java - 由于某种原因列表大小回到零

Question

我正在将数据从云 firestore 加载到自定义 recyclerview 适配器中。根据我的日志，我从 firestore 获取数据很好，在我的 get 方法中，日志显示我的列表大小在增加。在我的 get 方法之外，mWorkoutList 返回到大小 0。我无法弄清楚为什么会发生这种情况。

public class ViewWorkoutsActivity extends AppCompatActivity implements WorkoutAdapter.WorkoutAdapterOnClickHandler {

private final String TAG = this.getClass().getName();
private FirebaseFirestore db;
private String mUid;
private List<Set> mSetList;
private RecyclerView mRecyclerView;
private WorkoutAdapter mWorkoutAdapter;
private List<OtherWorkout> mWorkoutList;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_view_workouts);
    mWorkoutList = new ArrayList<>();
    mUid = getIntent().getStringExtra("uid");
    db = FirebaseFirestore.getInstance();
    initDatabaseData();

    mRecyclerView = findViewById(R.id.recycler_view);

    LinearLayoutManager linearLayoutManager
            = new LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false);
    mRecyclerView.setLayoutManager(linearLayoutManager);
    mRecyclerView.setHasFixedSize(true);
    mWorkoutAdapter = new WorkoutAdapter(this);
    mRecyclerView.setAdapter(mWorkoutAdapter);
    Log.i("LOG", "workout list size HERE: " + mWorkoutList.size());
    mWorkoutAdapter.setWorkoutData(mWorkoutList);
}

@Override
public void onClick(OtherWorkout workout) {

}

public void initDatabaseData(){
    CollectionReference collectionReference2 = db.collection("users").document(mUid).collection("workouts");
    collectionReference2.get()
            .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                @Override
                public void onComplete(@NonNull Task<QuerySnapshot> task) {
                    if(task.isSuccessful()){
                        for (QueryDocumentSnapshot document : task.getResult()){
                            OtherWorkout workout = document.toObject(OtherWorkout.class);
                            mWorkoutList.add(workout);
                            Log.i("LOG", "workout list size: " + mWorkoutList.size());
                        }
                    }
                }
            });
}
}

Answer 1

嗨艾伦，您正在调用 initDatabaseData();在 oncreate() 方法中，该方法正在发出网络请求并获取数据。因此，获取数据大约需要 2-3 秒。但在那 2-3 秒内，您的 initDatabaseData() 下面的代码被调用

 LinearLayoutManager linearLayoutManager
        = new LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false);
mRecyclerView.setLayoutManager(linearLayoutManager);
mRecyclerView.setHasFixedSize(true);
mWorkoutAdapter = new WorkoutAdapter(this);
mRecyclerView.setAdapter(mWorkoutAdapter);
Log.i("LOG", "workout list size HERE: " + mWorkoutList.size());
mWorkoutAdapter.setWorkoutData(mWorkoutList);

因此，直到数据还没有到来之前，列表大小将显示 0，然后在 onComplete() 中获取数据后，列表大小将增加()。所以基本上添加一行 mWorkoutAdapter.notifyDatasetChanged();这里:-

if(task.isSuccessful()){
                    for (QueryDocumentSnapshot document : task.getResult()){
                        OtherWorkout workout = document.toObject(OtherWorkout.class);
                        mWorkoutList.add(workout);
                        Log.i("LOG", "workout list size: " + mWorkoutList.size());
                    }mWorkoutAdapter.notifyDatasetChanged();

                }

这将使用新值刷新您的适配器。