ViewModel的使用-ViewModel与RecycleView(七)

一、前言

本篇只是对RecycleView中使用ViewModel进行一个记录。确定使用哪种方案，在面临这两种情况可以考虑两种方案:

1、在RecyclView中使用List进行填充

2、在RecycleView中使用ViewModel进行填充

(上面对两种例子的代码实际上不能编译通过，只是起到一种辅助说明的意思。表示RecycleView中是使用一个包含很多ViewMode的列表，还是使用一个ViewMode，但是里面包含一个普通的列表)

二、结论

经过查看多方Android官方提供的示例代码发现官方采用的均是第二种写法，第一种写法从没出现。哪怕是位于androidx.recyclerview.widget包下面的ListAdapter中的示例也是没有使用第一种写法。虽然有些人使用第一种方式，不过遵循官方编写风格还是使用第二种比较好

三、ListAdapter源码

为了能帮助阅读，这里直接将ListAdapter的源码贴出来，如下:

package androidx.recyclerview.widget;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;

import java.util.List;

/**
 * {@link RecyclerView.Adapter RecyclerView.Adapter} base class for presenting List data in a
 * {@link RecyclerView}, including computing diffs between Lists on a background thread.
 * 

 * This class is a convenience wrapper around {@link AsyncListDiffer} that implements Adapter common
 * default behavior for item access and counting.
 * 

 * While using a LiveData is an easy way to provide data to the adapter, it isn't required
 * - you can use {@link #submitList(List)} when new lists are available.
 * 

 * A complete usage pattern with Room would look like this:
 * 
 * {@literal @}Dao
 * interface UserDao {
 *     {@literal @}Query("SELECT * FROM user ORDER BY lastName ASC")
 *     public abstract LiveData> usersByLastName();
 * }
 *
 * class MyViewModel extends ViewModel {
 *     public final LiveData> usersList;
 *     public MyViewModel(UserDao userDao) {
 *         usersList = userDao.usersByLastName();
 *     }
 * }
 *
 * class MyActivity extends AppCompatActivity {
 *     {@literal @}Override
 *     public void onCreate(Bundle savedState) {
 *         super.onCreate(savedState);
 *         MyViewModel viewModel = new ViewModelProvider(this).get(MyViewModel.class);
 *         RecyclerView recyclerView = findViewById(R.id.user_list);
 *         UserAdapter adapter = new UserAdapter();
 *         viewModel.usersList.observe(this, list -> adapter.submitList(list));
 *         recyclerView.setAdapter(adapter);
 *     }
 * }
 *
 * class UserAdapter extends ListAdapter {
 *     public UserAdapter() {
 *         super(User.DIFF_CALLBACK);
 *     }
 *     {@literal @}Override
 *     public void onBindViewHolder(UserViewHolder holder, int position) {
 *         holder.bindTo(getItem(position));
 *     }
 *     public static final DiffUtil.ItemCallback DIFF_CALLBACK =
 *             new DiffUtil.ItemCallback() {
 *         {@literal @}Override
 *         public boolean areItemsTheSame(
 *                 {@literal @}NonNull User oldUser, {@literal @}NonNull User newUser) {
 *             // User properties may have changed if reloaded from the DB, but ID is fixed
 *             return oldUser.getId() == newUser.getId();
 *         }
 *         {@literal @}Override
 *         public boolean areContentsTheSame(
 *                 {@literal @}NonNull User oldUser, {@literal @}NonNull User newUser) {
 *             // NOTE: if you use equals, your object must properly override Object#equals()
 *             // Incorrectly returning false here will result in too many animations.
 *             return oldUser.equals(newUser);
 *         }
 *     }
 * }
 *
 * Advanced users that wish for more control over adapter behavior, or to provide a specific base
 * class should refer to {@link AsyncListDiffer}, which provides custom mapping from diff events
 * to adapter positions.
 *
 * @param  Type of the Lists this Adapter will receive.
 * @param  A class that extends ViewHolder that will be used by the adapter.
 */
public abstract class ListAdapter<T, VH extends RecyclerView.ViewHolder>
        extends RecyclerView.Adapter<VH> {
    final AsyncListDiffer<T> mDiffer;
    private final AsyncListDiffer.ListListener<T> mListener =
            new AsyncListDiffer.ListListener<T>() {
        @Override
        public void onCurrentListChanged(
                @NonNull List<T> previousList, @NonNull List<T> currentList) {
            ListAdapter.this.onCurrentListChanged(previousList, currentList);
        }
    };

    @SuppressWarnings("unused")
    protected ListAdapter(@NonNull DiffUtil.ItemCallback<T> diffCallback) {
        mDiffer = new AsyncListDiffer<>(new AdapterListUpdateCallback(this),
                new AsyncDifferConfig.Builder<>(diffCallback).build());
        mDiffer.addListListener(mListener);
    }

    @SuppressWarnings("unused")
    protected ListAdapter(@NonNull AsyncDifferConfig<T> config) {
        mDiffer = new AsyncListDiffer<>(new AdapterListUpdateCallback(this), config);
        mDiffer.addListListener(mListener);
    }

    /**
     * Submits a new list to be diffed, and displayed.
     * 

     * If a list is already being displayed, a diff will be computed on a background thread, which
     * will dispatch Adapter.notifyItem events on the main thread.
     *
     * @param list The new list to be displayed.
     */
    public void submitList(@Nullable List<T> list) {
        mDiffer.submitList(list);
    }

    /**
     * Set the new list to be displayed.
     * 

     * If a List is already being displayed, a diff will be computed on a background thread, which
     * will dispatch Adapter.notifyItem events on the main thread.
     * 

     * The commit callback can be used to know when the List is committed, but note that it
     * may not be executed. If List B is submitted immediately after List A, and is
     * committed directly, the callback associated with List A will not be run.
     *
     * @param list The new list to be displayed.
     * @param commitCallback Optional runnable that is executed when the List is committed, if
     *                       it is committed.
     */
    public void submitList(@Nullable List<T> list, @Nullable final Runnable commitCallback) {
        mDiffer.submitList(list, commitCallback);
    }

    protected T getItem(int position) {
        return mDiffer.getCurrentList().get(position);
    }

    @Override
    public int getItemCount() {
        return mDiffer.getCurrentList().size();
    }

    /**
     * Get the current List - any diffing to present this list has already been computed and
     * dispatched via the ListUpdateCallback.
     * 

     * If a null List, or no List has been submitted, an empty list will be returned.
     * 

     * The returned list may not be mutated - mutations to content must be done through
     * {@link #submitList(List)}.
     *
     * @return The list currently being displayed.
     *
     * @see #onCurrentListChanged(List, List)
     */
    @NonNull
    public List<T> getCurrentList() {
        return mDiffer.getCurrentList();
    }

    /**
     * Called when the current List is updated.
     * 

     * If a null List is passed to {@link #submitList(List)}, or no List has been
     * submitted, the current List is represented as an empty List.
     *
     * @param previousList List that was displayed previously.
     * @param currentList new List being displayed, will be empty if {@code null} was passed to
     *          {@link #submitList(List)}.
     *
     * @see #getCurrentList()
     */
    public void onCurrentListChanged(@NonNull List<T> previousList, @NonNull List<T> currentList) {
    }
}

四、参考链接

1. architecture-components-samples

2. architecture-samples

3. Android MVVM：RecyclerView

4. Flexible RecyclerView Adapter with MVVM and Data Binding