Swift Programming: The Big Nerd Ranch Guide, 3rd Edition

Reference Cycles with Closures

Sometimes assets appreciate or depreciate in value. To demonstrate some closure-related memory management behavior, you will teach Asset instances to react to changes in their value using a provided closure.

In your Asset type, allow value to be mutable and react to value changes by executing a stored closure of type (Asset) -> Void.

Example 24.9. Reacting to changes in asset value (Asset.swift)

class Asset {
    let name: String
    let value: Double
    var value: Double {
        didSet {
            changeHandler(value - oldValue)
        }
    }
    weak var container: Vault?
    
    typealias ValueChangeHandler = (Double) -> Void
    var changeHandler: ValueChangeHandler = {_ in} init(name: String, value: Double) { self.name = name self.value = value } deinit { ...