NEW: Learn to build amazing SwiftUI apps for macOS with my new book! >>

Why can’t we call async functions using async var?

Paul Hudson    @twostraws   

Updated for Xcode 13.3

Swift’s async let syntax provides short, helpful syntax for running lots of work concurrently, allowing us to wait for them all later on. However, it only works as async let – it’s not possible to use async var.

If you think about it, this restriction makes sense. Consider pseudocode like this:

func fetchUsername() async -> String {
    // complex networking here
    "Taylor Swift"
}

async var username = fetchUsername()
username = "Justin Bieber"
print("Username is \(username)")

That attempts to create a variable asynchronously, then writes to it directly. Have we cancelled the async work? If not, when the async work completes will it overwrite our new value? Do we still need to use await when reading the value even after we’ve explicitly set it?

This kind of code would create all sorts of confusion, so it’s just not allowed – async let is our only option.

Hacking with Swift is sponsored by RevenueCat

SPONSORED Spend less time managing in-app purchase infrastructure so you can focus on building your app. RevenueCat gives everything you need to easily implement, manage, and analyze in-app purchases and subscriptions without managing servers or writing backend code.

Get Started

Sponsor Hacking with Swift and reach the world's largest Swift community!

Similar solutions…

BUY OUR BOOKS
Buy Pro Swift Buy Swift Design Patterns Buy Testing Swift Buy Hacking with iOS Buy Swift Coding Challenges Buy Swift on Sundays Volume One Buy Server-Side Swift (Vapor Edition) Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Advanced iOS Volume Three Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with macOS Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Server-Side Swift (Kitura Edition) Buy Beyond Code

Was this page useful? Let us know!

 
Unknown user

You are not logged in

Log in or create account
 

Link copied to your pasteboard.