本文出自 “阿敏其人” 簡書博客，轉載或引用請注明出處。

"從前有座山，山里有座廟，廟里有個老和尚他袈裟七彩燈...... "
推過來的歌曲真有意思！

首先，感謝Alex_Cin的留言，才有下文的產(chǎn)生。

之前寫過一小文，Android 程序打開即啟動指定頁面，比如密碼檢查頁。，實現(xiàn)方式麻煩瑣碎，后來Alex_Cin留言說可以考慮采用Application.ActivityLifecycleCallbacks來實現(xiàn) 監(jiān)聽程序處于前臺還是后臺 ，閱讀學習后，大呼過癮。很早就想把這個寫成博客記錄分享給各位朋友，但是因為個人原因遲遲未完成，現(xiàn)在補上。

關于監(jiān)聽程序處于前臺還是后臺

• 弄一個BaseActivity，onResume等方法加1減1？這樣不好，bug還是有的。
• 獲得所有程序的列表，判斷當前程序是否至于前臺？不好，而且新SDK有所限制。
• 廣播+服務，牛刀殺雞。

所以，Application.ActivityLifecycleCallbacks登場，清脆利索，簡單大氣。

老規(guī)矩，先看圖：

ges.gif

結構圖

Paste_Image.png

菜已上，酒已喝。說說菜怎么做的吧。

一、Application.ActivityLifecycleCallbacks

Application通過ActivityLifecycleCallbacks使用接口提供了一套回調(diào)方法，用于讓開發(fā)者對Activity的生命周期事件進行集中處理。 ActivityLifecycleCallbacks接口回調(diào)可以簡化監(jiān)測Activity的生命周期事件，在一個類中作統(tǒng)一處理。 ActivityLifecycleCallbacks使用要求API 14+ （Android 4.0+）。

既然是接口，那么來實現(xiàn)以下看看怎樣唄

隨便寫一個類，比如名為TestAppCb，實現(xiàn)Application.ActivityLifecycleCallbacks接口

public class TestAppCb implements Application.ActivityLifecycleCallbacks {
    @Override
    public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
        
    }

    @Override
    public void onActivityStarted(Activity activity) {

    }

    @Override
    public void onActivityResumed(Activity activity) {

    }

    @Override
    public void onActivityPaused(Activity activity) {

    }

    @Override
    public void onActivityStopped(Activity activity) {

    }

    @Override
    public void onActivitySaveInstanceState(Activity activity, Bundle outState) {

    }

    @Override
    public void onActivityDestroyed(Activity activity) {

    }
}

我們發(fā)現(xiàn)了很多跟Activity很聲明周期很相似的方法。作用不贅述，下面會附上一個名為ForegroundCallbacks的實現(xiàn)類，自行參考。

以往若需監(jiān)測Activity的生命周期事件代碼，你可能是這樣做的，重寫每一個Acivity的onResume()，然后作統(tǒng)計和處理，ActivityLifecycleCallbacks接口回調(diào)可以簡化這一繁瑣過程，在一個類中作統(tǒng)一處理。

二、具體使用

核心封裝類，根據(jù)下面的代碼的封裝，結合自定義的Application，就可以輕松地實現(xiàn)監(jiān)聽程序處于前臺還是后臺的需求。

public class ForegroundCallbacks implements Application.ActivityLifecycleCallbacks {
    public static final long CHECK_DELAY = 500;
    public static final String TAG = ForegroundCallbacks.class.getName();
    public interface Listener {
        public void onBecameForeground();
        public void onBecameBackground();
    }
    private static ForegroundCallbacks instance;
    private boolean foreground = false, paused = true;
    private Handler handler = new Handler();
    private List<Listener> listeners = new CopyOnWriteArrayList<Listener>();
    private Runnable check;
    public static ForegroundCallbacks init(Application application){
        if (instance == null) {
            instance = new ForegroundCallbacks();
            application.registerActivityLifecycleCallbacks(instance);
        }
        return instance;
    }
    public static ForegroundCallbacks get(Application application){
        if (instance == null) {
            init(application);
        }
        return instance;
    }
    public static ForegroundCallbacks get(Context ctx){
        if (instance == null) {
            Context appCtx = ctx.getApplicationContext();
            if (appCtx instanceof Application) {
                init((Application)appCtx);
            }
            throw new IllegalStateException(
                    "Foreground is not initialised and " +
                            "cannot obtain the Application object");
        }
        return instance;
    }
    public static ForegroundCallbacks get(){
        if (instance == null) {
            throw new IllegalStateException(
                    "Foreground is not initialised - invoke " +
                            "at least once with parameterised init/get");
        }
        return instance;
    }
    public boolean isForeground(){
        return foreground;
    }
    public boolean isBackground(){
        return !foreground;
    }
    public void addListener(Listener listener){
        listeners.add(listener);
    }
    public void removeListener(Listener listener){
        listeners.remove(listener);
    }
    @Override
    public void onActivityResumed(Activity activity) {
        paused = false;
        boolean wasBackground = !foreground;
        foreground = true;
        if (check != null)
            handler.removeCallbacks(check);
        if (wasBackground){
           L.d ("went foreground");

            for (Listener l : listeners) {
                try {
                    l.onBecameForeground();


                } catch (Exception exc) {
                    L.d ("Listener threw exception!:"+exc.toString());
                }
            }
        } else {
            L.d ("still foreground");
        }
    }
    @Override
    public void onActivityPaused(Activity activity) {
        paused = true;
        if (check != null)
            handler.removeCallbacks(check);
        handler.postDelayed(check = new Runnable(){
            @Override
            public void run() {
                if (foreground && paused) {
                    foreground = false;
                    L.d ("went background");
                    for (Listener l : listeners) {
                        try {
                            l.onBecameBackground();
                        } catch (Exception exc) {
                            L.d ("Listener threw exception!:"+exc.toString());
                        }
                    }
                } else {
                    L.d ("still foreground");
                }
            }
        }, CHECK_DELAY);
    }
    @Override
    public void onActivityCreated(Activity activity, Bundle savedInstanceState) {}
    @Override
    public void onActivityStarted(Activity activity) {}
    @Override
    public void onActivityStopped(Activity activity) {}
    @Override
    public void onActivitySaveInstanceState(Activity activity, Bundle outState) {}
    @Override
    public void onActivityDestroyed(Activity activity) {}
}

.
.

MyApplication 結合Application.ActivityLifecycleCallbacks

監(jiān)聽如此輕松

public class MyApplication extends Application{

    @Override
    public void onCreate() {
        super.onCreate();
        ForegroundCallbacks.init(this);

        ForegroundCallbacks.get().addListener(new ForegroundCallbacks.Listener() {
            @Override
            public void onBecameForeground() {
                L.d("當前程序切換到前臺");
                if(CacheUtils.getBoolean(getApplicationContext(), MyConst.GESTRUE_IS_LIVE)){
                    L.d("已經(jīng)開啟手勢鎖");
                    Intent intent = new Intent(getApplicationContext(), CheckGesPwdActivity.class);
                    intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP|Intent.FLAG_ACTIVITY_NEW_TASK);
                    startActivity(intent);
                }else{

                }

            }

            @Override
            public void onBecameBackground() {
                L.d("當前程序切換到后臺");
            }
        });

    }
}

如上，即可，手勢庫可以直接引用做個小demo。

整個過程實現(xiàn)起來都是相當輕松的，亮點是ForegroundCallbacks，里面的代碼值得咀嚼和學習。

本文粗糙了點，哈哈哈哈，不過我為什么感覺我還是挺良心的，哈哈哈哈。

附上github鏈接GestureLockMaster

三、單個Activity單獨使用？

就在一個頁面監(jiān)聽，當然可以。

public class TestLifeAc extends AppCompatActivity implements ForegroundCallbacks.Listener {

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_city_list);
        // 注冊監(jiān)聽
        ForegroundCallbacks.get(this).addListener(this);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        // 移除監(jiān)聽
        ForegroundCallbacks.get(this).removeListener(this);
    }

    @Override
    public void onBecameForeground(Activity activity) {
        // 切換為前臺
    }

    @Override
    public void onBecameBackground(Activity activity) {
        //切換為后臺
    }
}

四，參考學習

Is my Android app currently foreground or background?

本篇完。