Escaping и нез escaping-замыкания, автозамыкания

Обзор

Не сохраняется по умолчанию

// Non-escaping by default: used immediately inside the function.
func applyTwice(_ f: (Int) -> Int, to x: Int) -> Int {
    return f(f(x))   // called synchronously; not stored
}
let doubled = applyTwice({ $0 * 2 }, to: 3)
print(doubled)  // 12

Сохраняющийся closure

// Need @escaping when storing for later use:
var tasks: [() -> Void] = []

func addTask(_ work: @escaping () -> Void) {
    tasks.append(work)  // stored to run later
}

addTask { print("run later") }
tasks.forEach { $0() }

Асинхронное завершение

func fetchData(completion: @escaping (String) -> Void) {
    // simulate async: call completion in the future
    // (In real apps, use DispatchQueue or URLSession)
    completion("done")  // still escaping by contract (may be called later)
}
fetchData { result in print(result) }

Автоматическое создание closure

// @autoclosure converts an expression into () -> Bool automatically.
func logIf(_ condition: @autoclosure () -> Bool) {
    if condition() { print("Condition is true") }
}

let x = 3
logIf(x > 1)       // no braces needed; expression is turned into a closure

Автоматическое создание и сохранение closure

// You can combine @autoclosure with @escaping:
// (be careful: the expression is captured and evaluated later)
var checks: [() -> Bool] = []

func enqueueCheck(_ condition: @autoclosure @escaping () -> Bool) {
    checks.append(condition)   // stored to run later
}

var threshold = 10
enqueueCheck(threshold > 5)   // captures current variable by reference semantics
threshold = 3
print(checks.map { $0() })    // re-evaluates with updated threshold

Правило сохранения closure

Итоги