Head First Swift

Chapter 5. Closures: Fancy, Flexible Functions

Functions are useful, but sometimes you need more flexibility. Swift allows you to use a function as a type, just like an integer or string. This means you can create a function and assign it to a variable. Once it’s assigned to a variable, you can call the function using the variable, or pass the function to other functions as a parameter. When you create and use a function in this manner, it’s called a closure. Closures are useful because they can capture references to constants and variables from the context in which they are defined. This is called closing over a value, hence the name.

Meet the humble closure

This is a closure:

You can call pizzaCooked() just like a regular function:

At first glance, you might think a closure is just another way to write a function.

You’d be kind of correct, but closures allow you to do a lot of things that functions alone aren’t suited for. You’ll understand the power in a few pages.

Closures are like functions that you can pass around as variables. That’s the starting point to their usefulness, and it only goes up from there!

Closures are definitely a little bit confusing.

It’s normal ...