没有连接适配器；在 MVVM 架构中片段恢复后跳过 Recyclerview 中的布局问题

Question

我正在使用 MVVM 架构实现回收器视图。加载片段时，数据第一次在回收站视图中填充，但在返回后返回片段后，它不会在 resuming 片段后加载列表。

这是我对 Fragment HomeFragment.java

的实现

public class HomeFragment extends Fragment {

private HomeViewModel homeViewModel;
private RecyclerView rv;
private ProgressBar pb;
private DataAdapter adapter;
private List<DataModel> modelList;

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}

public View onCreateView(@NonNull LayoutInflater inflater,
                         ViewGroup container, Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.fragment_home, container, false);
    rv = v.findViewById(R.id.rv_home);
    pb = v.findViewById(R.id.home_pb);
    return v;
}

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

    modelList = new ArrayList<>();
    Log.d("Home Frag", "onViewCreated: called again after back");

    rv.setLayoutManager(new GridLayoutManager(getActivity(), 2));
    rv.setHasFixedSize(true);

    pb.setVisibility(View.VISIBLE);
    homeViewModel = new ViewModelProvider(this).get(HomeViewModel.class);
    homeViewModel.getUserMutableLiveData().observe(Objects.requireNonNull(getActivity()), (userListUpdateObserver));
    pb.setVisibility(View.GONE);

    view.setFocusableInTouchMode(true);
    view.requestFocus();
    view.setOnKeyListener(new View.OnKeyListener() {
        @Override
        public boolean onKey(View v, int keyCode, KeyEvent event) {
            Log.i("HomeFragment", "keyCode: " + keyCode);
            if (keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_UP) {
                Log.i("HomeFragment", "keyCode: " + keyCode);
                AlertDialog.Builder builder = new AlertDialog.Builder(getContext());
                builder.setTitle("Logout");
                builder.setMessage("Are you sure you want to Logout?");

                // add the buttons
                builder.setPositiveButton("Yes",
                        new DialogInterface.OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialog, int which) {
                                dialog.dismiss();
                                getActivity().finish();
                            }
                        });
                builder.setNegativeButton("No", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialog, int which) {
                        dialog.dismiss();
                    }
                });

                // create and show the alert dialog
                AlertDialog dialog = builder.create();
                dialog.show();
                return true;
            }
            return false;
        }
    });
}

Observer<List<Category>> userListUpdateObserver = new Observer<List<Category>>() {
    @Override
    public void onChanged(final List<Category> userArrayList) {
        for (int i = 0; i < userArrayList.size(); i++) {
            modelList.add(new DataModel(userArrayList.get(i).getTitle(), userArrayList.get(i).getImage(), userArrayList.get(i).getId(), 0));
        }
        adapter = new DataAdapter(getContext(), modelList, new DataAdapter.RecyclerViewClickListener() {
            @Override
            public void onClick(View view, int position) {
                homeViewModel.selectedId(modelList.get(position).getId());
                homeViewModel.selectedString(modelList.get(position).getText());
                SubCategoryFragment subCategoryFragment = new SubCategoryFragment();
                ManageFragments.replaceFragment((FragmentActivity) getContext(), subCategoryFragment);
            }
        });
        rv.setLayoutManager(new GridLayoutManager(getActivity(), 2));
        rv.setAdapter(adapter);
        adapter.notifyDataSetChanged();
    }
  };
}

这是我的 ViewModel class HomeViewModel.java

public class HomeViewModel extends ViewModel {

private MutableLiveData<List<Category>> userLiveData;
public static final MutableLiveData<Integer> selectedItemId = new MutableLiveData<Integer>();
private ApiInterface apiInterface;
public static final MutableLiveData<String> selectedString = new MutableLiveData<String>();

public HomeViewModel() {
    userLiveData = new MutableLiveData<List<Category>>();
    init();
}

public void selectedId(Integer id) {
    selectedItemId.setValue(id);
}

public void selectedString(String name) {
    selectedString.setValue(name);
}

MutableLiveData<List<Category>> getUserMutableLiveData() {
    return userLiveData;
}

private void init() {
    populateList();
}

private void populateList() {
    apiInterface = ApiRequest.createService(ApiInterface.class);
    Call<MainCategoryModel> call = apiInterface.getCategory();

    call.enqueue(new Callback<MainCategoryModel>() {
        @Override
        public void onResponse(Call<MainCategoryModel> call, Response<MainCategoryModel> response) {
            userLiveData.setValue(response.body().getData().getCategory());
        }

        @Override
        public void onFailure(Call<MainCategoryModel> call, Throwable t) {
            Log.d("TAG", "onFailure: " + t.getMessage());
        }
    });
  }
}

谢谢。

Answer 1

这里发生的事情是，在 HomeFragment 创建时 onViewCreated 被调用，因此它的内容。在回收视图中，片段生命周期中没有附加适配器。您正在将适配器附加到将被调用但 asynchronously 的观察者中的回收视图。因此你得到 No Adapter Attached: skipping layout error.

您只需要观察者将更改更新到您的适配器列表。要修复它，您需要在片段生命周期中的 Recycle View 上添加适配器，例如 onViewCreated 方法。请发现适配器代码的变化如下-

public class CategoryAdapter extends RecyclerView.Adapter<CategoryAdapter.CustomViewHolder> {

    private Context context;
    private List<Category> categoryList;
    private CategoryClickListener categoryClickListener;

    public CategoryAdapter(Context context) {
        this.context = context;
    }
    
    public interface CategoryClickListener {
        //modify the method according to your requirement
        void onCategoryClick(String id);
    }

    public void setUpCategoryListener(CategoryClickListener categoryClickListener ) {
        this.categoryClickListener = categoryClickListener ;
    }

    public CustomViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        return new CustomViewHolder(LayoutInflater.from(context).inflate(R.layout.item_layout, parent, false));
    }

    @Override
    public void onBindViewHolder(@NonNull CustomViewHolder holder, int position) {
    // setup binding of views
    }

    public void setCategoryList(List<Category> categoryList) {
        this.categoryList = categoryList;
        notifyDataSetChanged();
    }

    @Override
    public int getItemCount() {
        if (categoryList!= null)
            return categoryList.size();
        else
            return 0;
    }

    public class CustomViewHolder extends RecyclerView.ViewHolder {

        //item variable intializer
        public CustomViewHolder(@NonNull View itemView) {
            super(itemView);
            // find views here from item

            //putting click listener to register a click on item
            itemView.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    categoryClickListener.onCategoryClick(categoryList.get(getAdapterPosition()).getId());
                }
            });
       }
    }
}

现在您可以将适配器创建为 -

//make adapter as global variable of class; 
CategoryAdapter categoryAdapter;

//in onViewCreated()
rv.setLayoutManager(new GridLayoutManager(getActivity(), 2));
CategoryAdapter categoryAdapter = new CategoryAdapter(getContext());
rv.setAdapter(adapter);
categoryAdapter.setUpCategoryListener(new CategoryAdapter.CategoryClickListener(){
    @Override
    public void onCategoryClick(String id) {
        //here you can get the selected item id or any value you require 
        // your fragment transaction as well               
    }
});

在观察者下，你可以简单地使用CategoryAdapter的setCategoryList()方法-

Observer<List<Category>> userListUpdateObserver = new Observer<List<Category>>() {
    @Override
    public void onChanged(final List<Category> userArrayList) {
        categoryAdapter.setCategoryList(userArrayList);
    }
};

无需致电 notifyDataSetChanged()，因为已经处理 undersetCategoryList()。

甚至我们已经处理 getItemCount()

下适配器的 userArrayList 上的 NullPointerException

编码愉快！

Answer 2

这一行有问题：

homeViewModel.getUserMutableLiveData().observe(Objects.requireNonNull(getActivity()), (userListUpdateObserver));

您正在使用 activity 作为 LifecycleOwner。在 onViewCreated() 中使用的 LifecycleOwner 总是错误的。相反，您需要使用 getViewLifecycleOwner() - 与片段视图关联的 LifecycleOwner。

homeViewModel.getUserMutableLiveData().observe(getViewLifecycleOwner(), 
        userListUpdateObserver);

没有连接适配器；在 MVVM 架构中片段恢复后跳过 Recyclerview 中的布局问题

No adapter attached; skipping layout issue in Recycler Vierw after fragment resume in MVVM architecture

android

android-layout

android-fragments

android-fragmentactivity

android-recyclerview