Typealias

What is a Typealias?

• Adds a named alias for an existing type.
• Does not create a new type, it just refers to the existing type.
• You can use this named alias instead / as well as of the existing type name.

Grammar

• “I created a typealias definition for…”.

Creating a Typealias

• A typealias takes the form:

    typealias name = existing type

Using a Typealias

• Typealias’ are useful to rename a type that may be complex to read each time, or you want to redefine the name of.
• The existing type remains the same, this is just an alias for the existing type.

Example: Typealias String and for use as a Key

typealias Key = String

let analyticsKey: Key

Example: Typealias Int and use as an Identity

typealias Identity = Int

let identity: Identity = 0

Example: Typealias a completion handler to make the completion handler parameter types more readable

typealias ImageLoadingCompletedHandler = (() -> Void)

func loadImage(imageUrl: URL?, onCompleted: ImageLoadingCompletedHandler? = nil) {
...

Example: Typealias multiple protocols so that the type conforms to the typealias

typealias ButtonElement = Equatable & CustomStringConvertible

final class ButtonViewModel<Element: ButtonElement>  {
...

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