即使设置了适配器，RecyclerView 也不显示

Question

我陷入了这个问题，我不知道这有什么问题。我正在尝试使用 RecyclerView 并使用 Firebase 实时数据库中的数据填充它。我已经设置了我的适配器，但问题是 onBindViewHolder 并且 onCreateViewHolder 显然没有被调用。这是代码

WorkoutAdapter.java

public class WorkoutAdapter extends RecyclerView.Adapter<WorkoutAdapter.WorkoutViewHolder> {
private List<Workout> workoutList;

public WorkoutAdapter(List<Workout> workoutList) {
    this.workoutList = workoutList;
}

public WorkoutAdapter() {
}

public List<Workout> getWorkoutList() {
    return workoutList;
}

public void setWorkoutList(List<Workout> workoutList) {
    this.workoutList = workoutList;
}

@NonNull
@Override
public WorkoutViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
    Log.i("DEBUG MENU", "onCreateViewHolder");
    LayoutInflater layoutInflater = LayoutInflater.from(parent.getContext());
    ItemWorkoutBinding binding = ItemWorkoutBinding.inflate(layoutInflater, parent, false);
    return new WorkoutViewHolder(binding);
}

@Override
public void onBindViewHolder(@NonNull WorkoutViewHolder holder, int position) {
    Log.i("DEBUG MENU", "onBindViewHolder");
    Workout workout = workoutList.get(position);
    holder.bind(workout);
}

@Override
public int getItemCount() {
    return workoutList.size();
}

class WorkoutViewHolder extends RecyclerView.ViewHolder {
    private ItemWorkoutBinding binding;

    public WorkoutViewHolder(ItemWorkoutBinding binding) {
        super(binding.getRoot());
        this.binding = binding;
    }

    public void bind(Workout workout) {
        binding.setWorkout(workout);
        binding.executePendingBindings();
    }
}

WorkoutFragment.java

public class WorkoutPlanFragment extends Fragment {

private WorkoutPlanViewModel mViewModel;
private WorkoutPlanFragmentBinding binding;
private DatabaseReference mDatabase;
Plan plan;
List<Workout> workoutList;

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    mDatabase = FirebaseDatabase.getInstance().getReference();
}

@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,
                         @Nullable Bundle savedInstanceState) {
    WorkoutPlanViewModelFactory workoutPlanViewModelFactory = new WorkoutPlanViewModelFactory(workoutList);
    mViewModel = new ViewModelProvider(this, workoutPlanViewModelFactory).get(WorkoutPlanViewModel.class);
    binding = DataBindingUtil.inflate(inflater, R.layout.workout_plan_fragment, container, false);
    binding.setViewmodel(mViewModel);
    View view = binding.getRoot();
    assert getArguments() != null;
    plan = WorkoutPlanFragmentArgs.fromBundle(getArguments()).getPlan();
    ((AppCompatActivity) getActivity()).getSupportActionBar().setTitle(plan.getPlan_name());
    Glide.with(binding.getRoot().getContext()).load(plan.getUri()).into(binding.imageView2);
    return view;
}

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    binding.setPlan(plan);
    setupRvWorkout();
}

public void setupRvWorkout() {
    RecyclerView recyclerView = binding.workoutRV;
    LinearLayoutManager layoutManager = new LinearLayoutManager(getContext(), LinearLayoutManager.VERTICAL, false);
    recyclerView.setLayoutManager(layoutManager);
    mDatabase.child("exercise_workout").child("data").orderByChild("plan_id").equalTo(plan.getPlan_key()).addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            workoutList = new ArrayList<>();
            for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
                String workout_id = snapshot.child("workout_id").getValue(String.class);
                mDatabase.child("workout").child(workout_id).addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull DataSnapshot snapshot) {
                        Workout workout = snapshot.getValue(Workout.class);
                        workout.setWorkout_id(workout_id);
                        Log.d("GET", "workout_name: " + workout.getWorkout_name() + " " + workout.getWorkout_id());
                        workoutList.add(workout);
                    }

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

                    }
                });
            }
            WorkoutAdapter adapter = new WorkoutAdapter();
            recyclerView.setAdapter(adapter);
            adapter.setWorkoutList(workoutList);
            adapter.notifyDataSetChanged();
        }

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

        }
    });

}

就是这样，提前谢谢你！

编辑：将来自 firebase 的数据添加到锻炼列表中，证明如下：

data added to the list

exercise_plan collection

exercise_workout collection

workout collection

item_workout.xml

<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools">

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <TextView
            android:id="@+id/txt_workout"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginStart="8dp"
            android:layout_marginTop="8dp"
            android:layout_marginBottom="8dp"
            android:fontFamily="@font/roboto_medium"
            android:text="@{workout.workout_name}"
            android:textSize="24sp"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.0"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.0" />
    </androidx.constraintlayout.widget.ConstraintLayout>

    <data>

        <variable
            name="workout"
            type="com.example.zahfitclient.model.Workout" />
    </data>
</layout>

workout_plan_fragment.xml

<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools">

    <androidx.constraintlayout.widget.ConstraintLayout
        android:id="@+id/frameLayout2"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".ui.workoutplan.WorkoutPlanFragment">

        <ImageView
            android:id="@+id/imageView2"
            android:layout_width="0dp"
            android:layout_height="180dp"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            tools:src="@tools:sample/avatars" />

        <TextView
            android:id="@+id/txt_plannameWO"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="8dp"
            android:fontFamily="@font/roboto_slab_bold"
            android:text="@{plan.plan_name}"
            android:textSize="24sp"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/imageView2" />

        <TextView
            android:id="@+id/txt_planlevelWO"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="8dp"
            android:layout_marginEnd="16dp"
            android:fontFamily="@font/roboto_medium"
            android:text="@{plan.level_name}"
            android:textColor="?android:attr/textColorSecondary"
            app:layout_constraintEnd_toStartOf="@+id/view"
            app:layout_constraintHorizontal_bias="1.0"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/txt_plannameWO" />

        <TextView
            android:id="@+id/txt_plantypeWO"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginStart="16dp"
            android:layout_marginTop="8dp"
            android:fontFamily="@font/roboto_medium"
            android:text="@{plan.type_name}"
            android:textColor="?android:attr/textColorSecondary"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.0"
            app:layout_constraintStart_toEndOf="@+id/view"
            app:layout_constraintTop_toBottomOf="@+id/txt_plannameWO" />

        <View
            android:id="@+id/view"
            android:layout_width="2dp"
            android:layout_height="20dp"
            android:layout_marginStart="204dp"
            android:layout_marginTop="8dp"
            android:layout_marginEnd="204dp"
            android:background="@color/black"
            android:backgroundTint="@color/material_on_surface_emphasis_medium"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/txt_plannameWO" />

        <androidx.recyclerview.widget.RecyclerView
            android:id="@+id/workoutRV"
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:layout_marginTop="8dp"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/view" />

    </androidx.constraintlayout.widget.ConstraintLayout>

    <data>

        <variable
            name="plan"
            type="com.example.zahfitclient.model.Plan" />

        <variable
            name="viewmodel"
            type="com.example.zahfitclient.ui.workoutplan.WorkoutPlanViewModel" />
    </data>
</layout>

Answer 1

对notifyDataSetChanged 的调用需要在内部onDataSetChanged 内，因为您可以在其中将项目添加到视图以workoutList.add(workout) 显示的列表中：

mDatabase.child("exercise_workout").child("data").orderByChild("plan_id").equalTo(plan.getPlan_key()).addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
        workoutList = new ArrayList<>();
        for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
            String workout_id = snapshot.child("workout_id").getValue(String.class);
            mDatabase.child("workout").child(workout_id).addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot snapshot) {
                    Workout workout = snapshot.getValue(Workout.class);
                    workout.setWorkout_id(workout_id);
                    Log.d("GET", "workout_name: " + workout.getWorkout_name() + " " + workout.getWorkout_id());
                    workoutList.add(workout);
                    adapter.notifyDataSetChanged();
                }

                @Override
                public void onCancelled(@NonNull DatabaseError error) {
                    throw error.toException(); // don't ignore errors
                }
            });
        }
        WorkoutAdapter adapter = new WorkoutAdapter();
        recyclerView.setAdapter(adapter);
        adapter.setWorkoutList(workoutList);
    }

    @Override
    public void onCancelled(@NonNull DatabaseError error) {
        throw error.toException(); // don't ignore errors
    }
});

---

您可以稍微简化一下，只需在所有数据加载之外构建适配器：

WorkoutAdapter adapter = new WorkoutAdapter();
recyclerView.setAdapter(adapter);
workoutList = new ArrayList<>();
adapter.setWorkoutList(workoutList);
mDatabase.child("exercise_workout").child("data").orderByChild("plan_id").equalTo(plan.getPlan_key()).addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
        workoutList.clear();
        for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
            String workout_id = snapshot.child("workout_id").getValue(String.class);
            mDatabase.child("workout").child(workout_id).addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot snapshot) {
                    Workout workout = snapshot.getValue(Workout.class);
                    workout.setWorkout_id(workout_id);
                    Log.d("GET", "workout_name: " + workout.getWorkout_name() + " " + workout.getWorkout_id());
                    workoutList.add(workout);
                    adapter.notifyDataSetChanged();
                }

                @Override
                public void onCancelled(@NonNull DatabaseError error) {
                    throw error.toException(); // don't ignore errors
                }
            });
        }
    }

    @Override
    public void onCancelled(@NonNull DatabaseError error) {
        throw error.toException(); // don't ignore errors
    }
});

这更清楚地表明您可以在有数据更新时操作workoutList，只要在需要更新 UI 时调用notifyDataSetChanged()。

---

另见：

• getContactsFromFirebase() method return an empty list
• Setting Singleton property value in Firebase Listener

Answer 2

你需要改变这个：

WorkoutAdapter adapter = new WorkoutAdapter();
recyclerView.setAdapter(adapter);
adapter.setWorkoutList(workoutList);
adapter.notifyDataSetChanged();

到这里：

WorkoutAdapter adapter = new WorkoutAdapter();
adapter.setWorkoutList(workoutList);
recyclerView.setAdapter(adapter);
adapter.notifyDataSetChanged();

并在 onDataChange 方法中调用它

希望它会起作用。

Answer 3

@Override
 public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
         workoutList = new ArrayList < > ();
         for (DataSnapshot snapshot: dataSnapshot.getChildren()) {
             String workout_id = snapshot.child("workout_id").getValue(String.class);
             mDatabase.child("workout").child(workout_id).
             addListenerForSingleValueEvent(new ValueEventListener() {
                 @Override
                 public void onDataChange(@NonNull DataSnapshot snapshot) {
                     Workout workout = snapshot.getValue(Workout.class);
                     workout.setWorkout_id(workout_id);
                     Log.d("GET", "workout_name: " + workout.getWorkout_name() +
                     " " + workout.getWorkout_id());
                     workoutList.add(workout);
                 }

                 @Override
                 public void onCancelled(@NonNull DatabaseError error) {}
             });
         }
         WorkoutAdapter adapter = new WorkoutAdapter();
         recyclerView.setAdapter(adapter);
         adapter.setWorkoutList(workoutList);
         adapter.notifyDataSetChanged();

在for each loop中，您以异步方式填写列表，在foreach之后，您在调用adapter.setWorkoutList(workoutList)时立即调用adapter.setWorkoutList(workoutList)，列表没有数据，因为您以异步方式填写列表。您必须等到获取数据或使用同步方式获取数据并在所有数据获取后强制调用

adapter.setWorkoutList(workoutList);
adapter.notifyDataSetChanged();

为了实现这一点，你可以这样做：

 @Override
 public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
         workoutList = new ArrayList < > ();
         WorkoutAdapter adapter = new WorkoutAdapter();
         recyclerView.setAdapter(adapter);
         int[] num= {0};
         for (DataSnapshot snapshot: dataSnapshot.getChildren()) {
             String workout_id = snapshot.child("workout_id").getValue(String.class);
             mDatabase.child("workout").child(workout_id).
             addListenerForSingleValueEvent(new ValueEventListener() {
                 @Override
                 public void onDataChange(@NonNull DataSnapshot snapshot) {
                     Workout workout = snapshot.getValue(Workout.class);
                     workout.setWorkout_id(workout_id);
                     Log.d("GET", "workout_name: " + workout.getWorkout_name() +
                     " " + workout.getWorkout_id());
                     workoutList.add(workout);
                     num[0]=num[0]++;
                     if(num[0]==dataSnapshot.getChildren().size(){
                          adapter.setWorkoutList(workoutList);
                          adapter.notifyDataSetChanged();
                     }
                 }

                 @Override
                 public void onCancelled(@NonNull DatabaseError error) {}
             });
         }