java - Android ListAdapter.getItemCount() 总是返回 0

Question

如果 RecyclerView 为空，我会尝试显示 TextView。但是 ListAdapter 的 getItemCount() 总是返回 0，即使 RecyclerView 显示项目也是如此。

fragment .java

public class ProjectsFragment extends Fragment {

    private RepoViewModel repoViewModel;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        FloatingActionButton floatingActionButton = ((MainActivity) requireActivity()).getFloatingActionButton();
        floatingActionButton.setVisibility(View.VISIBLE);

        return inflater.inflate(R.layout.fragment_projects, container, false);
    }

    public void onViewCreated(@NonNull View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        TextView textview_no_project = view.findViewById(R.id.textview_no_project);

        RecyclerView recyclerView = view.findViewById(R.id.recyclerview);
        recyclerView.setLayoutManager(new LinearLayoutManager(requireActivity().getApplicationContext()));
        final RepoListAdapter adapter = new RepoListAdapter(new RepoListAdapter.RepoDiff());

        if (adapter.getItemCount() == 0) {
            recyclerView.setVisibility(View.INVISIBLE);
            textview_no_project.setVisibility(View.VISIBLE);
        }
        else {
            recyclerView.setVisibility(View.VISIBLE);
            textview_no_project.setVisibility(View.INVISIBLE);
        }
        recyclerView.setAdapter(adapter);
        repoViewModel = new ViewModelProvider(requireActivity()).get(RepoViewModel.class);
        repoViewModel.getAllRepos().observe(getViewLifecycleOwner(), adapter::submitList);
    }
}

RepoListAdapter.java

public class RepoListAdapter extends ListAdapter<Repo, RepoViewHolder> {

    public RepoListAdapter(@NonNull DiffUtil.ItemCallback<Repo> diffCallback) {
        super(diffCallback);
    }

    @NonNull
    @Override
    public RepoViewHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int viewType) {
        return RepoViewHolder.create(viewGroup);
    }

    @Override
    public void onBindViewHolder(RepoViewHolder holder, int position) {
        Repo current = getItem(position);
        holder.bind(current.getName());
    }

    public static class RepoDiff extends DiffUtil.ItemCallback<Repo> {

        @Override
        public boolean areItemsTheSame(@NonNull Repo oldItem, @NonNull Repo newItem) {
            return oldItem == newItem;
        }

        @Override
        public boolean areContentsTheSame(@NonNull Repo oldItem, @NonNull Repo newItem) {
            return oldItem.getName().equals(newItem.getName());
        }
    }
}

如有任何帮助，我们将不胜感激。

Answer 1

数据只会在 adapter::submitList 之后设置调用即在实时数据观察器调用之后。所以你需要在它之后进行更改。有几种方法可以做到这一点。以下是您需要将适配器创建为全局适配器或仅从 RecyclerView 获取它的一种方法。

public void onLoaded(List<Repo> list){
    if (list.size() == 0) {
        recyclerView.setVisibility(View.INVISIBLE);
        textview_no_project.setVisibility(View.VISIBLE);
    }
    else {
        recyclerView.setVisibility(View.VISIBLE);
        textview_no_project.setVisibility(View.INVISIBLE);
        adapter.submitList(list);
    }
}

设置onLoaded作为观察者 lambda。

repoViewModel.getAllRepos().observe(getViewLifecycleOwner(), this::onLoaded);

您还可以使用提交列表变体 submitList(@Nullable List<String> list, @Nullable Runnable commitCallback)这将为您提供回调，您可以在 Runnable 中做同样的事情.在这种情况下，这几乎是一回事。