Swift中声明协议中的class关键字的作用

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protocol GameMode:class{
    var userInterface:CCNode! {get}

    func gameplay(mainScene:MainScene,droppedFallingObject:FallingObject)
    func gameplay(mainScene:MainScene,caughtFallingObject:FallingObject)
    func gameplayStep(mainScene:MainScene,delta:CCTime)->GameOver
}

注意在声明协议名后直接跟了一个class关键字,这是神马意思?

查阅Swift编程手册得知协议中的class关键字用来限制该协议只能应用在类上,原话是这么说的:

“You can limit protocol adoption to class types (and not
 structures or enumerations) by adding the class keyword
 must always appear first in a protocol’s inheritance
 list, before any inherited protocols”

所以如果你无所谓这个限制,你大可不必加class关键字,没有什么区别!