当我尝试为 Room Db 和 Retrofit 实现 NetworkBoundResource 和 Resource 帮助程序类时，它工作得很好。但是，我需要使用 Retrofit only without Room 实现来自 RESTful 的搜索结果。 Resources 类很好，我不需要更改它。我想要做的是尝试删除此类中的数据库源。

public abstract class NetworkBoundResource<ResultType, RequestType> {
  private final AppExecutors appExecutors;

  private final MediatorLiveData<Resource<ResultType>> result = new MediatorLiveData<>();

  @MainThread
  public NetworkBoundResource(AppExecutors appExecutors) {
    this.appExecutors = appExecutors;
    result.setValue(Resource.loading(null));
    LiveData<ResultType> dbSource = loadFromDb();
    result.addSource(dbSource, data -> {
      result.removeSource(dbSource);
      if (shouldFetch(data)) {
        fetchFromNetwork(dbSource);
      } else {
        result.addSource(dbSource, newData -> setValue(Resource.success(newData)));
      }
    });
  }

  @MainThread
  private void setValue(Resource<ResultType> newValue) {
    if (!Objects.equals(result.getValue(), newValue)) {
      result.setValue(newValue);
    }
  }

  private void fetchFromNetwork(final LiveData<ResultType> dbSource) {
    LiveData<ApiResponse<RequestType>> apiResponse = createCall();
    // we re-attach dbSource as a new source, it will dispatch its latest value quickly
    result.addSource(dbSource, newData -> setValue(Resource.loading(newData)));
    result.addSource(apiResponse, response -> {
      result.removeSource(apiResponse);
      result.removeSource(dbSource);
      //noinspection ConstantConditions
      if (response.isSuccessful()) {
        appExecutors.diskIO().execute(() -> {
          saveCallResult(processResponse(response));
          appExecutors.mainThread().execute(() ->
              // we specially request a new live data,
              // otherwise we will get immediately last cached value,
              // which may not be updated with latest results received from network.
              result.addSource(loadFromDb(),
                  newData -> setValue(Resource.success(newData)))
          );
        });
      } else {
        onFetchFailed();
        result.addSource(dbSource,
            newData -> setValue(Resource.error(response.errorMessage, newData)));
      }
    });
  }

  protected void onFetchFailed() {
  }

  public LiveData<Resource<ResultType>> asLiveData() {
    return result;
  }

  @WorkerThread
  protected RequestType processResponse(ApiResponse<RequestType> response) {
    return response.body;
  }

  @WorkerThread
  protected abstract void saveCallResult(@NonNull RequestType item);

  @MainThread
  protected abstract boolean shouldFetch(@Nullable ResultType data);

  @NonNull
  @MainThread
  protected abstract LiveData<ResultType> loadFromDb();

  @NonNull
  @MainThread
  protected abstract LiveData<ApiResponse<RequestType>> createCall();
}

最佳答案

问题是任何加载的数据都必须首先通过数据库，然后将其从数据库加载到 UI，就像 NetworkBoundResource 所做的那样。因此，我所做的是分离持久数据库并创建一个临时字段来加载。

例如，如果我想编辑 original搜索方法，我建议:

public LiveData<Resource<List<Repo>>> search(String query) {
    return new NetworkBoundResource<List<Repo>, RepoSearchResponse>(appExecutors) {

        // Temp ResultType
        private List<Repo> resultsDb;

        @Override
        protected void saveCallResult(@NonNull RepoSearchResponse item) {
            // if you don't care about order
            resultsDb = item.getItems();
        }

        @Override
        protected boolean shouldFetch(@Nullable List<Repo> data) {
            // always fetch.
            return true;
        }

        @NonNull
        @Override
        protected LiveData<List<Repo>> loadFromDb() {
            if (resultsDb == null) {
                return AbsentLiveData.create();
            }else {
                return new LiveData<List<Repo>>() {
                    @Override
                    protected void onActive() {
                        super.onActive();
                        setValue(resultsDb);
                    }
                };
            }
        }

        @NonNull
        @Override
        protected LiveData<ApiResponse<RepoSearchResponse>> createCall() {
            return githubService.searchRepos(query);
        }

        @Override
        protected RepoSearchResponse processResponse(ApiResponse<RepoSearchResponse> response) {
            RepoSearchResponse body = response.body;
            if (body != null) {
                body.setNextPage(response.getNextPage());
            }
            return body;
        }
    }.asLiveData();
}

我运行了它，它起作用了。

编辑: 我创建了另一个更简单的类来处理这个问题(Daniel Wilson 的另一个答案具有更多功能并且已更新)。

但是，这个类没有依赖关系，被转换为基础只做fetch响应:

abstract class NetworkBoundResource<RequestType> {

    private val result = MediatorLiveData<Resource<RequestType>>()

    init {
        setValue(Resource.loading(null))
        fetchFromNetwork()
    }

    @MainThread
    private fun setValue(newValue: Resource<RequestType>) {
        if (result.value != newValue) {
            result.value = newValue
        }
    }

    private fun fetchFromNetwork() {
        val apiResponse = createCall()
        result.addSource(apiResponse) { response ->
            result.removeSource(apiResponse)

            when (response) {
                is ApiSuccessResponse -> {
                        setValue(Resource.success(processResponse(response)))
                }

                is ApiErrorResponse -> {
                    onFetchFailed()
                    setValue(Resource.error(response.errorMessage, null))

                }
            }
        }
    }

    protected fun onFetchFailed() {
    }

    fun asLiveData() = result as LiveData<Resource<RequestType>>

    @WorkerThread
    protected open fun processResponse(response: ApiSuccessResponse<RequestType>) = response.body

    @MainThread
    protected abstract fun createCall(): LiveData<ApiResponse<RequestType>>
}

所以在使用的时候，只能实现一个方法createCall():

fun login(email: String, password: String) = object : NetworkBoundResource<Envelope<User>>() {
    override fun createCall() = api.login(email, password)
}.asLiveData()

关于android - 没有 Room 的 NetworkBoundResource 帮助程序类，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/46923289/