java - Array List 在 addListenerForSingleValueEvent 监听器方法后变为空

Question

我正在从 firebase 数据库中获取数据到 recyclerview。我无法将数据获取到 RecyclerViewAdapter，因为 Array 列表在 addListenerForSingleValueEvent 方法后变为空。

@Override
    public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        final View view = inflater.inflate(R.layout.fragment_dashboard, container, false);
        database = FirebaseDatabase.getInstance();
        ArrayList<Item> arr = new ArrayList<Item>();


        DatabaseReference reference = database.getReference("NoticeBoard");

        reference.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                for (DataSnapshot singleSnapshot : dataSnapshot.getChildren()) {

                    String text1 = (String) singleSnapshot.child("Text1").getValue();
                    String text2 = (String) singleSnapshot.child("Text2").getValue();

                    arr.add(new Item(text1, text2));
                }
            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
            }
        });

        RecyclerViewAdapter adapter = new RecyclerViewAdapter(getContext(), arr);// Here the ArrayList arr is empty

        RecyclerView recyclerView = view.findViewById(R.id.dashboardRecyclerView);

        recyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
        recyclerView.setAdapter(adapter);

        return view;
    }

Answer 1

onDataChange() 是异步的，因此由于您在 addListenerForSingleValueEvent 之后调用 arraylist，它将返回一个空列表，因为尚未检索到数据。

要解决您的问题，您需要在onDatachange() 中添加arrayList:

        reference.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                for (DataSnapshot singleSnapshot : dataSnapshot.getChildren()) {

                    String text1 = (String) singleSnapshot.child("Text1").getValue();
                    String text2 = (String) singleSnapshot.child("Text2").getValue();
                    ArrayList<Item> arr = new ArrayList<Item>();
                    arr.add(new Item(text1, text2));

                    RecyclerViewAdapter adapter = new RecyclerViewAdapter(getContext(), arr);
                    RecyclerView recyclerView = view.findViewById(R.id.dashboardRecyclerView);

                    recyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
                    recyclerView.setAdapter(adapter);
                }
            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
            }
        });