Question

我看到了很多以下格式的例子

extension Protocolname where Self: UIViewController

协议扩展中的where Self是什么。我无法找到相关文档。

Answer 1

该语法为：https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Protocols.html#//apple_ref/doc/uid/TP40014097-CH25-ID521

考虑：

protocol Meh {
    func doSomething();
}

//Extend protocol Meh, where `Self` is of type `UIViewController`
//func blah() will only exist for classes that inherit `UIViewController`. 
//In fact, this entire extension only exists for `UIViewController` subclasses.

extension Meh where Self: UIViewController {
    func blah() {
        print("Blah");
    }

    func foo() {
        print("Foo");
    }
}

class Foo : UIViewController, Meh { //This compiles and since Foo is a `UIViewController` subclass, it has access to all of `Meh` extension functions and `Meh` itself. IE: `doSomething, blah, foo`.
    func doSomething() {
        print("Do Something");
    }
}

class Obj : NSObject, Meh { //While this compiles, it won't have access to any of `Meh` extension functions. It only has access to `Meh.doSomething()`.
    func doSomething() {
        print("Do Something");
    }
}

以下将给出编译器错误，因为Obj无法访问Meh扩展函数。

let i = Obj();
i.blah();

但是下面的内容会有效。

let j = Foo();
j.blah();

换句话说，Meh.blah()仅适用于UIViewController类型的类。

Answer 2

这是一个例子，解释了self：UIViewController

的用法

protocol SBIdentifiable {
    static var sbIdentifier: String { get }
}

extension SBIdentifiable where Self: UIViewController {
    static var sbIdentifier: String {
        return String(describing: self)
    }
}

extension UIVieWcontroller: SBIdentifiable { }

class ViewController: UIViewController {
  func loadView() {
  /*Below line we are using the sbIdentifier which will return the 
   ViewController class name.
    and same name we would mentioned inside ViewController 
    storyboard ID. So that we do not need to write the identifier everytime. 
   So here where Self: UIViewController means it will only conform the protocol of type UIViewController*/ 

  let viewController = self.instantiateViewController(withIdentifier: 
    self.sbIdentifier) as? SomeBiewController
  }
}

Answer 3

您可以在此处找到相同的示例：WWDC2015-408，（强烈建议观看，它说明了原因）

另外，另一个类似的示例是带有通用Where子句的扩展

struct Stack<Element> {
    var items = [Element]()
    mutating func push(_ item: Element) {
        items.append(item)
    }
    mutating func pop() -> Element {
        return items.removeLast()
    }
}

where子句为扩展名添加了一个要求，以便仅当堆栈中的项目是相等的时，扩展名才添加isTop（_ :)方法。

extension Stack where Element: Equatable {
    func isTop(_ item: Element) -> Bool {
        guard let topItem = items.last else {
            return false
        }
        return topItem == item
    }
}

什么是自我＆＃39;在协议扩展中

3 个答案: