有没有办法计算 recyclerView 中显示的项目数

Question

我想汇总recyclerView 中显示的项目数。目前我正在尝试通过TextView 实现这一目标。

这是我Adapterclass中的代码

@Override
public int getItemCount()
{
    final int count = options.getSnapshots() != null
            ? options.getSnapshots().size()
            : 0;

    return count;
}

这是我在实际课堂上尝试使用recyclerview 实现这一目标的方法：

attendanceNumbers = findViewById(R.id.attendanceNums);
    attendanceNumbers.setText(adapter.getItemCount()+ " students have attended this session");

这是我的完整适配器类

public class AttendanceAdapter extends FirestoreRecyclerAdapter<Attendance, AttendanceAdapter.AttendanceHolder>{

private AttendanceAdapter.OnItemClickListener listener;
private FirestoreRecyclerOptions<Attendance> options;


public AttendanceAdapter(@NonNull FirestoreRecyclerOptions<Attendance> options) {
    super(options);
    this.options = options;
}

@Override
public int getItemCount()
{
    final int count = options.getSnapshots() != null
            ? options.getSnapshots().size()
            : 0;

    return count;
}

@Override
protected void onBindViewHolder(@NonNull AttendanceHolder holder, int position, @NonNull Attendance model) {
    holder.textViewStudent.setText(model.getUserEmail());
    holder.textViewTime.setText(model.getSignInTime());
    holder.textViewSession.setText(model.getSessionID());
    holder.textViewUserUID.setText(model.getUserUID());

}

@NonNull
@Override
public AttendanceHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
    View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.attendance_item, parent, false);
    return new AttendanceAdapter.AttendanceHolder(v);
    }

public void deleteItem(int position){
    getSnapshots().getSnapshot(position).getReference().delete();
}

public int countItems(){
    return getItemCount();
}

class AttendanceHolder extends RecyclerView.ViewHolder {
    TextView textViewStudent;
    TextView textViewTime;
    TextView textViewSession;
    TextView textViewUserUID;

    public AttendanceHolder(View itemView){
        super(itemView);
        textViewStudent = itemView.findViewById(R.id.text_view_student_name);
        textViewTime = itemView.findViewById(R.id.text_view_course);
        textViewSession = itemView.findViewById(R.id.text_view_student_ID);
        textViewUserUID = itemView.findViewById(R.id.text_view_user_UID);


        itemView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int position = getAdapterPosition();
                if (position != RecyclerView.NO_POSITION && listener != null){
                    listener.onItemClick(getSnapshots().get(position),position);
                }

            }
        });
    }
    }

public interface  OnItemClickListener{
    void onItemClick(Attendance documentSnapshot, int position);
}

public void setSetOnItemClickListener(AttendanceAdapter.OnItemClickListener listener){
    this.listener = listener;
}

这里是 logcat 错误：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int com.dissertation.studentscanner.AttendanceAdapter.getItemCount()' on a null object reference

这是我在我的班级中创建适配器的地方：不是我的 adapterClass

private void setUpRecyclerView() {
    final Query query = attendanceRef.whereEqualTo("sessionID", "5I0PLcpgmwdcjKLYLgkU").orderBy("userEmail", Query.Direction.ASCENDING);

    FirestoreRecyclerOptions<Attendance> options = new FirestoreRecyclerOptions.Builder<Attendance>()
            .setQuery(query, Attendance.class).build();


    adapter = new AttendanceAdapter(options);

    RecyclerView recyclerView = findViewById(R.id.recycler_view);
    recyclerView.setHasFixedSize(true);
    recyclerView.setLayoutManager(new LinearLayoutManager(this));
    recyclerView.setAdapter(adapter);

    attendanceNumbers = findViewById(R.id.attendanceNums);
    attendanceNumbers.setText((adapter.getItemCount())+ " students have attended this session");

Answer 1

NullPointerException 是由您尝试调用的事实引起的

attendanceNumbers.setText(adapter.getItemCount()+ " students have attended this session");

在创建 adapter 变量之前。 在适配器创建后移动上述语句将解决问题。

另外，请确保

attendanceNumbers = findViewById(R.id.attendanceNums);

在设置attendanceNumbers 文本之前调用。

Answer 2

要求看起来你想观察计数

1) 添加学生

2) 删除学生等

为此，请创建如下回调

public interface CountObserver {
  void onStudentsCount(int count);
}

改变适配器类，传递回调

CountObserver countObserver;
public AttendanceAdapter(@NonNull FirestoreRecyclerOptions<Attendance> options,CountObserver listener) {
    super(options);
    this.options = options;
    this.countObserver = listener;
}

@Override
public int getItemCount()
{
    final int count = options.getSnapshots() != null
            ? options.getSnapshots().size()
            : 0;
    if(countObserver!=null){
       countObserver.onStudentsCount(count);
    }
    return count;
}

最后一步

CountObserver countObserver = new CountObserver(){
  public void onStudentsCount(int count){
      attendanceNumbers.setText(count+ " students have attended this session");
  }
}
adapter = new AttendanceAdapter(options,countObserver);

对于 NPE，只需确保您在 setContentView 之后调用 setUpRecyclerView 方法，并且在您的活动 xml 布局中定义了 textView id 为 attendanceNums 例如 android:id="@+id/attendanceNums"