擷取資料來源資料的實現---Architecting Android

標籤：

UserRepository，這個介面，描述了Repository提供給使用者的功能就是getUsers,getUser(ID)。使用者只管使用，其它細節無需理會。

/*** Interface that represents a Repository for getting {@link User} related data.*/public interface UserRepository {  /**   * Get an {@link rx.Observable} which will emit a List of {@link User}.   */  Observable<List<User>> getUsers();  /**   * Get an {@link rx.Observable} which will emit a {@link User}.   *   * @param userId The user id used to retrieve user data.   */  Observable<User> getUser(final int userId);}

 

UserRepository的一個實作類別。通過委託的方式，通過委託userDataStoreFactory，來實現資料擷取的功能。

/*** {@link UserRepository} for retrieving user data.*/@Singletonpublic class UserDataRepository implements UserRepository {  private final UserDataStoreFactory userDataStoreFactory ;  private final UserEntityDataMapper userEntityDataMapper ;  private final Func1<List<UserEntity> , List<User>> userListEntityMapper =      new Func1<List<UserEntity> , List<User>>() {        @Override public List<User> call (List<UserEntity> userEntities) {          return UserDataRepository. this.userEntityDataMapper .transform(userEntities) ;        }      };  private final Func1<UserEntity , User>      userDetailsEntityMapper = new Func1<UserEntity , User>() {    @Override public User call (UserEntity userEntity) {      return UserDataRepository. this.userEntityDataMapper .transform(userEntity) ;    }  };  /**   * Constructs a {@link UserRepository}.   *   * @param dataStoreFactory A factory to construct different data source implementations.   * @param userEntityDataMapper {@link UserEntityDataMapper}.   */  @Inject  public UserDataRepository(UserDataStoreFactory dataStoreFactory ,      UserEntityDataMapper userEntityDataMapper) {    this .userDataStoreFactory = dataStoreFactory;    this. userEntityDataMapper = userEntityDataMapper ;  }  @Override public Observable<List<User>> getUsers() {    //we always get all users from the cloud    final UserDataStore userDataStore = this.userDataStoreFactory .createCloudDataStore() ;    return userDataStore.getUserEntityList().map( userListEntityMapper );  }  @Override public Observable<User> getUser( int userId) {    final UserDataStore userDataStore = this.userDataStoreFactory .create(userId);    return userDataStore.getUserEntityDetails(userId).map( userDetailsEntityMapper );  }}

　　

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UserDataStore,UserDataStoreFactory

UserDataStoreFactory，選用不同的UserDataStore來實現擷取資料的功能。不同的UserDataStore實現代表不同的資料來源。

/*** Interface that represents a data store from where data is retrieved.*/public interface UserDataStore {  /**   * Get an {@link rx.Observable} which will emit a List of {@link UserEntity}.   */  Observable<List<UserEntity>> getUserEntityList();  /**   * Get an {@link rx.Observable} which will emit a {@link UserEntity} by its id.   *   * @param userId The id to retrieve user data.   */  Observable<UserEntity> getUserEntityDetails (final int userId) ;}

　UserDataStoreFactory,兩個create方法，提供了兩個不同的UserDataStore實現

/*** Factory that creates different implementations of {@link UserDataStore}.*/@Singletonpublic class UserDataStoreFactory {  private final Context context;  private final UserCache userCache;  @Inject  public UserDataStoreFactory(Context context , UserCache userCache) {    if (context == null || userCache == null) {      throw new IllegalArgumentException( "Constructor parameters cannot be null!!!") ;    }    this .context = context.getApplicationContext() ;    this. userCache = userCache;  }  /**   * Create {@link UserDataStore} from a user id.   */  public UserDataStore create( int userId) {    UserDataStore userDataStore;    if (! this.userCache .isExpired() && this.userCache .isCached(userId)) {      userDataStore = new DiskUserDataStore(this. userCache);    } else {      userDataStore = createCloudDataStore();    }    return userDataStore;  }  /**   * Create {@link UserDataStore} to retrieve data from the Cloud.   */  public UserDataStore createCloudDataStore() {    UserEntityJsonMapper userEntityJsonMapper = new UserEntityJsonMapper() ;    RestApi restApi = new RestApiImpl(this .context, userEntityJsonMapper) ;    return new CloudUserDataStore(restApi , this.userCache );  }}

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資料來源實現一覽

 

CloudUserDataStore資料來源，是通過網路擷取資料的。它只要實現了UserDataStore聲明的介面便可以，這樣，它就是一個UserDataStore了。

/*** {@link UserDataStore} implementation based on connections to the api (Cloud).*/public class CloudUserDataStore implements UserDataStore {  private final RestApi restApi;  private final UserCache userCache;  private final Action1<UserEntity> saveToCacheAction = new Action1<UserEntity>() {    @Override public void call(UserEntity userEntity) {      if (userEntity != null) {        CloudUserDataStore.this .userCache.put(userEntity) ;      }    }  };  /**   * Construct a {@link UserDataStore} based on connections to the api (Cloud).   *   * @param restApi The {@link RestApi} implementation to use.   * @param userCache A {@link UserCache} to cache data retrieved from the api.   */  public CloudUserDataStore(RestApi restApi , UserCache userCache) {    this .restApi = restApi ;    this. userCache = userCache;  }  @Override public Observable<List<UserEntity>> getUserEntityList() {    return this .restApi.getUserEntityList() ;  }  @Override public Observable<UserEntity> getUserEntityDetails (final int userId) {    return this.restApi.getUserEntityById(userId).doOnNext( saveToCacheAction);  }}

-------------------------------------------------------------------------------------------------------- 我從中可以學習到的內容是：1.先用介面，聲明你將要提供給使用者的功能。 2.而關於資料擷取方面的實現，則是按照如下的方式進行:實現一個工廠，這個工廠負責選擇不同的資料來源執行個體，然後通過方法暴露給調用者。  比如，UserDataStoreFactory，它提供了兩個方法，這兩個方法都是用來返回一個UserDataStore執行個體，至於執行個體是採用哪種實現的，使用者無需關心。採用什麼實現，這是在UserDataStoreFactory中控制的。 在這個例子中，有如下的UserDataStore實現：CloudUserDataStore, DiskUserDataStore 這兩個實現，就提供了資料來源的具體實現。其實，還可以添加，從資料庫擷取使用者列表，使用者詳情，可以添加一個DataBaseUserDataStore。總之，可以添加不同的資料來源，只要它們實現了UserDataStore介面所聲明的功能就可以。  -------------------------------------------------------------------------------------------參照上述的實現方式，假如我自己要實現一個登陸功能，那麼，我要怎麼做？ 登陸功能不適合上述的實現方式。上述的實現方式，適合於專門使用者擷取資料的情境。登陸功能，只是一個REST調用，不是用來擷取資料的。

擷取資料來源資料的實現---Architecting Android