What does the open keyword do?

Swift version: 5.10

Paul Hudson @twostraws May 28th 2019

When working with code from another module – e.g., UIKit or a module you wrote separate from your main app – Swift differentiates between public accessibility and public overridability. That is, someone can be public for folks to use, but not public for them to extend.

Here’s an example to demonstrate the difference:

open class User {
    open func login() { }
    public func playGame() { }
    public init() { }
}

If that were defined in its own module, any other code accessing it would be able to inherit from the User class because it’s marked open. Inside the child class, they could override login() because it’s also marked open, but they could not override playGame() because it’s marked only as public – it can be called, but not changed. If you remove open from the whole User class it can be used but not subclassed.

The open keyword is an effective way of stopping other developers from accidentally overriding functionality that’s critical to the way your app works. If you use it selectively, subclassers can add their own functionality or perhaps replace a few non-critical components, without altering the fundamental behavior of your class.