我在Swift操场上有以下示例,试图在Swift中实现一个副本构造函数:
class Shape : NSObject { var color : String override init() { color = "Red" } init(copyFrom: Shape) { color = copyFrom.color } } class Square : Shape { var length : Double override init() { super.init() length = 10.0 } init(copyFrom: Square) { /* Compilation error here! */ super.init(copyFrom: copyFrom) length = copyFrom.length } } let s : Square = Square() // {{color "Red"} length 10.0} let copy = Square(copyFrom: s) // {{color "Red"} length 10.0} s.color = "Blue" // {{color "Blue"} length 10.0} s // {{color "Blue"} length 10.0} copy // {{color "Red"} length 10.0}
问题在于这实际上并不能以当前形式进行编译。在子类中的init(copyFrom: Square) 方法上Square,报告此错误:
init(copyFrom: Square)
Square
Overriding method with selector 'initWithCopyFrom:' has incompatible type '(Square) -> Square'
如果它不是 构造函数 ,就好比是常规的,那么这个问题 就很有意义 了,您可以传递超类中期望的类型,但在子类中已重写该类型以限制更多: __func
func
let mySquare : Shape = Square() // Note the var is a SHAPE mySquare.someShapeMethod("Test") // If Square overrides someShapeMethod() to expect Int, compiler errors out to protect us here.
但是事实是它是 构造函数 ,这使我相信我应该能够重写它并提供不同的方法签名,因为在编译时绝对知道对象的类型是什么。
如果我更改Shape为不再扩展,此问题将消失NSObject。但是,由于包含在现有的Objective- C代码中,因此需要对其进行扩展NSObject。
Shape
NSObject
如何更新我的复制构造函数,以允许a Shape知道从a复制Shape,而允许a Square知道从a复制Square?
init(copyFrom: Square)是的重载,而不是覆盖init(copyFrom: Shape)。我的意思是,它们是不相关的方法,因为它们接受不同的类型。在Swift中这是可以接受的。在ObjC中,这是非法的。ObjC中没有重载。
init(copyFrom: Shape)
Swift初始化器不会自动继承。因此,在Swift中,您不能尝试将random复制Shape为Square。初始化器不可用。但是在ObjC中,初始化器 会 自动继承(您不能阻止它们这样做)。因此,如果您有一个方法initWithCopyFrom:(*Shape),则要求每个子类都愿意接受它。这意味着您可以(在ObjC中)尝试创建一个Circle作为Square的副本。那当然是胡说八道。
initWithCopyFrom:(*Shape)
如果这是NSObject子类,则应使用NSCopying。这是您的处理方式:
NSCopying
import Foundation class Shape : NSObject, NSCopying { // <== Note NSCopying var color : String required override init() { // <== Need "required" because we need to call dynamicType() below color = "Red" } func copyWithZone(zone: NSZone) -> AnyObject { // <== NSCopying // *** Construct "one of my current class". This is why init() is a required initializer let theCopy = self.dynamicType() theCopy.color = self.color return theCopy } } class Square : Shape { var length : Double required init() { length = 10.0 super.init() } override func copyWithZone(zone: NSZone) -> AnyObject { // <== NSCopying let theCopy = super.copyWithZone(zone) as Square // <== Need casting since it returns AnyObject theCopy.length = self.length return theCopy } } let s = Square() // {{color "Red"} length 10.0} let copy = s.copy() as Square // {{color "Red"} length 10.0} // <== copy() requires a cast s.color = "Blue" // {{color "Blue"} length 10.0} s // {{color "Blue"} length 10.0} copy // {{color "Red"}
迅捷3
class Shape: NSObject, NSCopying { required override init() { super.init() } func copy(with zone: NSZone? = nil) -> Any { let copy = type(of: self).init() return copy } } class Square: Shape { required override init() { super.init() } func copy(with zone: NSZone? = nil) -> Any { let copy = super.copy(with: zone) as! Square copy.foo = self.foo ...... return copy } }