一张图看Goodle Clean设计架构

　　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！

　　

　　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：

 /*
  * 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;

 /**
  * Use cases are the entry points to the domain layer.
  *
  * @param <Q> the request type
  * @param <P> the response type
  */
 public abstract class UseCase<Q extends UseCase.RequestValues, P extends UseCase.ResponseValue> {

     private Q mRequestValues;

     private UseCaseCallback<P> mUseCaseCallback;

     public void setRequestValues(Q requestValues) {
         mRequestValues = requestValues;
     }

     public Q getRequestValues() {
         return mRequestValues;
     }

     public UseCaseCallback<P> getUseCaseCallback() {
         return mUseCaseCallback;
     }

     public void setUseCaseCallback(UseCaseCallback<P> useCaseCallback) {
         mUseCaseCallback = useCaseCallback;
     }

     void run() {
        executeUseCase(mRequestValues);
     }

     protected abstract void executeUseCase(Q requestValues);

     /**
      * Data passed to a request.
      */
     public interface RequestValues {
     }

     /**
      * Data received from a request.
      */
     public interface ResponseValue {
     }

     public interface UseCaseCallback<R> {
         void onSuccess(R response);
         void onError();
     }
 }

　　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：

 /*
  * 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.tasks.domain.usecase;

 import android.support.annotation.NonNull;

 import com.example.android.architecture.blueprints.todoapp.UseCase;
 import com.example.android.architecture.blueprints.todoapp.data.source.TasksRepository;

 import static com.google.common.base.Preconditions.checkNotNull;

 /**
  * Marks a task as active (not completed yet).
  */
 public class ActivateTask extends UseCase<ActivateTask.RequestValues, ActivateTask.ResponseValue> {

     private final TasksRepository mTasksRepository;

     public ActivateTask(@NonNull TasksRepository tasksRepository) {
         mTasksRepository = checkNotNull(tasksRepository, "tasksRepository cannot be null!");
     }

     @Override
     protected void executeUseCase(final RequestValues values) {
         String activeTask = values.getActivateTask();
         mTasksRepository.activateTask(activeTask);
         getUseCaseCallback().onSuccess(new ResponseValue());
     }

     public static final class RequestValues implements UseCase.RequestValues {

         private final String mActivateTask;

         public RequestValues(@NonNull String activateTask) {
             mActivateTask = checkNotNull(activateTask, "activateTask cannot be null!");
         }

         public String getActivateTask() {
             return mActivateTask;
         }
     }

     public static final class ResponseValue implements UseCase.ResponseValue { }
 }

　　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：

 /*
  * 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;

 /**
  * Interface for schedulers, see {@link UseCaseThreadPoolScheduler}.
  */
 public interface UseCaseScheduler {

     void execute(Runnable runnable);

     <V extends UseCase.ResponseValue> void notifyResponse(final V response,
             final UseCase.UseCaseCallback<V> useCaseCallback);

     <V extends UseCase.ResponseValue> void onError(
             final UseCase.UseCaseCallback<V> useCaseCallback);
 }

 /*
  * 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 android.os.Handler;

 import java.util.concurrent.ArrayBlockingQueue;
 import java.util.concurrent.Executors;
 import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;

 /**
  * Executes asynchronous tasks using a {@link ThreadPoolExecutor}.
  * <p>
  * See also {@link Executors} for a list of factory methods to create common
  * {@link java.util.concurrent.ExecutorService}s for different scenarios.
  */
 public class UseCaseThreadPoolScheduler implements UseCaseScheduler {

     private final Handler mHandler = new Handler();

     public static final int POOL_SIZE = 2;

     public static final int MAX_POOL_SIZE = 4;

     public static final int TIMEOUT = 30;

     ThreadPoolExecutor mThreadPoolExecutor;

     public UseCaseThreadPoolScheduler() {
         mThreadPoolExecutor = new ThreadPoolExecutor(POOL_SIZE, MAX_POOL_SIZE, TIMEOUT,
                 TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(POOL_SIZE));
     }

     @Override
     public void execute(Runnable runnable) {
         mThreadPoolExecutor.execute(runnable);
     }

     @Override
     public <V extends UseCase.ResponseValue> void notifyResponse(final V response,
             final UseCase.UseCaseCallback<V> useCaseCallback) {
         mHandler.post(new Runnable() {
             @Override
             public void run() {
                 useCaseCallback.onSuccess(response);
             }
         });
     }

     @Override
     public <V extends UseCase.ResponseValue> void onError(
             final UseCase.UseCaseCallback<V> useCaseCallback) {
         mHandler.post(new Runnable() {
             @Override
             public void run() {
                 useCaseCallback.onError();
             }
         });
     }

 }

　　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：

 @Override
     public void activateTask(@NonNull Task activeTask) {
         checkNotNull(activeTask, "activeTask cannot be null!");
         mUseCaseHandler.execute(mActivateTask, new ActivateTask.RequestValues(activeTask.getId()),
                 new UseCase.UseCaseCallback<ActivateTask.ResponseValue>() {
                     @Override
                     public void onSuccess(ActivateTask.ResponseValue response) {
                         mTasksView.showTaskMarkedActive();
                         loadTasks(false, false);
                     }

                     @Override
                     public void onError() {
                         mTasksView.showLoadingTasksError();
                     }
                 });
     }

　　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！