Android判断APP是否在前台运行

public class MyActivityLifecycleCallbacks implements Application.ActivityLifecycleCallbacks { 
    private int foregroundActivities;
    private boolean hasSeenFirstActivity;
    private boolean isChangingConfiguration;
    @Override public void onActivityCreated(Activity activity,Bundle savedInstanceState) {
    }
 
    @Override public void onActivityStarted(Activity activity) {
        mForegroundActivities++;
        if (hasSeenFirstActivity && foregroundActivities == 1 && !isChangingConfiguration) {
            applicationDidEnterForeground(activity);
        }
        hasSeenFirstActivity = true;
        isChangingConfiguration = false;
    }
    @Override public void onActivityResumed(Activity activity) {
    }
 
    @Override public void onActivityPaused(Activity activity) {
    }
 
    @Override public void onActivityStopped(Activity activity) {
        foregroundActivities--;
        if (foregroundActivities == 0) {
            applicationDidEnterBackground(activity);
        }
        isChangingConfiguration = activity.isChangingConfigurations();
    }
 
    @Override public void onActivitySaveInstanceState(Activity activity,Bundle outState) {
    }
 
    @Override public void onActivityDestroyed(Activity activity) {
    }
}

实现了自己的类之后，就可以调用Application的registerActivityLifecycleCallback方法，将我们的类MyActivityLifecycleCallbacks注册进去，这样就可以实现对Activity生命周期的监听了。