Swift 2 协议(protocol)扩展没有正确调用覆盖的方法

Question

我在使用带有默认实现的 Swift 2 协议(protocol)扩展时遇到了一个问题。基本要点是我提供了一个协议(protocol)方法的默认实现，我在实现该协议(protocol)的类中重写了该方法。从基类调用该协议(protocol)扩展方法，然后基类调用我在派生类中重写的方法。结果是未调用覆盖的方法。

我试图将问题提炼到最小的可能的 Playground 上，这说明了下面的问题。

protocol CommonTrait: class {
    func commonBehavior() -> String
}

extension CommonTrait {
    func commonBehavior() -> String {
        return "from protocol extension"
    }
}

class CommonThing {
    func say() -> String {
        return "override this"
    }
}

class ParentClass: CommonThing, CommonTrait {
    override func say() -> String {
        return commonBehavior()
    }
}

class AnotherParentClass: CommonThing, CommonTrait {
    override func say() -> String {
        return commonBehavior()
    }
}

class ChildClass: ParentClass {
    override func say() -> String {
        return super.say()
        // it works if it calls `commonBehavior` here and not call `super.say()`, but I don't want to do that as there are things in the base class I don't want to have to duplicate here.
    }
    func commonBehavior() -> String {
        return "from child class"
    }
}

let child = ChildClass()
child.say() // want to see "from child class" but it's "from protocol extension”

Answer 1

不幸的是，协议(protocol)还没有这样的动态行为。

但您可以通过在 ParentClass 中实现 commonBehavior() 并在 ChildClass 中覆盖它来做到这一点(在类的帮助下) .您还需要 CommonThing 或另一个类来符合 CommonTrait ，它是 ParentClass 的父类(super class):

class CommonThing: CommonTrait {
    func say() -> String {
        return "override this"
    }
}

class ParentClass: CommonThing {
    func commonBehavior() -> String {
        // calling the protocol extension indirectly from the superclass
        return (self as CommonThing).commonBehavior()
    }

    override func say() -> String {
        // if called from ChildClass the overridden function gets called instead
        return commonBehavior()
    }
}

class AnotherParentClass: CommonThing {
    override func say() -> String {
        return commonBehavior()
    }
}

class ChildClass: ParentClass {
    override func say() -> String {
        return super.say()
    }

    // explicitly override the function
    override func commonBehavior() -> String {
        return "from child class"
    }
}
let parent = ParentClass()
parentClass.say()          // "from protocol extension"
let child = ChildClass()
child.say()                // "from child class"

由于这只是您问题的一个简短解决方案，我希望它适合您的项目。