1 類圖

在【FJU項目】1像素進程保活（二）中，涉及到的幾個類的類圖如下所示（僅供參考）：
實線箭頭：關聯
虛線箭頭：依賴

在上圖中，OnePixelManager裏面有太多的職責，違背了單一職責原則。裏面還有很多地方違背了其它的設計原則，稍後通過代碼進行分析。

2 代碼分析

2.1 之前的OnePixelManager類源碼如下：

package com.yds.jianshu.onepixel;

import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;

import java.lang.ref.WeakReference;

public class OnePixelManager {
    private static final String TAG = "[OnePixelManager]";
    private WeakReference<Activity> mActivity;
    private OnePixelReceiver onePixelReceiver;

    /**
     * 一像素廣播接收者註冊方法。該方法中初始化OnePixelReceiver，並添加了過濾條件
     * 屏幕息屏和亮屏。然後註冊該廣播接收者
     * @param context
     */
    public void registerOnePixelReceiver(Context context){
        IntentFilter filter = new IntentFilter();
        filter.addAction(Intent.ACTION_SCREEN_OFF);
        filter.addAction(Intent.ACTION_SCREEN_ON);
        onePixelReceiver = new OnePixelReceiver();
        context.registerReceiver(onePixelReceiver,filter);
    }

    /**
     * 對廣播接收者進行解註冊
     * @param context
     */
    public void unregisterOnePixelReceiver(Context context){
        if (null != onePixelReceiver){
            context.unregisterReceiver(onePixelReceiver);
        }
    }

    /**
     * 開啓一像素Activity
     * @param context
     */
    public void startOnePixelActivity(Context context){
        Intent intent = new Intent();
        intent.setClass(context,OnePixelActivity.class);
        context.startActivity(intent);
    }

    /**
     * 關閉一像素Activity
     */
    public void finishOnePixelActivity(){
        if(null!=mActivity){
            Activity activity = mActivity.get();
            if(null!=activity){
                activity.finish();
            }
            mActivity = null;
        }
    }

    /**
     * 使用弱引用獲取一像素的上下文
     * @param activity
     */
    public void setKeepAliveReference(OnePixelActivity activity){
        mActivity = new WeakReference<Activity>(activity);
    }

}

上述代碼中，總共有5個方法，但是職責卻不是單一的，上述方法的作用如下：

registerOnePixelReceiver：一像素廣播接收者註冊方法。該方法中初始化OnePixelReceiver，並添加了過濾條件屏幕息屏和亮屏。然後註冊該廣播接收者
unregisterOnePixelReceiver：對廣播接收者進行解註冊
startOnePixelActivity：開啓一像素Activity
finishOnePixelActivity：關閉一像素Activity
setKeepAliveReference：使用弱引用獲取一像素的上下文

上述可以大致分爲兩個職責，一個是對Activity進行管理，一個是對廣播接收者進行管理，但上述代碼將其封裝到了一個類中，不利於擴展，且耦合度比較高。所以可以將其拆分成兩個管理類，一個用於管理Activity的ActivityManager，一個用於管理Receiver的ReceiverManager類。

2.2 寫一個Manager接口，預留後用

package com.yds.jianshu.itf;

/**
 * Created by yds
 * on 2019/8/2.
 */
public interface Manager {
    //預留接口
}

寫一個抽象ActivityManager類

package com.yds.jianshu.manager;

import android.app.Activity;
import android.content.Context;

import com.yds.jianshu.itf.Manager;
import com.yds.jianshu.utils.ObjectUtil;

/**
 * Created by yds
 * on 2019/8/2.
 */
public abstract class ActivityManager implements Manager {
    public abstract void startActivity(Context context);
    public void finishActivity(Activity activity){
        ObjectUtil.requireNonNull(activity,"activity is null");
        activity.finish();
    }
}

一般Activity有兩種最常用的行爲，一個是開啓Activity，一個是關閉Activity，開啓Activity可以根據其需求寫具體的代碼，所以這裏將其設置爲抽象類型，關閉Activity幾乎是通用的，這裏就在抽象類裏實現。

2.3 具體的Activity管理類OnePixelActivityManager

package com.yds.jianshu.manager;

import android.app.Activity;
import android.content.Context;
import android.content.Intent;

import com.yds.jianshu.onepixel.OnePixelActivity;
import com.yds.jianshu.utils.ObjectUtil;

import java.lang.ref.WeakReference;

/**
 * Created by yds
 * on 2019/8/2.
 */
public class OnePixelActivityManager extends ActivityManager {
    private WeakReference<Activity> mActivity;
      /**
     * 將{@link OnePixelActivity}的引用保存起來，用於後面監測到屏幕亮時關閉{@link OnePixelActivity}，
     * 使用弱引用，是因爲如果O{@link OnePixelActivity}被回收了，那麼就不需要再保存該對象的引用用於
     * finish了
     * @param activity
     */
    public void registerActivityReference(Activity activity){
        ObjectUtil.requireNonNull(activity,"activity is null");
        mActivity = new WeakReference<Activity>(activity);
    }
    @Override
    public void startActivity(Context context) {
        ObjectUtil.requireNonNull(context,"context is null");
        Intent intent = new Intent();
        intent.setClass(context, OnePixelActivity.class);
        context.startActivity(intent);
    }
    public void finishOnePixelActivity(){
        ObjectUtil.requireNonNull(mActivity,"mActivity is null");
        Activity activity = mActivity.get();
        ObjectUtil.requireNonNull(activity,"activity is null");
        finishActivity(activity);
        mActivity = null;
    }
}

2.4 用於管理BroadcastReceiver的類

package com.yds.jianshu.manager;

import android.content.BroadcastReceiver;
import android.content.Context;

import com.yds.jianshu.itf.Manager;

/**
 * Created by yds
 * on 2019/8/2.
 */
public abstract class ReceiverManager implements Manager {
    public BroadcastReceiver receiver;
    public abstract void register(Context context);
    public abstract void unregister(Context context);

    public void createReceiver(BroadcastReceiver receiver){
        this.receiver = receiver;
    }

}

廣播接收者一般有兩個行爲，一個是註冊，一個是取消註冊。因爲註冊和取消註冊，都需要創建具體的Receiver實例，所以可以將其提取到父類中。

2.5 用於管理1像素receiver的OnePixelReceiverManager

package com.yds.jianshu.manager;

import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;

import com.yds.jianshu.onepixel.OnePixelReceiver;
import com.yds.jianshu.utils.ObjectUtil;

/**
 * Created by yds
 * on 2019/8/2.
 */
public class OnePixelReceiverManager extends ReceiverManager {
    public OnePixelReceiverManager(){
        createReceiver(new OnePixelReceiver());
    }

    @Override
    public void register(Context context) {
        ObjectUtil.requireNonNull(context,"context is null");
        ObjectUtil.requireNonNull(receiver,"receiver is null");
        IntentFilter filter = new IntentFilter();
        filter.addAction(Intent.ACTION_SCREEN_OFF);
        filter.addAction(Intent.ACTION_SCREEN_ON);
        context.registerReceiver(receiver,filter);
    }

    @Override
    public void unregister(Context context) {
        ObjectUtil.requireNonNull(context,"context is null");
        ObjectUtil.requireNonNull(receiver,"receiver is null");
        context.unregisterReceiver(receiver);
    }
}

上述代碼符合單一職責原則，單一職責原則有如下定義：

單一職責原則：一個對象應該只包含單一的職責，並且該職責被完整封裝在一個類中。

上述OnePixelReceiverManager符合單一職責，它主要是對OnePixelReceiver進行相關操作管理；OnePixelActivityManager 也符合單一職責，它主要是對OnePixelActivity進行相關管理。

上述代碼中，有這麼一段代碼：

public class OnePixelReceiverManager extends ReceiverManager {
    public OnePixelReceiverManager(){
        createReceiver(new OnePixelReceiver());
    }
}

public abstract class ReceiverManager implements Manager {
    public BroadcastReceiver receiver;
    public void createReceiver(BroadcastReceiver receiver){
        this.receiver = receiver;
    }
}

ReceiverManager 的createReceiver的參數是BroadcastReceiver ，在ReceiverManager的子類OnePixelReceiverManager 中調用createReceiver時，用OnePixelReceiver實例化了BroadcastReceiver，這符合里氏代換原則，其定義如下：

里氏代換原則：所有引用基類的地方必須能透明地使用其子類的對象。

在里氏代換原則中，應該將父類設計爲抽象類或者接口，讓子類繼承父類或實現父接口，並實現在父類中聲明方法，在運行時子類實例替換父類實例。更詳細可參考【設計模式】里氏代換原則

設計ActivityManager和ReceiverManager是爲了讓代碼符合開閉原則，因爲這兩個管理類中都提取了Activity和Receiver的共性方法，如果需要增加其它具體的Activity和Receiver管理類，只需創建一個新的具體類，而不需要修改ActivityManager和ReceiverManager代碼，符合開閉原則，開閉原則定義如下：

開閉原則：軟件實體應該對擴展開放，對修改關閉。

設計Manager接口也是如此，該接口是預留接口，後續可能會用到。

2.6 再看看OnePixelActivity裏修改部分

package com.yds.jianshu.onepixel;

import android.app.Activity;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.util.Log;
import android.view.Gravity;
import android.view.Window;
import android.view.WindowManager;

import com.yds.jianshu.manager.OnePixelActivityManager;

public class OnePixelActivity extends Activity{
    private static final String TAG = "[OnePixelActivity]";
    private OnePixelActivityManager manager;
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setWindowAttributes();
        initData();

        Log.e(TAG,"onCreate");
    }
    private void initData(){
        manager = new OnePixelActivityManager();
        manager.registerActivityReference(this);//將引用傳給OnePixelActivityManager
    }
    private void setWindowAttributes(){
        Window window = getWindow();
        window.setGravity(Gravity.LEFT|Gravity.TOP);
        WindowManager.LayoutParams params = window.getAttributes();
        params.x = 0;
        params.y = 0;
        params.height = 1;
        params.width = 1;
        window.setAttributes(params);
    }
    @Override
    protected void onDestroy() {
        super.onDestroy();
        Log.e(TAG,"onDestroy");
    }

    @Override
    protected void onStop() {
        super.onStop();
        Log.e(TAG,"onStop");
    }

    @Override
    protected void onPause() {
        super.onPause();
        Log.e(TAG,"onPause");
    }

    @Override
    protected void onStart() {
        super.onStart();
        Log.e(TAG,"onStart");
    }

    @Override
    protected void onResume() {
        super.onResume();
        Log.e(TAG,"onResume");
    }
}

2.7 OnePixelReceiver源碼

package com.yds.jianshu.onepixel;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;

import com.yds.jianshu.manager.OnePixelActivityManager;


public class OnePixelReceiver extends BroadcastReceiver {
    private static final String TAG = "[OnePixelReceiver]";
    private OnePixelActivityManager manager;
    @Override
    public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();
        manager = new OnePixelActivityManager();
        if (Intent.ACTION_SCREEN_ON.equals(action)){//如果亮屏，則關閉1像素Activity
            manager.finishOnePixelActivity();
        }else if(Intent.ACTION_SCREEN_OFF.equals(action)){//如果息屏，則開啓1像素Activity
            manager.startActivity(context);
        }
    }
}

2.8 OnePixelService源碼部分

package com.yds.jianshu.onepixel;

import android.app.Service;
import android.content.Intent;
import android.os.IBinder;

import com.yds.jianshu.manager.OnePixelReceiverManager;
import com.yds.jianshu.manager.ReceiverManager;

public class OnePixelService extends Service {
    private ReceiverManager manager;
    public OnePixelService() {
    }

    @Override
    public IBinder onBind(Intent intent) {
        // TODO: Return the communication channel to the service.
        throw new UnsupportedOperationException("Not yet implemented");
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        manager = new OnePixelReceiverManager();//符合里氏代換原則
        manager.register(this);//註冊廣播接收者
        return START_STICKY;
    }

    @Override
    public void onDestroy() {
        manager.unregister(this);
    }
}

在上述代碼中，使用了OnePixelReceiverManager來實例化ReceiverManager ，符合里氏代換原則。

2.9 MainActivity代碼

package com.yds.jianshu.mobile;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

import com.example.myapplication.R;
import com.yds.jianshu.onepixel.OnePixelService;

public class MainActivity extends AppCompatActivity{
    private static final String TAG = "[MainActivity]";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initData();
    }
    private void initData(){
        startOnePixelService();
    }

    private void startOnePixelService(){
        Intent intent = new Intent();
        intent.setClass(MainActivity.this, OnePixelService.class);
        startService(intent);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
    }
}

該部分代碼基本不變。

在此基礎上，還增加了一個工具類，用於對對象判空

package com.yds.jianshu.utils;

/**
 * Created by yds
 * on 2019/8/2.
 */
public class ObjectUtil {
    public static<T> T requireNonNull(T object,String message){
        if(object == null){
            throw new NullPointerException(message);
        }
        return object;
    }
}

3 重構後的UML圖

暫時不提供

【Android實例】用設計原則來重構1像素保活代碼

1 類圖

2 代碼分析

2.1 之前的OnePixelManager類源碼如下：

2.2 寫一個Manager接口，預留後用

2.3 具體的Activity管理類OnePixelActivityManager

2.4 用於管理BroadcastReceiver的類

2.5 用於管理1像素receiver的OnePixelReceiverManager

2.6 再看看OnePixelActivity裏修改部分

2.7 OnePixelReceiver源碼

2.8 OnePixelService源碼部分

2.9 MainActivity代碼

3 重構後的UML圖

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