Swift中声明协议中的class关键字的作用

最近在Cocos2D编程for Swift中看到以下一个代码片段:

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!