处理程序中的 Firebase 查询

Question

我正在尝试围绕（Firebase 查询）x（执行某些操作）功能包装服务。也就是说，我需要从 Firebase 检索数据，然后对于检索到的每个项目，执行某个类似下载文件的操作（它本身可以是 AsyncTask 或服务）。我的服务中有一个处理程序：

        @Override
    public void handleMessage(Message msg) {
        super.handleMessage(msg);
        FirebaseUser user= FirebaseAuth.getInstance().getCurrentUser();
        DatabaseReference ref= FirebaseDatabase.getInstance().getReference().child("users").child(user.getUid()).child("oj_goals");
        ref.orderByChild("ojTag").addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                OjGoal goal= dataSnapshot.getValue(OjGoal.class);
                Log.d("inside onChildAdded",goal.getMe());
                data.add(goal);
            }
            @Override
            public void onChildChanged(DataSnapshot dataSnapshot, String s) {
            }
            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {
            }
            @Override
            public void onChildMoved(DataSnapshot dataSnapshot, String s) {
            }
            @Override
            public void onCancelled(DatabaseError databaseError) {
            }
        });
        //FIXME: the below code continues to execute even if the Firebase-call has not yet completed
        //
        Intent intent= new Intent("com.example.sj.keymeasures.BAR_CHART_DATA_PREPARATION_FINISHED");
        /**
         * https://stackoverflow.com/questions/43800772/does-variable-order-matter-while-parcel-read-write-operation-in-parcelable
         */
        intent.putParcelableArrayListExtra("oj_goals",data);
        Log.d("[Handler]","sendingBroadcast with size "+data.size());
        sendBroadcast(intent);
        stopSelf(msg.arg1);
    }

现在您可以在代码中的 FIXME 中看到，我在另一端（在专用接收器内部）收到的内容是一个空列表。到目前为止，我知道这整件事是个坏主意，但是如果没有嵌套异步任务之类的奇怪东西，您将如何设计我要完成的工作（见第一段）？

Answer 1

很简单：加载子节点后需要执行的代码，应该在内部onChildAdded函数：

public void handleMessage(Message msg) {
    super.handleMessage(msg);
    FirebaseUser user= FirebaseAuth.getInstance().getCurrentUser();
    DatabaseReference ref= FirebaseDatabase.getInstance().getReference().child("users").child(user.getUid()).child("oj_goals");
    ref.orderByChild("ojTag").addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
            OjGoal goal= dataSnapshot.getValue(OjGoal.class);
            Log.d("inside onChildAdded",goal.getMe());
            data.add(goal);
            Intent intent= new Intent("com.example.sj.keymeasures.BAR_CHART_DATA_PREPARATION_FINISHED");
            intent.putParcelableArrayListExtra("oj_goals",data);
            Log.d("[Handler]","sendingBroadcast with size "+data.size());
            sendBroadcast(intent);
            stopSelf(msg.arg1);
        }
        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) {
        }
        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {
        }
        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) {
        }
        @Override
        public void onCancelled(DatabaseError databaseError) {
            throw databaseError.toException(); // don't ignore errors
        }
    });
}