类符合协议的隐藏逻辑

Question

Are there any ways to hide that class conforms to some protocol? Like in Objective-C - just used to add Protocol in .m file and other classes (from another files) didn't see it.

For example. I have a test cell which has a textfield. I want to hide, that this cell conforms to protocol. Something like that:

class TestCell: UITableViewCell {

}

fileprivate extension TestCell : UITextFieldDelegate {

}

But compiler swears me. Any elegant solution?

Answer 1

This capability has been stated by the Swift team as "unlikely" to be implemented. Here is the original thread about it: https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160229/011666.html

The specific statement about this particular scenario was:

Private conformances

Right now, a protocol conformance can be no less visible than the minimum of the conforming type's access and the protocol's access. Therefore, a public type conforming to a public protocol must provide the conformance publicly. One could imagine removing that restriction, so that one could introduce a private conformance:

 public protocol P { } public struct X { } extension X : internal P { … } // X conforms to P, but only within this module 

The main problem with private conformances is the interaction with dynamic casting. If I have this code:

 func foo(value: Any) { if let x = value as? P { print(“P”) } } foo(X()) 

Under what circumstances should it print “P”? If foo() is defined within the same module as the conformance of X to P? If the call is defined within the same module as the conformance of X to P? Never? Either of the first two answers requires significant complications in the dynamic casting infrastructure to take into account the module in which a particular dynamic cast occurred (the first option) or where an existential was formed (the second option), while the third answer breaks the link between the static and dynamic type systems—none of which is an acceptable result.