Closures as parameters

Because closures can be used just like strings and integers, you can pass them into functions. The syntax for this can hurt your brain at first, so we’re going to take it slow.

First, here’s our basic driving() closure again

let driving = {
    print("I'm driving in my car")
}

If we wanted to pass that closure into a function so it can be run inside that function, we would specify the parameter type as () -> Void. That means “accepts no parameters, and returns Void” – Swift’s way of saying “nothing”.

So, we can write a travel() function that accepts different kinds of traveling actions, and prints a message before and after:

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

We can now call that using our driving closure, like this:

travel(action: driving)

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!