What does the typealias keyword do?

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Main Source: 🔗 Ace the iOS Interview

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TL/DR

The typealias keyword allows us to define our own keyword to represent an existing data type in our application. This does not create a new type - it simply provides a new name for an existing type.

An excellent example is Swift's Codable protocol which is implemented as a typealias.

public typealias Codable = Decodable & Encodable

By the way, did you notice that you can combine protocols with & in Swift?

Now, whenever the compiler sees Codable,it will replace it with Decodable & Encodable and continue compilation.

Imagine we were trying to represent time on a clock. We could use typealias to make the tuple representing our clock easier to work with:

typealias ClockTime = (hours: Int, min: Int)

func drawHands(clockTime: ClockTime) {
print(clockTime.hours) // 3
print(clockTime.min) // 30
}

ClockTime( 3 , 30 )

If you wish to make the typealias accessible throughout your codebase, declare it outside of any class or enclosing type. Otherwise, the typealias will be limited in scope as any other variable would be.

// Accessible across the codebase
typealias ClockTime = (hours: Int, min: Int)

class HelloWorld {
// Only available within this class
typealias Greeting = String

func sayGreeting(greeting: Greeting) {}
}

It's easy to overuse typealias and thereby limit the discoverability of your code, so it’s important to exercise a little restraint.