是什麼「其中自我」的協議擴展

Question

我看到這樣的例子很多下面的格式

是什麼協議擴展where Self

extension Protocolname where Self: UIViewController

什麼是where Self這裏指的是，找不到關於這個的文檔。

Answer 1

這語法是：https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Protocols.html#//apple_ref/doc/uid/TP40014097-CH25-ID521

考慮：

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"); 
    } 
} 

下面將給出一個編譯器錯誤，因爲的OBJ沒有獲得咩擴展功能。

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

但是，下面的工作。

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

換句話說，Meh.blah()只提供給那些UIViewController類型的類。

Answer 2

這裏是說明一個例子有什麼用，其中自我：UIViewController中

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 
    } 
}