监测app从后台到前台，从前台到后台

    // app从后台进入前台都会调用这个方法
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(applicationBecomeActive) name:UIApplicationWillEnterForegroundNotification object:nil];
    
    // 添加检测app进入后台的观察者
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(applicationEnterBackground) name: UIApplicationDidEnterBackgroundNotification object:nil];