Hands-On Design Patterns with Swift

Using weak and unowned

Swift provides us with two keywords that indicate how we want to extend the lifetime of an object in a closure. While both prevent creating retain cycles, they are fundamentally different.

Using weak will wrap the captured value inside of an optional, indicating that the instance may have been deallocated before the closure was executed:

class MyClass {    var running = false    func run() {        running = true        DispatchQueue.main.asyncAfter(deadline: .now() + 10) { [weak self] in            self?.running = false        }    }}var instance: MyClass? = MyClass()instance?.run()instance = nil

In this execution, instance will immediately be deallocated when set to nil.

Using unowned indicates that the variable won't be owned by the block. Another mechanism ...