xcode 如何在 Swift 中对结构进行子类化？
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How do I subclass a struct in Swift?
提问by Oleksandr
struct MyRect : CGRect {...}
Is it possible at all in swift to subclass a Structure?
是否有可能迅速对结构进行子类化？
I have already found out on Apple official website an example: This example extends the CGRect structure to contain a computed area property:
我已经在苹果官网上找到了一个例子：这个例子扩展了 CGRect 结构以包含一个计算区域属性：
extension CGRect {
var area: CGFloat {
return width * height
}
}
let rect = CGRect(x: 0.0, y: 0.0, width: 10.0, height: 50.0)
let area = rect.area
How can i define a subclass of a structure ?
如何定义结构的子类？
回答by Sam Clewlow
It is not possible to subclass a structin Swift, only classes can be subclassed. An extensionis not a subclass, it's just adding additional functionality on to the existing struct, this is comparable to a category in Objective-C.
struct在 Swift 中无法对 a 进行子类化，只能对类进行子类化。Anextension不是子类，它只是在现有的 上添加附加功能struct，这与 Objective-C 中的类别相当。
Please see the Apple Swift documentation hereto read about the differences between structand class.
请参阅这里的苹果斯威夫特文档阅读有关之间的差异struct和class。
回答by James Hays
This is a bit old, but I ran across it and thought I'd post a common pattern I use when needed. For example, an Application I was working on had the need to do some work with AspectRatios. Well, in my mind, an AspectRatio is nothing for than a CGSize with some additional functionality. I wanted a little separation between an AspectRatio and a CGSize, so I created a TypeAlias and then created an extension of the TypeAlias.
这有点旧，但我遇到了它，并认为我会发布一个在需要时使用的通用模式。例如，我正在处理的一个应用程序需要使用 AspectRatios 做一些工作。好吧，在我看来，AspectRatio 只不过是具有一些附加功能的 CGSize。我想在 AspectRatio 和 CGSize 之间做一点分离，所以我创建了一个 TypeAlias，然后创建了一个 TypeAlias 的扩展。
Something like this:
像这样的东西：
public typealias AspectRatio = CGSize
public extension AspectRatio {
enum Orientation {
case landscape
case portrait
case square
}
static let square = AspectRatio(width: 1.0, height: 1.0)
static let standard = AspectRatio(width: 4.0, height: 3.0)
static let wideScreen = AspectRatio(width: 16.0, height: 9.0)
static let theater = AspectRatio(width: 21.0, height: 9.0)
static let imax = AspectRatio(width: 1.9, height: 1.0)
func aspectRatio(precision: Int? = nil) -> CGFloat {
let aspectRatio = width / height
guard let precision = precision else {
return aspectRatio
}
let multiplier = pow(10, Double(precision))
let value = (Double(aspectRatio) * multiplier).rounded() / multiplier
return CGFloat(value)
}
var orientation: Orientation {
switch self.aspectRatio() {
case let aspect where aspect > 1.0:
return .landscape
case let aspect where aspect < 1.0:
return .portrait
default:
return .square
}
}
func flipped() -> AspectRatio {
return AspectRatio(width: height, height: width)
}
...
}
Then I could easily define variables like such:
然后我可以轻松地定义这样的变量：
var aspectRatio: AspectRatio = .imax
回答by acecilia
Since Swift 5.1 it is now possible to do something similar to what you asked for: using composition, with KeyPathand dynamicMemberLookup. Have a look: https://medium.com/@acecilia/struct-composition-using-keypath-and-dynamicmemberlookup-kind-of-struct-subclassing-but-better-6ce561768612?source=friends_link&sk=da479578032c0b46c1c69111dfb6054e
从 Swift 5.1 开始，现在可以执行类似于您要求的操作：使用组合、使用KeyPath和dynamicMemberLookup。看看：https: //medium.com/@acecilia/struct-composition-using-keypath-and-dynamicmemberlookup-kind-of-struct-subclassing-but-better-6ce561768612?source=friends_link&sk=da479578032c0b46c1c69114edfb605
