Swift — Closures

Basic closure

let greet = { (name: String) -> String in
    return "Hello, \(name)!"
}
print(greet("Alice")) // Hello, Alice!

Trailing closure

let nums = [3, 1, 4, 1, 5, 9]

// Trailing closure
let sorted = nums.sorted { $0 < $1 }
print(sorted) // [1, 1, 3, 4, 5, 9]

// Multiple trailing closures
UIView.animate(withDuration: 0.3, animations: {
    view.alpha = 0
}, completion: { _ in
    view.removeFromSuperview()
})

Shorthand argument names

let nums = [1, 2, 3, 4, 5]

let doubled = nums.map { $0 * 2 }
print(doubled) // [2, 4, 6, 8, 10]

let sum = nums.reduce(0) { $0 + $1 }
print(sum) // 15

Capturing values

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

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

Closures as parameters

func perform(_ operation: (Int, Int) -> Int, on a: Int, _ b: Int) -> Int {
    operation(a, b)
}

print(perform(+, on: 3, 4))    // 7
print(perform(*, on: 3, 4))    // 12
print(perform(-, on: 10, 3))   // 7

@escaping closures

var completionHandlers: [() -> Void] = []

func fetchData(completion: @escaping () -> Void) {
    completionHandlers.append(completion)
    completion()
}

fetchData {
    print("Completed")
}

Mini Practice

Write Swift code that:

1. Creates a closure and passes it to a function
2. Uses trailing closure syntax
3. Demonstrates value capture in a closure
4. Uses @escaping for deferred execution

Up Next

In the next lesson, you'll learn about Arrays — working with collections in Swift.