如何等到从 Firebase 检索到我的数据？ [复制]

Question

我的问题是，在结束我的主要方法之前，我需要等待通过方法“onDataChange”检索来自 Firebase 的数据。

我的代码是（CalendarDbHelper）：

public synchronized ArrayList<Day> listDays()
    {
       final  CountDownLatch done = new CountDownLatch(1);
        db.addValueEventListener(new ValueEventListener() {

            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                if(dataSnapshot.exists())
                {
                    for(DataSnapshot postSnapShot:dataSnapshot.getChildren())
                    {
                        Day day=postSnapShot.getValue(Day.class);
                        listDays.add(day);

                    }
                }
               done.countDown();
            }


            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }

        });

        try {
            done.await(); 
        } catch(InterruptedException e) {
            e.printStackTrace();
        }
        return listDays;

    }

如您所见，我已经在尝试使用 CountDownLatch 等待，但它不起作用：卡住了。

我需要活动“CalendarActivity”的listDays，我像这样使用它：

 //Real initialization of the database
        db = FirebaseDatabase.getInstance().getReference();
        mDatabase = new CalendarDbHelper(db);
        final List<Day> allDays = mDatabase.listDays();



        //Activating things to click on
        if(allDays.size() > 0){
            calendarRV.setVisibility(View.VISIBLE);
            calendarRVAdapter = new CalendarAdapter(this, allDays, new CalendarAdapter.OnListItemClickListener() {

                @Override
                public void onListItemClick(int clickedItemIndex) {
                    String id = allDays.get(clickedItemIndex).getId();
                    MyCustomAlertDialog(id);
                }
            });
            calendarRV.setAdapter(calendarRVAdapter);
        }else {
            calendarRV.setVisibility(View.GONE);
            Toast.makeText(this, "There is no product in the database. Start adding now", Toast.LENGTH_LONG).show();
        }

Answer 1

如果这是你的“主要方法”，你应该在onDataChange被触发时放置返回数据

Answer 2

试试这个。

 public synchronized ArrayList<Day> listDays()
    {
   final  CountDownLatch done = new CountDownLatch(1);
    db.addValueEventListener(new ValueEventListener() {

        @Override
        public void onDataChange(DataSnapshot dataSnapshot) 
{
            if(dataSnapshot.exists())
            {
                for(DataSnapshot postSnapShot:dataSnapshot.getChildren())
                {
                    Day day=postSnapShot.getValue(Day.class);
                    listDays.add(day);

                }

             // Do whatever you want to do with your list from here..

            }
           done.countDown();
        }


        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {

        }

    });

    try {
        done.await(); 
    } catch(InterruptedException e) {
        e.printStackTrace();
    }
    return listDays;

}