如何使Codable类对所有属性使用decodeIfPresent而无需在自定义初始化程序中键入所有属性？一个例子:classBook:Codable{varname:String="Defaultname"varpages:Int=1requiredinit(fromdecoder:Decoder)throws{letcontainer=trydecoder.container(keyedBy:CodingKeys.self)forkeyincontainer.allKeys{//WhattodoheretocalldecodeIfPresent?}}}本质上，我想自己复制自动创建的

我第一次使用Swift4的Codable协议(protocol)，我无法理解Decodable的decodeIfPresent的使用。///Decodesavalueofthegiventypeforthegivenkey,ifpresent.//////Thismethodreturns`nil`ifthecontainerdoesnothaveavalueassociatedwith`key`,orifthevalueisnull.Thedifferencebetweenthesestatescanbedistinguishedwitha`contains(_:)`call.///