How to make optional protocol methods

Paul Hudson    February 12th 2018    @twostraws

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 Printable {
    @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.

Available from iOS 8.0 – learn more in my book Swift Design Patterns

Did this solution work for you? Please pass it on!

Tweet

Other people are reading…

• How to detect when the system is under pressure and you should reduce your work
• How to convert radians to degrees
• What’s the difference between a protocol and a class?
• What are the changes in Swift 2.2?
• How to create a custom debug description

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.