之前用一张图分析了Google给出的MVP架构,但是在Google给出的所有案例里面除了基本的MVP架构还有其它几种架构,今天就来分析其中的Clean架构。同样的,网上介绍Clean架构的文章很多,我也就不用文字过多叙述了,还是用一张类图来分析一下Clean架构的这个案例吧。好了,先直接上图!
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上完图,再说一说我对Clean架构的一个理解吧。对比前一篇文章的MVP架构图可以看出,clean在一定程度上继承了mvp的设计思想,但是其抽象程度比mvp更高。初次看这个demo的时候,确实被震撼了一下——原来用Java可以这样写代码!!!跟之前用的一些项目框架和我自己平时写的一些代码对比一下,只能感叹clean的这种设计思想真不是一般的程序员可以想出来的。它对接口、抽象类和实现类之间的实现、继承、调用关系发挥到了一个比较高的层次,它并不是像我们平时写代码那样很直白地写下来,而是充分利用了面向对象的封装性、继承性和多态性,是对面向对象思想的一个高度理解。其实,要说clean复杂,它确实有些难理解,可是如果你真的理解了面向对象思想,那么又会觉得这样的设计完全在情理之中。
举个例子,在这个案例里面,对实体类的设计就进行了高度的抽象与封装。首先,为所有的实体类设计了基类——UseCase,UseCase的代码如下:
1 /* 2 * Copyright 2016, The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 *10 * Unless required by applicable law or agreed to in writing, software11 * distributed under the License is distributed on an "AS IS" BASIS,12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.13 * See the License for the specific language governing permissions and14 * limitations under the License.15 */16 17 package com.example.android.architecture.blueprints.todoapp;18 19 /**20 * Use cases are the entry points to the domain layer.21 *22 * @param <Q> the request type23 * @param <P> the response type24 */25 public abstract class UseCase<Q extends UseCase.RequestValues, P extends UseCase.ResponseValue> {26 27 private Q mRequestValues;28 29 private UseCaseCallback<P> mUseCaseCallback;30 31 public void setRequestValues(Q requestValues) {32 mRequestValues = requestValues;33 }34 35 public Q getRequestValues() {36 return mRequestValues;37 }38 39 public UseCaseCallback<P> getUseCaseCallback() {40 return mUseCaseCallback;41 }42 43 public void setUseCaseCallback(UseCaseCallback<P> useCaseCallback) {44 mUseCaseCallback = useCaseCallback;45 }46 47 void run() {48 executeUseCase(mRequestValues);49 }50 51 protected abstract void executeUseCase(Q requestValues);52 53 /**54 * Data passed to a request.55 */56 public interface RequestValues {57 }58 59 /**60 * Data received from a request.61 */62 public interface ResponseValue {63 }64 65 public interface UseCaseCallback<R> {66 void onSuccess(R response);67 void onError();68 }69 }
实体基类UseCase的设计用了泛型和接口,仅仅设计了两个字段mRequestValues和mUseCaseCallback。其中,mRequestValues代表数据请求参数,用泛型进行了封装,它其实也是一个类的对象;mUseCaseCallback代表请求结果,同样的,它也是一个类的对象,只不过这个类是用接口的形式进行抽象和封装的。同时,UseCase中定义抽象方法executeUseCase()作为实体操作的入口。
接下来,我们随便看一个UseCase的实现类,就拿ActivateTask来说,ActivateTask继承了UseCase,其实现代码如下:
1 /* 2 * Copyright 2016, The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 *10 * Unless required by applicable law or agreed to in writing, software11 * distributed under the License is distributed on an "AS IS" BASIS,12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.13 * See the License for the specific language governing permissions and14 * limitations under the License.15 */16 17 package com.example.android.architecture.blueprints.todoapp.tasks.domain.usecase;18 19 import android.support.annotation.NonNull;20 21 import com.example.android.architecture.blueprints.todoapp.UseCase;22 import com.example.android.architecture.blueprints.todoapp.data.source.TasksRepository;23 24 import static com.google.common.base.Preconditions.checkNotNull;25 26 /**27 * Marks a task as active (not completed yet).28 */29 public class ActivateTask extends UseCase<ActivateTask.RequestValues, ActivateTask.ResponseValue> {30 31 private final TasksRepository mTasksRepository;32 33 public ActivateTask(@NonNull TasksRepository tasksRepository) {34 mTasksRepository = checkNotNull(tasksRepository, "tasksRepository cannot be null!");35 }36 37 @Override38 protected void executeUseCase(final RequestValues values) {39 String activeTask = values.getActivateTask();40 mTasksRepository.activateTask(activeTask);41 getUseCaseCallback().onSuccess(new ResponseValue());42 }43 44 public static final class RequestValues implements UseCase.RequestValues {45 46 private final String mActivateTask;47 48 public RequestValues(@NonNull String activateTask) {49 mActivateTask = checkNotNull(activateTask, "activateTask cannot be null!");50 }51 52 public String getActivateTask() {53 return mActivateTask;54 }55 }56 57 public static final class ResponseValue implements UseCase.ResponseValue { }58 }
可以看到,在ActivateTask 中,实现了父类UseCase的两个接口RequestValues 和ResponseValue ,这两个类将分别作为最终的实体请求对象类和返回结果对象类,同时,UseCase中的抽象方法executeUseCase()也被实现。因为实现的代码里面加入了泛型和接口,所以看起来会比较复杂,但是说到底无非就是继承和实现的关系,仅此而已。通过这种面向接口的设计方式,可以让我们的代码看起来结构更清晰、更统一。
接下来,我们可以看一下这个项目中的任务执行类UseCaseThreadPoolScheduler,同样,UseCaseThreadPoolScheduler的设计采用了面向接口的方式,它实现了seCaseScheduler接口,UseCaseScheduler和UseCaseThreadPoolScheduler的实现分别如下:
1 /* 2 * Copyright 2016, The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 *10 * Unless required by applicable law or agreed to in writing, software11 * distributed under the License is distributed on an "AS IS" BASIS,12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.13 * See the License for the specific language governing permissions and14 * limitations under the License.15 */16 17 package com.example.android.architecture.blueprints.todoapp;18 19 /**20 * Interface for schedulers, see {@link UseCaseThreadPoolScheduler}.21 */22 public interface UseCaseScheduler {23 24 void execute(Runnable runnable);25 26 <V extends UseCase.ResponseValue> void notifyResponse(final V response,27 final UseCase.UseCaseCallback<V> useCaseCallback);28 29 <V extends UseCase.ResponseValue> void onError(30 final UseCase.UseCaseCallback<V> useCaseCallback);31 }
1 /* 2 * Copyright 2016, The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 *10 * Unless required by applicable law or agreed to in writing, software11 * distributed under the License is distributed on an "AS IS" BASIS,12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.13 * See the License for the specific language governing permissions and14 * limitations under the License.15 */16 17 package com.example.android.architecture.blueprints.todoapp;18 19 import android.os.Handler;20 21 import java.util.concurrent.ArrayBlockingQueue;22 import java.util.concurrent.Executors;23 import java.util.concurrent.ThreadPoolExecutor;24 import java.util.concurrent.TimeUnit;25 26 /**27 * Executes asynchronous tasks using a {@link ThreadPoolExecutor}.28 * <p>29 * See also {@link Executors} for a list of factory methods to create common30 * {@link java.util.concurrent.ExecutorService}s for different scenarios.31 */32 public class UseCaseThreadPoolScheduler implements UseCaseScheduler {33 34 private final Handler mHandler = new Handler();35 36 public static final int POOL_SIZE = 2;37 38 public static final int MAX_POOL_SIZE = 4;39 40 public static final int TIMEOUT = 30;41 42 ThreadPoolExecutor mThreadPoolExecutor;43 44 public UseCaseThreadPoolScheduler() {45 mThreadPoolExecutor = new ThreadPoolExecutor(POOL_SIZE, MAX_POOL_SIZE, TIMEOUT,46 TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(POOL_SIZE));47 }48 49 @Override50 public void execute(Runnable runnable) {51 mThreadPoolExecutor.execute(runnable);52 }53 54 @Override55 public <V extends UseCase.ResponseValue> void notifyResponse(final V response,56 final UseCase.UseCaseCallback<V> useCaseCallback) {57 mHandler.post(new Runnable() {58 @Override59 public void run() {60 useCaseCallback.onSuccess(response);61 }62 });63 }64 65 @Override66 public <V extends UseCase.ResponseValue> void onError(67 final UseCase.UseCaseCallback<V> useCaseCallback) {68 mHandler.post(new Runnable() {69 @Override70 public void run() {71 useCaseCallback.onError();72 }73 });74 }75 76 }
可以看出,UseCaseThreadPoolScheduler实现了UseCaseScheduler中的三个抽象方法。
接下来,我们再看看UseCaseHandler这个类,在UseCaseHandler中,通过子类实例化父类的形式,用UseCaseThreadPoolScheduler实例化了UseCaseScheduler对象。UseCaseHandler的代码如下:
/* * Copyright 2016, The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */package com.example.android.architecture.blueprints.todoapp;import com.example.android.architecture.blueprints.todoapp.util.EspressoIdlingResource;/** * Runs {@link UseCase}s using a {@link UseCaseScheduler}. */public class UseCaseHandler { private static UseCaseHandler INSTANCE; private final UseCaseScheduler mUseCaseScheduler; public UseCaseHandler(UseCaseScheduler useCaseScheduler) { mUseCaseScheduler = useCaseScheduler; } public <T extends UseCase.RequestValues, R extends UseCase.ResponseValue> void execute( final UseCase<T, R> useCase, T values, UseCase.UseCaseCallback<R> callback) { useCase.setRequestValues(values); useCase.setUseCaseCallback(new UiCallbackWrapper(callback, this)); // The network request might be handled in a different thread so make sure // Espresso knows // that the app is busy until the response is handled. EspressoIdlingResource.increment(); // App is busy until further notice mUseCaseScheduler.execute(new Runnable() { @Override public void run() { useCase.run(); // This callback may be called twice, once for the cache and once for loading // the data from the server API, so we check before decrementing, otherwise // it throws "Counter has been corrupted!" exception. if (!EspressoIdlingResource.getIdlingResource().isIdleNow()) { EspressoIdlingResource.decrement(); // Set app as idle. } } }); } public <V extends UseCase.ResponseValue> void notifyResponse(final V response, final UseCase.UseCaseCallback<V> useCaseCallback) { mUseCaseScheduler.notifyResponse(response, useCaseCallback); } private <V extends UseCase.ResponseValue> void notifyError( final UseCase.UseCaseCallback<V> useCaseCallback) { mUseCaseScheduler.onError(useCaseCallback); } private static final class UiCallbackWrapper<V extends UseCase.ResponseValue> implements UseCase.UseCaseCallback<V> { private final UseCase.UseCaseCallback<V> mCallback; private final UseCaseHandler mUseCaseHandler; public UiCallbackWrapper(UseCase.UseCaseCallback<V> callback, UseCaseHandler useCaseHandler) { mCallback = callback; mUseCaseHandler = useCaseHandler; } @Override public void onSuccess(V response) { mUseCaseHandler.notifyResponse(response, mCallback); } @Override public void onError() { mUseCaseHandler.notifyError(mCallback); } } public static UseCaseHandler getInstance() { if (INSTANCE == null) { INSTANCE = new UseCaseHandler(new UseCaseThreadPoolScheduler()); } return INSTANCE; }}
从上面的代码中,我们可以看到,声明的变量mUseCaseScheduler是UseCaseScheduler的对象,但是在构建UseCaseHandler对象的时候,传入的参数却是UseCaseThreadPoolScheduler对象,即用UseCaseThreadPoolScheduler实例化了UseCaseScheduler对象。然后,对mUseCaseScheduler的所有操作都转化成了对UseCaseThreadPoolScheduler对象的操作。
然后,我们仔细看UseCaseHandler的实现的代码,我们会发现其实对实体进行操作的入口就是execute()方法!因为这个方法里面调用了UseCase的run(),而UseCase的run()最终调用了UseCase的executeUseCase()。通过刚刚的分析,我们应该知道,我们实际上操作的实体应该是UseCase的实现类,而不是UseCase类本身,那么这中间是通过什么方式将对UseCase的操作转移到UseCase的实现类上面的呢?我们会发现UseCaseHandler的execute()传入了UseCase对象作为参数,好的,那么我们就看看execute()是在哪里被调用的吧!
经过追踪,我们看到在TasksPresenter类中调用了此方法,调用处的代码如下:
1 @Override 2 public void activateTask(@NonNull Task activeTask) { 3 checkNotNull(activeTask, "activeTask cannot be null!"); 4 mUseCaseHandler.execute(mActivateTask, new ActivateTask.RequestValues(activeTask.getId()), 5 new UseCase.UseCaseCallback<ActivateTask.ResponseValue>() { 6 @Override 7 public void onSuccess(ActivateTask.ResponseValue response) { 8 mTasksView.showTaskMarkedActive(); 9 loadTasks(false, false);10 }11 12 @Override13 public void onError() {14 mTasksView.showLoadingTasksError();15 }16 });17 }
可以看到,我们传入的参数实际上是UseCase的实现类ActivateTask的对象,到这里,我们就明白啦!原来也是子类实例化父类的方式。
上面我只是简单粗略地讲述了一下项目中部分模块的代码,仅仅是举个例子,更多的东西需要大家自己用面向对象的思想去理解。我说这些的目的就是想告诉大家,充分运面向对象的思想就可以设计出很多看似复杂的架构和项目,但是不管再怎么复杂的代码也肯定是有迹可循的,我们只要抓住了这些设计思想的本质,多看几遍代码,一定会豁然开朗!
原标题:一张图看Goodle Clean设计架构
关键词:架构