app退以后台，进入前台

// app退到后台

[[NSNotificationCenter
defaultCenter]
addObserver:self
selector:@selector(appDidEnterBackground)
name:UIApplicationWillResignActiveNotification
object:nil];

// app进入前台

[[NSNotificationCenter
defaultCenter]
addObserver:self
selector:@selector(appDidEnterPlayGround)
name:UIApplicationDidBecomeActiveNotification
object:nil];