Non-'@objc' method does not satisfy requirement of '@objc' protocol

Question

I've put the following into a Playground to try and understand this and I just don't:

import Foundation

@objc protocol Sample {

    var value: Int { get set }

    func increase()
    func decrese()

}

extension Sample {

    func increase() {
        value += 1
    }

    func decrease() {
        value -= 1
    }

}

class Test: Sample {

    var value: Int = 0

}

The error appears next to the class declaration for Test saying:

Non-'@objc' method 'increase()' does not satisfy requirement of '@objc' protocol 'Sample'

If I re-declare increase() and decrease() in the class then the warning is silenced. Or also if I remove the declarations from the protocol. Could someone please explain?

EDIT

I do need an Objective-C class to conform to this protocol as well, hence the @objc at the start.

Answer 1

The problem is that you're defining these methods in a protocol extension. This is used to define a “default implementation” for a protocol (ie if a type doesn't implement the method, the protocol's implementation will be called).

But Objective-C doesn't have the concept of default implementations for protocols. So it doesn't makes sense to declare the protocol as @objc and have default implementations within the Swift protocol extension. An Objective-C class conforming to this protocol would never be able to enjoy these Swift default implementations.