一張圖看Goodle Clean設計架構_ZenDei技術網路在線

之前用一張圖分析了Google給出的MVP架構，但是在Google給出的所有案例裡面除了基本的MVP架構還有其它幾種架構，今天就來分析其中的Clean架構。同樣的，網上介紹Clean架構的文章很多，我也就不用文字過多敘述了，還是用一張類圖來分析一下Clean架構的這個案例吧。好了，先直接上圖！上完 ...

　　之前用一張圖分析了Google給出的MVP架構，但是在Google給出的所有案例裡面除了基本的MVP架構還有其它幾種架構，今天就來分析其中的Clean架構。同樣的，網上介紹Clean架構的文章很多，我也就不用文字過多敘述了，還是用一張類圖來分析一下Clean架構的這個案例吧。好了，先直接上圖！

　　上完圖，再說一說我對Clean架構的一個理解吧。對比前一篇文章的MVP架構圖可以看出，clean在一定程度上繼承了mvp的設計思想，但是其抽象程度比mvp更高。初次看這個demo的時候，確實被震撼了一下——原來用Java可以這樣寫代碼！！！跟之前用的一些項目框架和我自己平時寫的一些代碼對比一下，只能感嘆clean的這種設計思想真不是一般的程式員可以想出來的。它對介面、抽象類和實現類之間的實現、繼承、調用關係發揮到了一個比較高的層次，它並不是像我們平時寫代碼那樣很直白地寫下來，而是充分利用了面向對象的封裝性、繼承性和多態性，是對面向對象思想的一個高度理解。其實，要說clean複雜，它確實有些難理解，可是如果你真的理解了面向對象思想，那麼又會覺得這樣的設計完全在情理之中。

　　舉個例子，在這個案例裡面，對實體類的設計就進行了高度的抽象與封裝。首先，為所有的實體類設計了基類——UseCase，UseCase的代碼如下：

 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, software
11  * 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 and
14  * 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 type
23  * @param <P> the response type
24  */
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 }

　　實體基類UseCase的設計用了泛型和介面，僅僅設計了兩個欄位mRequestValues和mUseCaseCallback。其中，mRequestValues代表數據請求參數，用泛型進行了封裝，它其實也是一個類的對象；mUseCaseCallback代表請求結果，同樣的，它也是一個類的對象，只不過這個類是用介面的形式進行抽象和封裝的。同時，UseCase中定義抽象方法executeUseCase()作為實體操作的入口。

　　接下來，我們隨便看一個UseCase的實現類，就拿ActivateTask來說，ActivateTask繼承了UseCase，其實現代碼如下：

 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, software
11  * 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 and
14  * 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     @Override
38     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，實現了父類UseCase的兩個介面RequestValues 和ResponseValue ，這兩個類將分別作為最終的實體請求對象類和返回結果對象類，同時，UseCase中的抽象方法executeUseCase()也被實現。因為實現的代碼裡面加入了泛型和介面，所以看起來會比較複雜，但是說到底無非就是繼承和實現的關係，僅此而已。通過這種面向介面的設計方式，可以讓我們的代碼看起來結構更清晰、更統一。

　　接下來，我們可以看一下這個項目中的任務執行類UseCaseThreadPoolScheduler，同樣，UseCaseThreadPoolScheduler的設計採用了面向介面的方式，它實現了seCaseScheduler介面，UseCaseScheduler和UseCaseThreadPoolScheduler的實現分別如下：

 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, software
11  * 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 and
14  * 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, software
11  * 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 and
14  * 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 common
30  * {@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     @Override
50     public void execute(Runnable runnable) {
51         mThreadPoolExecutor.execute(runnable);
52     }
53 
54     @Override
55     public <V extends UseCase.ResponseValue> void notifyResponse(final V response,
56             final UseCase.UseCaseCallback<V> useCaseCallback) {
57         mHandler.post(new Runnable() {
58             @Override
59             public void run() {
60                 useCaseCallback.onSuccess(response);
61             }
62         });
63     }
64 
65     @Override
66     public <V extends UseCase.ResponseValue> void onError(
67             final UseCase.UseCaseCallback<V> useCaseCallback) {
68         mHandler.post(new Runnable() {
69             @Override
70             public void run() {
71                 useCaseCallback.onError();
72             }
73         });
74     }
75 
76 }

　　可以看出，UseCaseThreadPoolScheduler實現了UseCaseScheduler中的三個抽象方法。

　　接下來，我們再看看UseCaseHandler這個類，在UseCaseHandler中，通過子類實例化父類的形式，用UseCaseThreadPoolScheduler實例化了UseCaseScheduler對象。UseCaseHandler的代碼如下：

/*
 * 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;
    }
}

　　從上面的代碼中，我們可以看到，聲明的變數mUseCaseScheduler是UseCaseScheduler的對象，但是在構建UseCaseHandler對象的時候，傳入的參數卻是UseCaseThreadPoolScheduler對象，即用UseCaseThreadPoolScheduler實例化了UseCaseScheduler對象。然後，對mUseCaseScheduler的所有操作都轉化成了對UseCaseThreadPoolScheduler對象的操作。

　　然後，我們仔細看UseCaseHandler的實現的代碼，我們會發現其實對實體進行操作的入口就是execute()方法！因為這個方法裡面調用了UseCase的run()，而UseCase的run()最終調用了UseCase的executeUseCase()。通過剛剛的分析，我們應該知道，我們實際上操作的實體應該是UseCase的實現類，而不是UseCase類本身，那麼這中間是通過什麼方式將對UseCase的操作轉移到UseCase的實現類上面的呢？我們會發現UseCaseHandler的execute()傳入了UseCase對象作為參數，好的，那麼我們就看看execute()是在哪裡被調用的吧！

　　經過追蹤，我們看到在TasksPresenter類中調用了此方法，調用處的代碼如下：

 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                     @Override
13                     public void onError() {
14                         mTasksView.showLoadingTasksError();
15                     }
16                 });
17     }

　　可以看到，我們傳入的參數實際上是UseCase的實現類ActivateTask的對象，到這裡，我們就明白啦！原來也是子類實例化父類的方式。

　　上面我只是簡單粗略地講述了一下項目中部分模塊的代碼，僅僅是舉個例子，更多的東西需要大家自己用面向對象的思想去理解。我說這些的目的就是想告訴大家，充分運面向對象的思想就可以設計出很多看似複雜的架構和項目，但是不管再怎麼複雜的代碼也肯定是有跡可循的，我們只要抓住了這些設計思想的本質，多看幾遍代碼，一定會豁然開朗！