未获取 Firebase 数据库数据

Question

我正在开发应用程序来显示存储在 firebase 数据库中的用户分数，但运行应用程序后它不会在列表视图中显示数据

下面是我的代码：-

listViewLeaderBoard = (ListView) findViewById(R.id.listView_leader_board);

        reference = FirebaseDatabase.getInstance().getReference("score");

        leaderBoards = new ArrayList<>();

        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                for (DataSnapshot postSnapshot : dataSnapshot.getChildren()){
                    LeaderBoard leaderBoard = postSnapshot.getValue(LeaderBoard.class);
                    leaderBoards.add(0,leaderBoard);
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });
        //Collections.sort(leaderBoards);
        CustomLeaderBoardAdapter adapter = new CustomLeaderBoardAdapter(getApplicationContext(),R.layout.leader_board_list_layout,leaderBoards);
        listViewLeaderBoard.setAdapter(adapter);
        adapter.notifyDataSetChanged();

数据库快照：-

输出：-

Answer 1

试试这个：

listViewLeaderBoard = (ListView) findViewById(R.id.listView_leader_board);

    reference = FirebaseDatabase.getInstance().getReference("score");

    leaderBoards = new ArrayList<>();

    reference.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for (DataSnapshot postSnapshot : dataSnapshot.getChildren()){
                LeaderBoard leaderBoard = postSnapshot.getValue(LeaderBoard.class);
                leaderBoards.add(leaderBoard);
         CustomLeaderBoardAdapter adapter = new CustomLeaderBoardAdapter(getApplicationContext(),R.layout.leader_board_list_layout,leaderBoards);
    listViewLeaderBoard.setAdapter(adapter);
    adapter.notifyDataSetChanged();
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

Answer 2

要解决这个问题，移动这行代码：

List<LeaderBoard> leaderBoards = new ArrayList<>();

在onDataChange() 方法内，否则它将一直存在，因为此方法的异步行为甚至在您尝试将LeaderBoard 类的那些对象添加到列表之前就被调用。

另外，leaderBoards.clear(); 也不再需要了。您的代码应如下所示：

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference scoreRef = rootRef.child("score");
ValueEventListener eventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        List<String> list = new ArrayList<>();
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            String imageUrl = ds.child("imageUrl").getValue(String.class);
            String name = ds.child("name").getValue(String.class);
            long score = ds.child("score").getValue(Long.class);
            Log.d("TAG", image + " / " + Urlname + " / " + score);
            list.add(image + " / " + Urlname + " / " + score);
        }
        Log.d("TAG", list);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {}
};
scoreRef.addListenerForSingleValueEvent(eventListener);

Answer 3

您可以在 Firebase 控制台中查找和更改数据库的规则。只需选择您的项目，单击左侧的“数据库”部分，然后选择“规则”选项卡。我不知道您是否这样做了试试这个，希望对你有帮助。

{
"rules": {
".read": true,
".write": true
}
}