未连接适配器； MVVM体系结构中的片段恢复后，跳过了Recycler Vierw中的布局问题

问题描述

我正在使用MVVM体系结构实现回收者视图。装入片段时，数据是第一次在回收器视图中填充，但是在按回键后又回到该片段之后，恢复片段后，该数据不会加载列表。

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类 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。

解决方法

问题出在这一行：

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

您正在使用活动作为LifecycleOwner。在onViewCreated()中始终使用错误的LifecycleOwner。相反，您需要使用getViewLifecycleOwner()-与Fragment的视图关联的LifecycleOwner。

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

,

这里发生的是，在创建HomeFragment时，onViewCreated被调用，因此其内容也是如此。在“回收”视图上，片段生命周期中没有连接适配器。您正在将适配器附加到观察者中的回收视图，该观察者将被称为，但 asynchronously 。因此，您会得到 No Adapter Attached: skipping layout error 。

您只需要观察者即可将更改更新到适配器的列表。 要解决此问题，您需要以片段生命周期（例如onViewCreated方法）在回收视图上添加适配器。请找到适配器代码中的更改，如下所示-

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               
    }
});

并且在观察者下，您只需使用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undersetCategoryList()了。

即使我们已经为NullPointerException下的适配器处理了userArrayList上的getItemCount()

快乐编码！