Swift version: 5.6
One of the many safety features of Swift is its type safety, which means it must know what kind of data is being held by all values at all times. However, sometimes you know information that Swift doesn’t: you know that the
UIViewController you have a reference to is in fact your custom
PictureViewController subclass, and you want to treat it like one.
Typecasting is the process of passing on your extra knowledge to Swift – a way of saying “that thing you thought was an X is actually a Y”. Swift won’t let you typecast completely unrelated types, which means you can’t say things like “that thing you thought was a string is actually an integer”, but is instead mainly used for subclasses.
As an example, here’s how we create new view controllers from a storyboard:
let vc = storyboard?.instantiateViewController(withIdentifier: "Detail") as? DetailViewController
as? is a typecast - we’re passing on to Swift the knowledge that the view controller we expect to be created is in fact a
DetailViewController. If the typecast fails then
vc will be nil, but if it succeeds then Swift will be able to use it as a
SPONSORED Play is the first native iOS design tool created for designers and engineers. You can install Play for iOS and iPad today and sign up to check out the Beta of our macOS app with SwiftUI code export. We're also hiring engineers!
Available from iOS 8.0
This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.
Link copied to your pasteboard.