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What is the @objc attribute?

Written by Paul Hudson    @twostraws

By default Swift generates code that is only available to other Swift code, but if you need to interact with the Objective-C runtime – all of UIKit, for example – you need to tell Swift what to do.

That’s where the @objc attribute comes in: when you apply it to a class or method it instructs Swift to make those things available to Objective-C as well as Swift code. So, any time you want to call a method from a UIBarButtonItem or a Timer, you’ll need to mark that method using @objc so it’s exposed – both of those, and many others, are Objective-C code.

Don’t worry: if you forget to add @objc when it’s needed, your code simply won’t compile – it’s not something you can forget by accident and introduce a bug.

To expose a method to Objective-C, just write @objc before its name like this:

class MyController: UIViewController {
    @objc func authenticateUser() {

    }
}

That whole class is automatically Objective-C friendly because it inherits from UIViewController, but if you need it you can also explicitly make a class open to Objective-C by marking it @objc.

Available from iOS 8.0

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