如何將If Ifse陳述放在這裏？

Question

如何使用，如果在這裏else語句，我已經嘗試了一些，但它的錯誤是不能分配的值類型（） - >鍵入的UIViewController - > UIScrollView的

Answer 1

您正在尋找「固定 - 鑄造模型「： https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Patterns.html

if let viewController = controller as? PhotoStreamController { 
    return viewController.photoStream 
} else if let viewController = controller as? CollectionViewController { 
    return viewController.collectionView 
} else { 
    return nil 
} 

或

switch controller { 
case let viewController as PhotoStreamController: 
    return viewController.photoStream 
case let viewController as CollectionViewController: 
    return viewController.collectionView 
default: 
    return nil 
}