Using closures as parameters when they accept parameters

This is where closures can start to be read a bit like line noise: a closure you pass into a function can also accept its own parameters.

We’ve been using () -> Void to mean “accepts no parameters and returns nothing”, but you can go ahead and fill the () with the types of any parameters that your closure should accept.

To demonstrate this, we can write a travel() function that accepts a closure as its only parameter, and that closure in turn accepts a string:

func travel(action: (String) -> Void) {
    print("I'm getting ready to go.")
    action("London")
    print("I arrived!")
}

Now when we call travel() using trailing closure syntax, our closure code is required to accept a string:

travel { (place: String) in
    print("I'm going to \(place) in my car")
}

SPONSORED With Sentry’s error and performance monitoring for iOS, you see mobile vitals that actually matter, can solve any latency issues quickly, and learn how each release is performing over time.

Learn More

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