ios监听程序回到前台，viewController获取事件

法在接下来的随笔中实现)

//增加监听
[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(appHasGoneInForeground:)
                                      name:UIApplicationWillEnterForegroundNotification
                                           object:nil];  
//别忘了删除监听
[[NSNotificationCenter defaultCenter] removeObserver:self];   

UIApplicationDidEnterBackgroundNotification  //进入后台

UIApplicationWillEnterForegroundNotification //回到程序