for i in stride(from: 0, to: 5, by: 1){
   print("i:\(i)")
}
//결과는 아래와 같습니다.
i:0
i:1
i:2
i:3
i:4

for x in stride(from: 5, to: 0, by: -1){
  print("x:\(x)")
}
//결과는 아래와 같습니다. 거꾸로 진행됩니다.
x:5
x:4
x:3
x:2
x:1

extension LensViewController: UIScrollViewDelegate {
    func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
  	let bounds = lensCollectionView.bounds
  	let x = bounds.origin.x + bounds.width / 2 // 우측으로 swip하면 bounds.origin.x 값이 계속 커짐.
    print("bounds x: \(bounds)")
    let y = bounds.height / 2
    let point = CGPoint(x: x, y: y)
    guard let idx = lensCollectionView.indexPathForItem(at: point) else {return}
	
    //snapping 동작을 위해서 정확히 가운데로 이동하게 함.
    lensCollectionView.scrollToItem(at: idx , at: .centeredHorizontally , animated: true )
  }
}

import  UserNotifications


func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

    UIApplication.shared.setMinimumBackgroundFetchInterval(UIApplication.backgroundFetchIntervalMinimum)
    if #available(iOS 10.0 , *){ //10이후에만 작동하므로.
        UNUserNotificationCenter.current().requestAuthorization(options: .badge) { (granted, error) in
            if error != nil {
                // success!
            }
        }
    }
    
    return true
}

func application(_ application: UIApplication, performFetchWithCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
    
    print("haha 실행됨.")
    UIApplication.shared.applicationIconBadgeNumber = 20
   //completionHandler(.newData)
   //completionHandler(.noData)
}