#57: Capturing Values in Swift Closures

Closures can capture variables from the surrounding scope and keep using them later.

func makeCounter() -> () -> Int {
    var value = 0
    return {
        value += 1
        return value
    }
}

let counter = makeCounter()
counter() // 1
counter() // 2

Here the closure keeps value alive even after makeCounter() has returned.

This behavior is incredibly useful, but it is also why closures can keep objects alive longer than expected.