Capturing Values

// CAPTURING VALUES
// Closures can capture and store references to variables
// from the surrounding context where they're defined

// BASIC EXAMPLE
func makeCounter() -> () -> Int {
    var count = 0  // This variable is "captured"

    let counter = {
        count += 1
        return count
    }

    return counter
}

let counter1 = makeCounter()
print(counter1())  // 1
print(counter1())  // 2
print(counter1())  // 3

// Each counter has its OWN captured count!
let counter2 = makeCounter()
print(counter2())  // 1 (new counter, new count)
print(counter1())  // 4 (original counter continues)

// CAPTURING MULTIPLE VALUES
func makeMultiplier(factor: Int) -> (Int) -> Int {
    // 'factor' is captured
    return { number in
        number * factor
    }
}

let double = makeMultiplier(factor: 2)
let triple = makeMultiplier(factor: 3)

print(double(5))  // 10
print(triple(5))  // 15

// CAPTURING BY REFERENCE
var total = 0

let addToTotal = { (amount: Int) in
    total += amount  // Captures 'total' by reference
}

addToTotal(10)
addToTotal(5)
print(total)  // 15 (original variable is modified!)

// CAPTURE LIST - Capture by value
var value = 10

let captureByReference = { print(value) }
let captureByValue = { [value] in print(value) }

value = 20

captureByReference()  // 20 (sees the change)
captureByValue()      // 10 (captured original value)

// PRACTICAL EXAMPLE: Button actions
func createButton(title: String) -> () -> Void {
    // title is captured
    return {
        print("Button '\(title)' was pressed!")
    }
}

let loginAction = createButton(title: "Login")
let signupAction = createButton(title: "Sign Up")

loginAction()   // Button 'Login' was pressed!
signupAction()  // Button 'Sign Up' was pressed!

// CLOSURE CAPTURES ARE STRONG BY DEFAULT
// This can cause memory issues (retain cycles)
// We'll learn about [weak self] later!