Article

Handling Dynamically Typed Methods and Objects in Swift

Cast instances of the Objective-C id type to a specific Swift type.

Overview

In Objective-C, the id type represents objects that are instances of any Objective-C class. The id type is instead imported by Swift as the Any type. When you pass a Swift instance to an Objective-C API, it's bridged as an id parameter so that it's usable in the API as an Objective-C object. When id values are imported into Swift as Any, the runtime automatically handles bridging back to either class references or value types.

var x: Any = "hello" as String
x as? String   // String with value "hello"
x as? NSString // NSString with value "hello"
 
x = "goodbye" as NSString
x as? String   // String with value "goodbye"
x as? NSString // NSString with value "goodbye"

Downcast Objects to Call Methods and Access Properties

When you work with objects of type Any where you know the underlying type, it's often useful to downcast those objects to the underlying type. However, because the Any type can refer to any type, a downcast to a more specific type isn't guaranteed by the compiler to succeed.

You can use the conditional type cast operator (as?), which returns an optional value of the type you are trying to downcast to:

let userDefaults = UserDefaults.standard
let lastRefreshDate = userDefaults.object(forKey: "LastRefreshDate") // lastRefreshDate is of type Any?
if let date = lastRefreshDate as? Date {
    print("\(date.timeIntervalSinceReferenceDate)")
}

If you're completely certain about the type of the object, you can use the forced downcast operator (as!) instead.

let myDate = lastRefreshDate as! Date
let timeInterval = myDate.timeIntervalSinceReferenceDate

However, if a forced downcast fails, a runtime error is triggered:

let myDate = lastRefreshDate as! String // Error

See Also

Language Interoperability

Objective-C and C Code Customization

Apply macros to your Objective-C APIs to customize how they're imported into Swift.

Migrating Your Objective-C Code to Swift

Learn the recommended steps to migrate your code.

Cocoa Design Patterns

Adopt and interoperate with Cocoa design patterns in your Swift apps.

Using Objective-C Runtime Features in Swift

Use selectors and key paths to interact with dynamic Objective-C APIs.

Imported C and Objective-C APIs

Use native Swift syntax to interoperate with types and functions in C and Objective-C.