Closure basics

Closures are so named because they have the ability to “close over” the variables and constants within the closure’s own scope. This simply means that a closure can access, store and manipulate the value of any variable or constant from the surrounding context. Variables and constants within the body of a closure are said to have been captured by the closure.

You may ask, “If closures are functions without names, then how do you use them?” To use a closure, you first have to assign it to a variable or constant.

Here’s a declaration of a variable that can hold a closure:

var multiplyClosure: (Int, Int) -> Int

multiplyClosure takes two Int values and returns an Int. Notice that this is exactly the same as a variable declaration for a function. Like I said, a closure is simply a function without a name. The type of a closure is a function type.

You assign a closure to a variable like so:

multiplyClosure = { (a: Int, b: Int) -> Int in return a * b

}

This looks similar to a function declaration, but there’s a subtle difference. There’s the same parameter list, -> symbol and return type. But in the case of closures, these elements appear inside braces, and there is an in keyword after the return type.

With your closure variable defined, you can use it just as if it were a function, like so:

let result = multiplyClosure(4, 2)

As you’d expect, result equals 8.

Notice how the closure has no external names for the parameters And you can’t even set them like you can with functions.