我实际上想按最近的顺序从Firebase数据库检索数据

Question

这是我的数据库参考：

firebaseDatabase = FirebaseDatabase.getInstance();
    myRef = (DatabaseReference) firebaseDatabase.getReference().child("Notifications").orderByChild("Date");

这是我的Firebase控制台的快照：

Answer 1

您可以按以下方式检索最近的数据：

List<Notifications> notificationList = new ArrayList();
firebaseDatabase = FirebaseDatabase.getInstance();  
myRef = (DatabaseReference) firebaseDatabase.getReference().child("Notifications").orderByKey();
 myRef.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
                        Notifications notification = snapshot.getValue(Notifications.class);
                        notificationList.add(0,notification);
                }

            }

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

            }
        });

使用firebase，您应该执行一些查询逻辑客户端。降序和升序在Firebase上不可用。

Answer 2

这是我从Firebase检索数据的方法：

1）制定帮助方法以附加和分离监听器。每当您的实时数据库发生更改时，就会调用此代码。

// Firebase instance variables
private FirebaseDatabase mFirebaseDatabase;
private DatabaseReference mNotificationsDatabaseReference;
private ChildEventListener mChildEventListener;

private void attachDatabaseReadListener() {
        if (mChildEventListener == null) {
            mChildEventListener = new ChildEventListener() {
                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String s) {

                    Notification currentNotification= dataSnapshot.getValue(Notification.class); //deserialize data (make it an object again)
                    mNotificationAdapter.add(currentNotification);

                    //Store your data here in what order you like.
                }

                public void onChildChanged(DataSnapshot dataSnapshot, String s) {
                }

                public void onChildRemoved(DataSnapshot dataSnapshot) {
                }

                public void onChildMoved(DataSnapshot dataSnapshot, String s) {
                }

                public void onCancelled(DatabaseError databaseError) {
                }
            };
            mNotificationsDatabaseReference.addChildEventListener(mChildEventListener);
        }
    }



private void detachDatabaseReadListener() {
    if (mChildEventListener != null) {
        mNotificationsDatabaseReference.removeEventListener(mChildEventListener);
        mChildEventListener = null;
    }
}

2）不要忘记在onResume和onPause上附加和分离侦听器。

@Override
protected void onResume() {
    super.onResume();
    attachDatabaseReadListener();  
}

@Override
protected void onPause() {
    super.onPause();
    mNotificationAdapter.clear(); //Delete data in adapter which populates a listview. Otherwise you will end up with double items.
    detachDatabaseReadListener();
}

有关其他信息和帮助，请阅读official documentation

如果您真的想了解Firebase的基础知识，我建议您看看这个令人惊叹的tutorial（由Google制造）。