Can I extend a final class in Swift?

Question

I'm using a third-party library for a new app that I'm making using Swift. The author of the class/library has made it final using the final keyword, probably to optimise and to prevent overriding its properties and methods.

Example:

final public class ExampleClass {
   // Properties and Methods here
}

Is it possible for me extend the class and add some new properties and methods to it without overriding the defaults?

Like so:

extension ExampleClass {
    // New Properties and Methods inside
}

Willodeanwilloughby

Answer 1

An extension may not contain stored properties but you can add methods inside.

Lp

Answer 2

Extensions (like Objective-C categories) don't allow stored properties.
Methods and computed properties are fine though.

A common (but IMO hacky) workaround in Objective-C was to use associated objects to gain storage within categories. This also works in Swift if you import ObjectiveC.
This answer contains some details.

Alula

Let's remind that it only works for associating NSObject subclasses. – Superclass

Answer 3

Yes, you can extend a final class. That extension has to follow the usual rules for extensions, otherwise it's nothing special.

Flamenco

Answer 4

While you cannot create new stored properties in extensions you can add methods and computed properties.

Example computed property:

extension ExampleClass { 

  // computed properties do not have a setter, only get access
  var asInt: Int? { 
    Int(aStringPropertyOnTheClass) 
  }
}

Lodged