Capturing More Than `self`

A common pattern when using closures in Swift is to add [weak self] in the captures list to hold a weak reference to self and avoid a retain cycle. This is then often followed by the following:

guard let self = self else { return }

But I often forget that capture lists can capture other variables in the current scope, so I thought I'd highlight some other use cases.

When creating a closure in mutating function of a struct capturing self is not possible:

struct Foo {
    var bar: Bool
    mutating func createClosure() -> () -> Bool {
        return { // Error: Escaping closure captures mutating 'self' parameter
            return self.bar
        }
    }
}
var foo = Foo(bar: true)
let closure = foo.createClosure()
closure()

To work around this you can capture the property:

struct Foo {
    var bar: Bool
    mutating func createClosure() -> () -> Bool {
        return { [bar] in
            return bar
        }
    }
}
var foo = Foo(bar: true)
let closure = foo.createClosure()
closure() // true

Another use case is capturing a variable a time of closure creation, when it can also be useful to rename the variable:

class Foo {
    var bar: Bool
    init(bar: Bool) { self.bar = bar }
    func doSomething() {
        let closure = { [weak self, originalBar = bar] in
            guard let self = self else { return }
            if originalBar != self.bar {
                print("Bar has changed")
            } else {
                print("Bar did not change")
            }
        }
        bar.toggle()
        closure()
    }
}
let foo = Foo(bar: true)
foo.doSomething()

The above code will print Bar has changed.

To read more in-depth information about closures read the Swift language guide page on closures.