之前有个需求，App从前台切换到后台，不需要做什么操作；从后台切换到前台，需要先验证屏锁密码。于是乎，搜索了一下，“利用Application监听App处于前台和后台”，找到了一个国外的big god的解决方案。不要问我是怎么找到的，做开发必备的技能，擅长检索，也是一种能力。致谢steveliles

...

废话不说了，直接上代码。

<pre>

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){
        LogUtil.e("went foreground");
        for (Listener l : listeners) {
            try {
                l.onBecameForeground();
            } catch (Exception exc) {
                LogUtil.e("Listener threw exception!", exc);
            }
        }
    } else {
        LogUtil.e("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;
                LogUtil.e("went background");
                for (Listener l : listeners) {
                    try {
                        l.onBecameBackground();
                    } catch (Exception exc) {
                        LogUtil.e("Listener threw exception!", exc);
                    }
                }
            } else {
                LogUtil.e("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) {}

}

</pre>