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。他们都有自己的功能
根据谷歌关于分页库的文章
https://developer.android.com/topic/libraries/architecture/paging.html
1.要从本地数据库加载数据,您需要使用 DataSource My Dao
@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.我的VM代码从数据库+网络加载数据
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();
}
}