Swift:设置协议(protocol)的可选属性

Question

如何设置协议(protocol)的可选属性？例如 UITextInputTraits 有许多可选的读/写属性。当我尝试以下操作时出现编译错误(无法分配给“textInputTraits”中的“keyboardType”):

func initializeTextInputTraits(textInputTraits: UITextInputTraits) {
  textInputTraits.keyboardType = .Default
}

通常在访问协议(protocol)的可选属性时，您会添加一个问号，但这在分配值时不起作用(错误:无法分配给该表达式的结果):

textInputTraits.keyboardType? = .Default

协议(protocol)如下:

protocol UITextInputTraits : NSObjectProtocol {
  optional var keyboardType: UIKeyboardType { get set }
}

Answer 1

这在 Swift 中是不可能的(现在？)。引用自 ADF thread :

Optional property requirements, and optional method requirements that return a value, will always return an optional value of the appropriate type when they are accessed or called, to reflect the fact that the optional requirement may not have been implemented.

So it's no surprise to get optional values easily. However, setting a property requires implementation to be guaranteed.