How to make optional protocol methods

Swift version: 5.1

Paul Hudson @twostraws May 28th 2019

By default, all methods listed in a Swift protocol must be implementing in a conforming type. However, there are two ways you can work around this restriction depending on your need.

The first option is to mark your protocol using the @objc attribute. While this means it can be adopted only by classes, it does mean you mark individual methods as being optional like this:

@objc protocol ObjcPrintable {
    @objc optional func canPrint() -> Bool
}

If possible, the second option is usually better: write default implementations of the optional methods that do nothing, like this:

protocol Printable {
    func canPrint() -> Bool
}

extension Printable {
    func canPrint() -> Bool {
        return true
    }
}

Remember, optional methods exist because you can provide sensible default behavior without them. In the above example it seems fair to make Printable things return true from canPrint() by default, because if someone wants to write an authentication layer for specific things they can implement their own version.