//后台进前台通知 UIApplicationDidBecomeActiveNotification
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(didBecomeActive) name:UIApplicationDidBecomeActiveNotification object:nil];

//进入后台UIApplicationDidEnterBackgroundNotification
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(didEnterBackground) name:UIApplicationDidEnterBackgroundNotification object:nil];

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