Firebase - 尝试将每个用户的数据检索到列表中

Question

我的应用中的每个用户都可以发送和获取好友请求.当用户检查他的朋友请求时,我希望应用程序通过向他发送朋友请求的每个用户并从中检索他的信息Realtime Database.

这是我的代码,以实现这一目标:

 public void check_For_Friends_And_Requests(){
        String loggedUser=SaveSharedPreference.getLoggedEmail(getApplicationContext());


            final DatabaseReference mDatabase= FirebaseDatabase.getInstance().getReference();
            final DatabaseReference userRef=mDatabase.child("Users").child(loggedUser);

            userRef.addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    if (dataSnapshot.exists()){

                        final List<User> friendRequestsReceived_UserList=new ArrayList<>();

                        for (DataSnapshot postSnapshot: dataSnapshot.child("friend_requests_received").getChildren()) {

                            final String senderEmail=postSnapshot.getKey();

                            Toast.makeText(getApplicationContext(),
                                    senderEmail, Toast.LENGTH_SHORT).show();

                            if (senderEmail!=null){
                                mDatabase.child("Users").child(senderEmail).addListenerForSingleValueEvent(new ValueEventListener() {
                                    @Override
                                    public void onDataChange(DataSnapshot dataSnapshot) {
                                        Toast.makeText(getApplicationContext(),
                                                dataSnapshot.child("name").getValue(String.class), Toast.LENGTH_SHORT).show();

                                        friendRequestsReceived_UserList.add(
                                                new User(
                                                        senderEmail,
                                                        dataSnapshot.child("name").getValue(String.class),
                                                       dataSnapshot.child("level").getValue(Integer.class),
                                                        dataSnapshot.child("skill").getValue(Double.class)));

                                    }

                                    @Override
                                    public void onCancelled(DatabaseError databaseError) {

                                    }
                                });

                            }
                        }

                        UserListAdapter friendRequestsReceived_Adapter =
                        new UserListAdapter(getApplicationContext(),
                                R.layout.friend_requests_received_listview_row,
                                friendRequestsReceived_UserList);

                        friendRequestsReceived_ListView.setAdapter(friendRequestsReceived_Adapter);

                    }

                    else
                    connectionErrorGoToMain();
                }

                @Override
                public void onCancelled(DatabaseError databaseError) {
                    connectionErrorGoToMain();
                }
            });


    }

我有这个代码2 ValueEventListeners.我将用户信息添加到内部列表中.问题是在此过程结束时列表为空.

我想使用以下行填充包含此信息的列表视图:

 UserListAdapter friendRequestsReceived_Adapter =
                        new UserListAdapter(getApplicationContext(),
                                R.layout.friend_requests_received_listview_row,
                                friendRequestsReceived_UserList);

                        friendRequestsReceived_ListView.setAdapter(friendRequestsReceived_Adapter);

当我把它们放在内部监听器中时,它工作正常,但我不想为列表中的每个用户设置适配器,只能在for循环之后.

我正在附加我的数据库结构的屏幕截图(我不需要获取所有参数):

Answer 1

该列表为空，因为您是friendRequestsReceived_UserList在内部onDataChange()方法外部声明的。onDataChange()发生这种情况是由于在将这些新对象添加到列表之前调用的方法的异步行为。因此，为了解决这个问题，只需将列表的声明移到内部onDataChange()方法中，如下所示：

if (senderEmail!=null){
mDatabase.child("Users").child(senderEmail).addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        final List<User> friendRequestsReceived_UserList=new ArrayList<>(); //Moved here

    Toast.makeText(getApplicationContext(), dataSnapshot.child("name").getValue(String.class), Toast.LENGTH_SHORT).show();

    friendRequestsReceived_UserList.add(
        new User(
            senderEmail,
            dataSnapshot.child("name").getValue(String.class),
                dataSnapshot.child("level").getValue(Integer.class),
            dataSnapshot.child("skill").getValue(Double.class)));
            UserListAdapter friendRequestsReceived_Adapter =

     new UserListAdapter(getApplicationContext(), R.layout.friend_requests_received_listview_row, friendRequestsReceived_UserList);
     friendRequestsReceived_ListView.setAdapter(friendRequestsReceived_Adapter);

    }

    @Override
    public void onCancelled(DatabaseError databaseError) {

    }
});

正如您可能看到的，我也在内部方法中设置了适配器。如果您想在之外使用该列表，我建议您阅读我在这篇文章onDataChange()中的回答。