Swift 协议作为 Class 的通用参数

Question

我正在尝试创建一个将协议作为通用参数的 class (ProtocolPrinter)。为什么不能编译？

import Foundation

@objc protocol MyProtocol {
    func foo()
}

class MyConformingClass: MyProtocol {
    func foo() {
        print("Foo!")
    }
}

class ProtocolPrinter<T: Protocol> {
    func printT() {
        print("T: \(T.self)")
    }

    func dosomethingWithObject(_ object: T) {
        if let object = object as? MyProtocol {
            object.foo()
        } else {
            print("I don't know what object this is: \(object).")
        }
    }
}

let x = MyConformingClass()
x.foo() // Foo!

let myProtocolMeta: Protocol = MyProtocol.self // No error.

ProtocolPrinter<MyProtocol>()                        // 'ProtocolPrinter' requires that 'MyProtocol' inherit from 'Protocol'
ProtocolPrinter<MyProtocol.self>()                   // (treats "<" and ">" as operators)
ProtocolPrinter<MyProtocol.Type>()                   // 'ProtocolPrinter' requires that 'MyProtocol.Type' inherit from 'Protocol'
ProtocolPrinter<MyProtocol.Type.self>()              // (treats "<" and ">" as operators)
ProtocolPrinter<MyProtocol.Protocol>()               // type 'MyProtocol.Protocol' does not confor
ProtocolPrinter<MyProtocol.Protocol.self>()          // (treats "<" and ">" as operators)
ProtocolPrinter<MyProtocol.Protocol.Type>()          // type 'MyProtocol.Protocol.Type' does not conform to protocol 'MyProtocol'
ProtocolPrinter<MyProtocol.Protocol.Type.self>()     // (treats "<" and ">" as operators)
ProtocolPrinter<MyProtocol.Protocol.Protocol>()      // cannot use 'Protocol' with non-protocol type 'MyProtocol.Protocol'
ProtocolPrinter<MyProtocol.Protocol.Protocol.Type>() // cannot use 'Protocol' with non-protocol type 'MyProtocol.Protocol'

Answer 1

事实证明，您只需要指定一个 @objc protocol，您的所有其他协议（也必须是 @objc protocols）都符合，例如：

import Foundation

@objc protocol SuperProtocol {}
@objc protocol MyProtocol: SuperProtocol {
    func foo()
}

class MyConformingClass: MyProtocol {
    func foo() {
        print("Foo!")
    }
}

class ProtocolPrinter<T: SuperProtocol> {
    func printT() {
        print("T: \(T.self)")
    }

    func dosomethingWithObject(_ object: T) {
        if let object = object as? MyProtocol {
            object.foo()
        } else {
            print("I don't know what object this is: \(object).")
        }
    }
}

let x = MyConformingClass()
x.foo() // Foo!
MyProtocol.Protocol.self
let myProtocolMeta: Protocol = MyProtocol.self

ProtocolPrinter<MyProtocol>().dosomethingWithObject(MyConformingClass()) // Foo!

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