在Application的onCrate（）里面執行此方法，直接一個方法就搞定了

/**
 * 判斷在前臺還是后臺
 */
public int count = 0;
private void frontOrBack() {
    //前后臺切換判斷
    registerActivityLifecycleCallbacks(new Application.ActivityLifecycleCallbacks() {
        @Override
        public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
        }
        @Override
        public void onActivityStarted(Activity activity) {
            if (count == 0) {
                Log.v("vergo", "**********切到前臺**********");
            }
            count++;
        }
        @Override
        public void onActivityResumed(Activity activity) {
        }
        @Override
        public void onActivityPaused(Activity activity) {
        }
        @Override
        public void onActivityStopped(Activity activity) {
            count--;
            if (count == 0) {
                Log.v("vergo", "**********切到后臺**********");
            }
        }
        @Override
        public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
        }
        @Override
        public void onActivityDestroyed(Activity activity) {
        }
    });
}