Can Swift Method Defined on Extensions on Protocols Accessed in Objective-c - objective-c

Question

Is it possible to call methods defined in a protocol extension in Swift from Objective-C?
For example:
protocol Product {
var price:Int { get }
var priceString:String { get }
}
extension Product {
var priceString:String {
get {
return "$\(price)"
}
}
}
class IceCream : Product {
var price:Int {
get {
return 2
}
}
}
The price string of an instance of IceCream is '$2' and can be accessed in Swift, however the method is not visible in Objective-C. The compiler throws the error 'No visible #interface for 'IceCream' declares the selector ...'.
In my configuration, if the method is defined directly in the Swift object's implementation, everything works as expected. i.e.:
protocol Product {
var price:Int { get }
var priceString:String { get }
}
class IceCream : Product {
var price:Int {
get {
return 2
}
}
var priceString:String {
get {
return "$\(price)"
}
}
}

Answer 1

I am nearly certain the answer to this is "no", although I haven't found official Apple documentation that spells it out.
Here is a message from the swift-evolution mailing list discussing the proposal to use dynamic dispatch for all method calls, which would provide calling semantics more like Objective-C:
Again, the sole exception to this is protocol extensions. Unlike any other construct in the language, protocol extension methods are dispatched statically in a situation where a virtual dispatch would cause different results. No compiler error prevents this mismatch. (https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151207/001707.html)
Protocol extensions are a Swift-only language feature, and as such are not visible to objc_msgSend().

Answer 2

If it is ok to remove the priceString from the protocol, and only have it in your extension, you can call the protocol extension by casting IceCream to a Product in a helper extension.
#objc protocol Product {
var price:Int { get }
}
extension Product {
var priceString:String {
return "$\(price)"
}
}
// This is the trick
// Helper extension to be able to call protocol extension from obj-c
extension IceCream : Product {
var priceString:String {
return (self as Product).priceString
}
}
#objc class IceCream: NSObject {
var price: Int {
return 2
}
}

Answer 3

Protocol extension does not work with #objc protocol, however you can extend the class in swift as a workaround.
#objc protocol Product {
var price: NSNumber? { get }
var priceString:String { get }
}
...
// IceCream defined in Objective-C that does not extend Product
// but has #property(nonatomic, strong, nullable) (NSNumber*)price
// to make it confirm to Product
...
extension IceCream: Product {
var priceString:String {
get {
return "$\(price ?? "")"
}
}
}
This code is not clean at all, but it works.