Android判断APP是在前台还是后台

1 继承Application.ActivityLifecycleCallbacks ，需要API > 14 , 现在基本上Android机都能达到这个要求。
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 listeners = new CopyOnWriteArrayList();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);}@Overridepublic void onActivityResumed(Activity activity) {    paused = false;    boolean wasBackground = !foreground;    foreground = true;    if (check != null)        handler.removeCallbacks(check);    if (wasBackground) {        A.e("went foreground");        for (Listener l : listeners) {            try {                l.onBecameForeground();            } catch (Exception exc) {                A.e("Listener threw exception!", exc);            }        }    } else {        A.e("still foreground");    }}@Overridepublic 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;                A.e("went background");                for (Listener l : listeners) {                    try {                        l.onBecameBackground();                    } catch (Exception exc) {                        A.e("Listener threw exception!", exc);                    }                }            } else {                A.e("still foreground");            }        }    }, CHECK_DELAY);}@Overridepublic void onActivityCreated(Activity activity, Bundle savedInstanceState) {}@Overridepublic void onActivityStarted(Activity activity) {}@Overridepublic void onActivityStopped(Activity activity) {}@Overridepublic void onActivitySaveInstanceState(Activity activity, Bundle outState) {}@Overridepublic void onActivityDestroyed(Activity activity) {}

}

2 在Application中注册事件：
callback = ForegroundCallbacks.init(this);
callback.addListener(new Listener() {

        @Override        public void onBecameForeground() {            startService(new Intent(AppContext.this, CheckVipService.class));            System.out.println("---------------------------- 前台");        }        @Override        public void onBecameBackground() {            System.out.println("---------------------------- 后台");        }    });

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