ios swift OperationQueue mvc ActivityViewController collectionviewflowlayout collectionviewdelegate collectionviewdatasource CollectionViewCell jsondecoder jsoncoder Human Interface Guideline CollectionView URLSessionDataTask target-action stackview Codable urlSession dispatchqueue actionsheet NotificationCenter segue Auto Layout Tabbar ScrollView DatePicker UIKit HIG 동시성 modal 재사용 ats 화면전환 gesture DataSource GCD 공유하기 operation delegate TextField Decoding foundation alert target cocoa stack date 내보내기 encoding Singleton Concurrency 계층 tableView presentation Queue view Photos Notification Navigation Action