swift protocol 与类继承结合时的bug

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”

实现链条缺失时会使用protocol的缺省实现。不能路由到真正的实现。

https://stackoverflow.com/questions/31795158/swift-2-protocol-extension-not-calling-overridden-method-correctly

Unfortunately protocols don't have such an dynamic behavior (yet).

But you can do that (with the help of classes) by implementing commonBehavior() in the ParentClass and overriding it in the ChildClass. You also need CommonThing or another class to conform to CommonTrait which is then the superclass of ParentClass: