beaconMessageList = null;
            FirebaseHelper.this.messageByBeaconListener.onSuccess(msgs);
          }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
          FirebaseHelper.this.messageByBeaconListener.onFail(databaseError);
        }
      });
    }
  }

  @Override
  public void onCancelled(DatabaseError databaseError) {
    FirebaseHelper.this.messageByBeaconListener.onFail(databaseError);
  }
});

请参阅以下答案以获取对单个元素数组的解释: https://stackoverflow.com/a/5977866/209103

See this answer for an explanation of the single-element-array: https://stackoverflow.com/a/5977866/209103

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