iOS 监听用户手动截屏的通知

#if __IPHONE_OS_VERSION_MAX_ALLOWED > __IPHONE_8_0
    if ([[UIDevice currentDevice].systemVersion floatValue] >= 8.0){
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(userDidTakeScreenshot:) name:UIApplicationUserDidTakeScreenshotNotification object:nil];
    }
#endif