What is trailing closure syntax?

Swift version: 5.6

Paul Hudson    @twostraws    May 28th 2019

Trailing closure syntax is a little piece of syntactic sugar that makes particularly common code more pleasant to read and write. Many functions in iOS accept multiple parameters where the final parameter is a closure. For example, if you've done animation in iOS you'll be familiar with this method:

public class func animate(withDuration: TimeInterval, animations: () -> Void)

That accepts an animation duration as its first parameter, and a closure containing animation instructions as its second.

One way of calling this method is like this:

UIView.animate(withDuration: 1, animations: { [unowned self] in
    self.view.backgroundColor = UIColor.red
})

While that is perfectly valid Swift code, it's harder to read than it ought to be. If a closure is the last parameter to a method, as seen here, Swift allows you write your code like this instead:

UIView.animate(withDuration: 1) { [unowned self] in
    self.view.backgroundColor = UIColor.red
}

That's shorter, and avoids the double closing }) code.

This functionality is available wherever a closure is the final parameter to a function. For testing purposes, we could write a simple one like this:

func greetThenRunClosure(name: String, closure: () -> ()) {
    print("Hello, \(name)!")
    closure()
}

That prints a message, then runs a closure. Because the closure is the final parameter to the function, we can call it using trailing closure syntax like this:

greetThenRunClosure(name: "Paul") {
    print("The closure was run")
}

SAVE 50% To celebrate WWDC23, all our books and bundles are half price, so you can take your Swift knowledge further without spending big! Get the Swift Power Pack to build your iOS career faster, get the Swift Platform Pack to builds apps for macOS, watchOS, and beyond, or get the Swift Plus Pack to learn advanced design patterns, testing skills, and more.

Save 50% on all our books and bundles!

Available from iOS 7.0

Similar solutions…

• What’s the difference between leading, trailing, left, and right anchors?
• What is a closure?
• What is an escaping closure?
• How to write a closure that returns a value
• What’s the difference between a function and a closure?

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.