NEW: Subscribe to Hacking with Swift+ and accelerate your learning! >>

Why does Swift have trailing closure syntax?

Paul Hudson    @twostraws   

Updated for Xcode 12.0

Trailing closure syntax is designed to make Swift code easier to read, although some prefer to avoid it.

Let’s start with a simple example first. Here’s a function that accepts a Double then a closure full of changes to make:

func animate(duration: Double, animations: () -> Void) {
    print("Starting a \(duration) second animation…")

(In case you were wondering, that’s a simplified version of a real and very common UIKit function!)

We can call that function without a trailing closure like this:

animate(duration: 3, animations: {
    print("Fade out the image")

That’s very common. Many people don’t use trailing closures, and that’s OK. But many more Swift developers look at the }) at the end and wince a little – it isn’t pleasant.

Trailing closures allow us to clean that up, while also removing the animations parameter label. That same function call becomes this:

animate(duration: 3) {
    print("Fade out the image")

Trailing closures work best when their meaning is directly attached to the name of the function – you can see what the closure is doing because the function is called animate().

If you’re not sure whether to use trailing closures or not, my advice is to start using them everywhere. Once you’ve given them a month or two you’ll have enough usage to look back and decide more clearly, but hopefully you get used to them because they are really common in Swift!

Hacking with Swift is sponsored by Fernando Olivares

SPONSORED Ever ask for help and your reviewer immediately notices issues you missed? Fernando Olivares is a 10-year veteran from Big Nerd Ranch and Lambda School who can help hone your app building skills, and he's just launched a new book that does just that – use the code "hacking" to save $5!

Buy the book

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

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!

Average rating: 3.9/5

Link copied to your pasteboard.