Firebase onChildAdded在没有数据时不会调用 [英] Firebase onChildAdded not called when there is no data

问题描述

我想检查里面是否有孩子。当我有数据的时候，当没有孩子没有被调用时，它会进入 OnChildAdded else。我想知道是否有任何数据？如何检查这个？

  ref.orderByKey（）。startAt（todayDate）.endAt（tomorrowDate）.addChildEventListener（new ChildEventListener （）{
 @Override 
 public void onChildAdded（DataSnapshot dataSnapshot，String s）{
 System.out.println（孩子是否存在？+ dataSnapshot.child（matches） （DataSnapshot postSnapshot：dataSnapshot.getChildren（））{
 AvailableRidesFirebaseObject rides = postSnapshot.getValue（AvailableRidesFirebaseObject.class）; 
 
 AvailableRidesFirebaseObject availableRidesFirebaseObject = new AvailableRidesFirebaseObject（）; 
 availableRidesFirebaseObject.setAdded_on（rides.getAdded_on（））; 
 availableRidesFirebaseObject.setCode（rides.getCode（））; 
 availableRidesFirebaseObject.setTime（rides.getTime（））; 
 availableRidesFirebaseObject.setDate_of_ride（rides.getDate_of_ride（ ））; 
 availableRidesFirebaseObject.setDay（rides.getDay（））; 
 availableRidesFirebaseObject.setDropoff_location（rides.getDropoff_location（））; 
 availableRidesFirebaseObject.setDropoff_coordinate（rides.getDropoff_coordinate（））; 
 availableRidesFirebaseObject.setFullname（rides.getFullname（））; 
 availableRidesFirebaseObject.setOffered_by（rides.getOffered_by（））; 
 availableRidesFirebaseObject.setPickup_location（rides.getPickup_location（））; 
 availableRidesFirebaseObject.setPickup_coordinate（rides.getPickup_coordinate（））; 
 availableRidesFirebaseObject.setSeats（rides.getSeats（））; 
 availableRidesFirebaseObject.setShare（rides.getShare（））; 
 availableRidesFirebaseObject.setVechile_type（rides.getVechile_type（））; 
 availableRidesList.add（availableRidesFirebaseObject）; 
 
 
 filteredList.add（rides）; 
 Logger.e（list，filteredList.toString（））; 
} 
 
尝试{
 adapter = new RecyclerViewAdapter（getActivity（），availableRidesList）; 
 availableRideListView.setHasFixedSize（true）; 
 RecyclerView.LayoutManager mLayoutManager = new LinearLayoutManager（getContext（））; 
 availableRideListView.setLayoutManager（mLayoutManager）; 
 availableRideListView.setAdapter（adapter）; 
 loading.setVisibility（View.GONE）; 
 
 catch（NullPointerException ne）{
 ne.printStackTrace（）; 
 /*if(availableRidesList.size（）== 0）{
 loading.setVisibility（View.GONE）; 
 
 
 
 
 @Override 
 public void onChildChanged（DataSnapshot dataSnapshot，String s）{
 
 
 $ b $ @覆盖
 public void onChildRemoved（DataSnapshot dataSnapshot）{
 
} 
 
 @Override 
 public void onChildMoved（DataSnapshot dataSnapshot，String s）{
 
} 
 $ b $ @Override 
 public void onCancelled（FirebaseError firebaseError）{
 
 } 
 
 
 
 
}）; 
  

解决方案

顾名思义 onChildAdded （）只会在子集被添加到集合时被调用。如果没有孩子，就不会被调用。

为了检测没有孩子的情况，可以附加一个值事件监听器。

 查询查询= ref.orderByKey（）
 .startAt（todayDate）
 .endAt（tomorrowDate）
 .limitToFirst（1）; 
 query.addValueEventListener（new ValueEventListener（）{
 public void onDataChange（DataSnapshot snapshot）{$ b $ if（snapshot.getChildrenCount（）== 0）{
 System.out.println （No children）; 
}; 
} 
 public void onCancelled（DatabaseError error）{
 throw error.toException（）; 
} 
 }）; 
  

I want to check if there is child inside or not.When I have data then it goes inside OnChildAdded else when there is no child that is not being called? I want to know if there is any data or not? How do I check this?

ref.orderByKey().startAt(todayDate).endAt(tomorrowDate).addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
            System.out.println("does the child exist? " + dataSnapshot.child("matches").exists());
            for (DataSnapshot postSnapshot: dataSnapshot.getChildren()) {
                AvailableRidesFirebaseObject rides = postSnapshot.getValue(AvailableRidesFirebaseObject.class);

                AvailableRidesFirebaseObject availableRidesFirebaseObject = new AvailableRidesFirebaseObject();
                availableRidesFirebaseObject.setAdded_on(rides.getAdded_on());
                availableRidesFirebaseObject.setCode(rides.getCode());
                availableRidesFirebaseObject.setTime(rides.getTime());
                availableRidesFirebaseObject.setDate_of_ride(rides.getDate_of_ride());
                availableRidesFirebaseObject.setDay(rides.getDay());
                availableRidesFirebaseObject.setDropoff_location(rides.getDropoff_location());
                availableRidesFirebaseObject.setDropoff_coordinate(rides.getDropoff_coordinate());
                availableRidesFirebaseObject.setFullname(rides.getFullname());
                availableRidesFirebaseObject.setOffered_by(rides.getOffered_by());
                availableRidesFirebaseObject.setPickup_location(rides.getPickup_location());
                availableRidesFirebaseObject.setPickup_coordinate(rides.getPickup_coordinate());
                availableRidesFirebaseObject.setSeats(rides.getSeats());
                availableRidesFirebaseObject.setShare(rides.getShare());
                availableRidesFirebaseObject.setVechile_type(rides.getVechile_type());
                availableRidesList.add(availableRidesFirebaseObject);


                filteredList.add(rides);
                Logger.e("list" ,filteredList.toString());
            }

            try{
                adapter = new RecyclerViewAdapter(getActivity(), availableRidesList);
                availableRideListView.setHasFixedSize(true);
                RecyclerView.LayoutManager mLayoutManager = new LinearLayoutManager(getContext());
                availableRideListView.setLayoutManager(mLayoutManager);
                availableRideListView.setAdapter(adapter);
                loading.setVisibility(View.GONE);

            }catch(NullPointerException ne){
                ne.printStackTrace();
                /*if(availableRidesList.size()==0){
                    loading.setVisibility(View.GONE);
                }*/
            }

        }

        @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(FirebaseError firebaseError) {

        }




    });

解决方案

As its name implies onChildAdded() will only be called if a child is added to the collection. If there are no children, it will not be called.

To detect the situation where there are no children, you can attach a value-event listener.

Query query = ref.orderByKey()
                 .startAt(todayDate)
                 .endAt(tomorrowDate)
                 .limitToFirst(1);
query.addValueEventListener(new ValueEventListener() {
  public void onDataChange(DataSnapshot snapshot) {
    if (snapshot.getChildrenCount() == 0) {
      System.out.println("No children");
    };
  }
  public void onCancelled(DatabaseError error) {
    throw error.toException();
  }
});

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