我发现了一个有趣的行为，似乎是一个错误…

基于行为描述了以下文章：

https://medium.com/ios-os-x-development/swift-protocol-extension-method-
dispatch-6a6bf270ba94

http://nomothetis.svbtle.com/the-ghost-of-swift-bugs-
future

当添加时SomeSuperclass，输出不是我期望的，而不是直接采用协议。

protocol TheProtocol {
    func method1()
}

extension TheProtocol {
    func method1() {
        print("Called method1 from protocol extension")
    }
    func method2NotInProtocol() {
        print("Called method2NotInProtocol from protocol extension")
    }
}

// This is the difference - adding a superclass
class SomeSuperclass: TheProtocol {
}

// It works as expected when it simply adopts TheProtocol, but not when it inherits from a class that adopts the protocol
class MyClass: SomeSuperclass {
    func method1() {
        print("Called method1 from MyClass implementation")
    }
    func method2NotInProtocol() {
        print("Called method2NotInProtocol from MyClass implementation")
    }
}

let foo: TheProtocol = MyClass()
foo.method1()  // expect "Called method1 from MyClass implementation", got "Called method1 from protocol extension"
foo.method2NotInProtocol()  // Called method2NotInProtocol from protocol extension

您知道这是错误还是设计使然？一位同事建议混合继承和协议扩展可能无法按预期工作。我打算使用协议扩展来提供默认的实现…如果我做不到，那么我将不得不标记它@objc并返回到可选协议。