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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…")
    animations()
}

(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!

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