NEW! Master Swift design patterns with my latest book! >>

How to make optional protocol methods

Paul Hudson       @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!

Other people are reading…

About the Swift Knowledge Base

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

Learn Swift faster!

Take your Swift learning to the next level: buy the Hacking with Swift e-book and get bonus material to help you learn faster!

MASTER SWIFT NOW
Buy Practical iOS 12 Buy Pro Swift Buy Swift Design Patterns Buy Practical iOS 11 Buy Swift Coding Challenges Buy Server-Side Swift (Vapor Edition) Buy Server-Side Swift (Kitura Edition) Buy Hacking with macOS Buy Advanced iOS Volume One Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with Swift Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Beyond Code

Was this page useful? Let me know!

Click here to visit the Hacking with Swift store >>