检索当前用户的 DataSnapshot Firebase

Question

更新：我决定重新编辑这个问题，以便更好地澄清并使用更集中的代码。

我遇到的问题是我有一个 UserList 对象的 ArrayList，这些对象是从以前的活动中选择的用户，我们称之为selectedUsers。我有另一个对象，我想将 selectedUsers 以及 Firebase 中的当前用户分组到其中。这将被称为mGroupMembers，其中包含 GroupMember 对象。

我能够将selectedUsers 放入mGroupMembers 的ArrayList 中，但我无法将当前用户（由他们的ID 引用）放入所述ArrayList。我认为问题在于 Firebase。不知道为什么当我调用addListenerForSingleValueEvent时，当前用户的数据没有添加到mGroupMembersArrayList中？

这是我的代码：

public class TestActivity extends AppCompatActivity {

    //Tag
    private static String TAG = "TestActivity";

    private List<UserList> selectedUsers;
    private TextView mPickedText;
    private TextView mCurrentText;
    private TextView mMembCountText;
    private List<GroupMembers> mGroupMembers;
    private String memberNames;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_test);

        //Selected users from previous activity.
        selectedUsers = (List<UserList>) getIntent().getSerializableExtra("users");

        //Text Views
        mPickedText = (TextView) findViewById(R.id.first_text);
        mCurrentText = (TextView) findViewById(R.id.second_text);
        mMembCountText = (TextView) findViewById(R.id.third_text);

        //GroupMembs
        mGroupMembers = new ArrayList<>();

        //Current user.
        final String currentUserID = FirebaseAuth.getInstance().getCurrentUser().getUid();
        DatabaseReference mRef = FirebaseDatabase.getInstance().getReference().child("users").child(currentUserID);
        mRef.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                UserList user = dataSnapshot.getValue(UserList.class);
                GroupMembers member;

                //If the current user...
                if (Objects.equals(user.getUid(), currentUserID)) {
                    mCurrentText.setText("Current: " + user.getName());
                    member = new GroupMembers(user.getName(), user.getUsername(), user.getEmail(), user.getUid());

                    Log.d(TAG, "Member object contains...Name: " + user.getName() + "\n" +
                            "Username: " + user.getUsername() + "\n" +
                            "Email: " + user.getEmail() +
                            "uid: " + user.getUid());

                    selectedUsers.add(user);

                    mGroupMembers.add(member);

                    for (GroupMembers members : mGroupMembers) {
                        Log.d(TAG, "Member object contains...Name: " + members.getName() + "\n" +
                                "Username: " + members.getUsername() + "\n" +
                                "Email: " + members.getEmail() +
                                "uid: " + members.getUid());
                    }
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });

        //Picked user.
        for (UserList selected : selectedUsers) {
            GroupMembers member = new GroupMembers();

            member.setName(selected.getName());
            member.setUsername(selected.getUsername());
            member.setEmail(selected.getEmail());
            member.setUid(selected.getUid());

            mGroupMembers.add(member);

            mPickedText.setText("Selected: " + selected.getName());
        }

        for (GroupMembers members : mGroupMembers) {
            memberNames = members.getName();
        }



        mMembCountText.setText("Group Members: " + mGroupMembers.size() + " ");
    }
}

这里是 Firebase 数据库

{
  "users" : {
    "S3l3yRh8TqRYNeTWQ7tKEdUe2K03" : {
      "email" : "antwon@domain.com",
      "name" : "Antwon ",
      "profilePicture" : " ",
      "uid" : "S3l3yRh8TqRYNeTWQ7tKEdUe2K03",
      "username" : "ant01"
    },
    "af7KgxP7QgZT3d6adtgwBRwnsaI2" : {
      "email" : "bill@domain.com",
      "name" : "Bill",
      "profilePicture" : " ",
      "uid" : "af7KgxP7QgZT3d6adtgwBRwnsaI2",
      "username" : "Bill123"
    }
  }
}

当我记录 mGroupMembers ArrayList 时会发生以下情况：

D/TestActivity: Member object contains...Name: Antwon 
                Username: ant01
                Email: antwon@domain.comuid: S3l3yRh8TqRYNeTWQ7tKEdUe2K03
D/TestActivity: Member object contains...Name: Bill
                Username: Bill123
                Email: bill@domain.comuid: af7KgxP7QgZT3d6adtgwBRwnsaI2
D/TestActivity: Member object contains...Name: Antwon 
                Username: ant01
                Email: antwon@domain.comuid: S3l3yRh8TqRYNeTWQ7tKEdUe2K03

正在添加当前用户（Antwon），但为什么我运行程序时没有显示？现在程序只显示所选用户的名称以及当前用户的名称。然后它显示mGroupMembers 的大小。大小当然只有 1，而应该是 2。

Answer 1

意外行为是 Firebase 侦听器的异步特性造成的。在从 Firebase 服务器检索数据快照之前，onDataChange() 不会执行。这通常需要几毫秒，并且会在添加侦听器之后的语句之后发生。对于您的代码，此 for 语句在 onDataChange() 触发并添加当前用户之前执行：

    //Picked user.
    for (UserList selected : selectedUsers) {
        GroupMembers member = new GroupMembers();

        member.setName(selected.getName());
        member.setUsername(selected.getUsername());
        member.setEmail(selected.getEmail());
        member.setUid(selected.getUid());

        mGroupMembers.add(member);

        mPickedText.setText("Selected: " + selected.getName());
    }

this answer 中提供了有关 Firebase 异步加载数据的进一步说明。