java - android 列表在第一次运行时在 recyclerview 中获得 0 值

Question

大家好，我的问题是，当我第一次运行我的应用程序时，我的适配器列表值变为 0。然后，当我单击后退按钮并再次返回到我的应用程序时，它工作正常。我正在尝试显示来自 firebase 的数据。感谢您的帮助。

这是我的用户助手类

public class UserHelper {
private FirebaseDatabase firebaseDatabase;
private DatabaseReference databaseReference;
private Context context;
private String success = "Add Success.";
private String error = "Something went wrong.";
private String message;
public UserHelper(Context context) {
    this.context = context;
    initFireBase();
}

private void initFireBase(){
    FirebaseApp.initializeApp(context);
    firebaseDatabase = FirebaseDatabase.getInstance();
    databaseReference = firebaseDatabase.getReference();
}


public List<User> getData(){
    final List<User> list = new ArrayList<>();
    databaseReference.child("User_table").addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            if (list.size()>0)
                list.clear();
            for (DataSnapshot snapshot : dataSnapshot.getChildren()){
                    User user = snapshot.getValue(User.class);
                    list.add(user);
            }
        }

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

        }
    });
    return list;
}

然后这是我的mainactivity显示代码

@Override
public void displayData(List<User> list) {
    Adapter adapter= new Adapter(list, new Adapter.OnItemClickListener() {
        @Override
        public void onClickItem(User user) {
            presenter.clickItem(user);
        }
    });
  recyclerView.setLayoutManager(new LinearLayoutManager(this));
    recyclerView.setAdapter(adapter);
  adapter.notifyDataSetChanged();
}

Answer 1

ValueEventListener 是一个异步调用。您需要在 onDataChange 中调用 notifyDataSetChanged()。问题是您的 getData() 返回空白列表，稍后会调用回调。改变你的方法如下:

 public void getData(){
    databaseReference.child("User_table").addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            final List<User> list = new ArrayList<>();
            for (DataSnapshot snapshot : dataSnapshot.getChildren()){
                User user = snapshot.getValue(User.class);
                list.add(user);
            }
            displayData(list);
        }
        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {
        // Handle error here
        }
    });
}

@Override
public void displayData(List<User> list) {
    Adapter adapter= new Adapter(list, new Adapter.OnItemClickListener() {
        @Override
        public void onClickItem(User user) {
            presenter.clickItem(user);
        }
    });
    recyclerView.setLayoutManager(new LinearLayoutManager(this));
    recyclerView.setAdapter(adapter);
}