如何将 Android 分页库与 NetworkBoundResource 集成

Question

我的应用正在使用 Android 的 Architecture components 库，并显示从具有无限滚动效果的分页 REST api 获取的项目列表。

我想要做的是将Paging Library 与NetworkBoundResource 结合使用，这样当用户向下滚动列表时，会从数据库中获取下一个项目并显示它们（如果存在），并且同时调用 API 来更新 DB 中的项目。

我找不到这两种模式同居的任何例子。

这是 DAO：

@Query("SELECT * FROM items ORDER BY id DESC")
LivePagedListProvider<Integer,MyItem> loadListPaginated();

这是我的NetworkBoundResource 实现：

public class PagedListNetworkBoundResource extends NetworkBoundResource<PagedList<MyItem>, List<MyItem>> {

    @Override
    protected void saveCallResult(@NonNull List<MyItem> items) {
        // Inserting new items into DB
        dao.insertAll(items);
    }

    @Override
    protected boolean shouldFetch(@Nullable PagedList<MyItem> data) {
        return true;
    }

    @NonNull
    @Override
    protected LiveData<PagedList<MyItem>> loadFromDb() {
        return Transformations.switchMap(dao.loadListPaginated().create(INITIAL_LOAD_KEY, PAGE_SIZE),
                new Function<PagedList<MyItem>, LiveData<List<MyItem>>>() {

            @Override
            public LiveData<PagedList<MyItem>> apply(final PagedList<MyItem> input) {
                // Here I must load nested objects, attach them, 
                // and return the fully loaded items
            }
        });
    }

    @NonNull
    @Override
    protected LiveData<ApiResponse<List<MyItem>>> createCall() {
        // I don't get the current paged list offset to perform a call to the API
        return ...;
    }
}

Answer 1

我还搜索了很多关于 NetworkBoundResource 的信息，我得出的结论是 NetworkBoundResource 和 Paging Lib 彼此不相关。它们都有自己的功能

根据谷歌给出的关于分页库的文章 https://developer.android.com/topic/libraries/architecture/paging.html

1.从本地数据库加载数据需要使用 DataSource 我的道

@Dao
public interface UserDao {
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    void insert(User... user);
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    void insert(List<User> users);

    @Query("Select * from User ")
    public abstract DataSource.Factory<Integer,User> getList();
}

2.然后从网络请求数据我们需要用LivePagedListBuilder实现BoundaryCallback类

public class UserBoundaryCallback extends PagedList.BoundaryCallback<User> {
    public static final String TAG = "ItemKeyedUserDataSource";
    GitHubService gitHubService;
    AppExecutors executors;
    private MutableLiveData networkState;
    private MutableLiveData initialLoading;
    public UserBoundaryCallback(AppExecutors executors) {
        super();
        gitHubService = GitHubApi.createGitHubService();
        this.executors = executors;
        networkState = new MutableLiveData();
        initialLoading = new MutableLiveData();
    }
    public MutableLiveData getNetworkState() {
        return networkState;
    }

    public MutableLiveData getInitialLoading() {
        return initialLoading;
    }

    @Override
    public void onZeroItemsLoaded() {
        //super.onZeroItemsLoaded();
        fetchFromNetwork(null);
    }

    @Override
    public void onItemAtFrontLoaded(@NonNull User itemAtFront) {
        //super.onItemAtFrontLoaded(itemAtFront);
    }

    @Override
    public void onItemAtEndLoaded(@NonNull User itemAtEnd) {
        // super.onItemAtEndLoaded(itemAtEnd);
        fetchFromNetwork(itemAtEnd);
    }
    public void fetchFromNetwork(User user) {
        if(user==null) {
            user = new User();
            user.userId = 1;
        }
        networkState.postValue(NetworkState.LOADING);
        gitHubService.getUser(user.userId,20).enqueue(new Callback<List<User>>() {
            @Override
            public void onResponse(Call<List<User>> call, Response<List<User>> response) {
                executors.diskIO().execute(()->{
                    if(response.body()!=null)
                        userDao.insert(response.body());
                        networkState.postValue(NetworkState.LOADED);
                });
            }

            @Override
            public void onFailure(Call<List<User>> call, Throwable t) {
                String errorMessage;
                errorMessage = t.getMessage();
                if (t == null) {
                    errorMessage = "unknown error";
                }
                Log.d(TAG,errorMessage);
                networkState.postValue(new NetworkState(Status.FAILED, errorMessage));
            }
        });
    }

}

3.My VM Code 从 DB + Network 加载数据

public class UserViewModel extends ViewModel {

    public LiveData<PagedList<User>> userList;
    public LiveData<NetworkState> networkState;
    AppExecutors executor;
    UserBoundaryCallback userBoundaryCallback;

    public UserViewModel() {
        executor = new AppExecutors();
    }
    public void init(UserDao userDao)
    {
        PagedList.Config pagedListConfig =
                (new PagedList.Config.Builder()).setEnablePlaceholders(true)
                        .setPrefetchDistance(10)
                        .setPageSize(20).build();
        userBoundaryCallback = new UserBoundaryCallback(executor);
        networkState = userBoundaryCallback.getNetworkState();
        userList = (new LivePagedListBuilder(userDao.getList(), pagedListConfig).setBoundaryCallback(userBoundaryCallback))
                .build();
    }
}

Answer 2

这假定回调中的每个项目都包含一个索引/偏移量。通常情况并非如此 - 项目可能只包含 ID。