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！