2021 Swift
ARC in Swift: Basics and beyond
Learn about the basics of object lifetimes and ARC in Swift. Dive deep into what language features make object lifetimes observable, consequences of relying on observed object lifetimes and some safe techniques to fix them.
Watch at developer.apple.com ↗Code shown on screen · 13 snippets
ARC Example
class Traveler {
var name: String
var destination: String?
}
func test() {
let traveler1 = Traveler(name: "Lily")
let traveler2 = traveler1
traveler2.destination = "Big Sur"
print("Done traveling")
}Reference Cycle Example
class Traveler {
var name: String
var account: Account?
func printSummary() {
if let account = account {
print("\(name) has \(account.points) points")
}
}
}
class Account {
var traveler: Traveler
var points: Int
}
func test() {
let traveler = Traveler(name: "Lily")
let account = Account(traveler: traveler, points: 1000)
traveler.account = account
traveler.printSummary()
}Weak Reference Example
class Traveler {
var name: String
var account: Account?
func printSummary() {
if let account = account {
print("\(name) has \(account.points) points")
}
}
}
class Account {
weak var traveler: Traveler?
var points: Int
}
func test() {
let traveler = Traveler(name: "Lily")
let account = Account(traveler: traveler, points: 1000)
traveler.account = account
traveler.printSummary()
}Accessing an object via weak reference
class Traveler {
var name: String
var account: Account?
}
class Account {
weak var traveler: Traveler?
var points: Int
func printSummary() {
print("\(traveler!.name) has \(points) points")
}
}
func test() {
let traveler = Traveler(name: "Lily")
let account = Account(traveler: traveler, points: 1000)
traveler.account = account
account.printSummary()
}Accessing an object via optional binding of weak reference
class Traveler {
var name: String
var account: Account?
}
class Account {
weak var traveler: Traveler?
var points: Int
func printSummary() {
if let traveler = traveler {
print("\(traveler.name) has \(points) points")
}
}
}
func test() {
let traveler = Traveler(name: "Lily")
let account = Account(traveler: traveler, points: 1000)
traveler.account = account
account.printSummary()
}Safe techniques for handling weak references - withExtendedLifetime()
func test() {
let traveler = Traveler(name: "Lily")
let account = Account(traveler: traveler, points: 1000)
traveler.account = account
withExtendedLifetime(traveler) {
account.printSummary()
}
}
func test() {
let traveler = Traveler(name: "Lily")
let account = Account(traveler: traveler, points: 1000)
traveler.account = account
account.printSummary()
withExtendedLifetime(traveler) {}
}
func test() {
let traveler = Traveler(name: "Lily")
let account = Account(traveler: traveler, points: 1000)
defer {withExtendedLifetime(traveler) {}}
traveler.account = account
account.printSummary()
}Safe techniques for handling weak references - Redesign to access via strong reference
class Traveler {
var name: String
var account: Account?
func printSummary() {
if let account = account {
print("\(name) has \(account.points) points")
}
}
}
class Account {
private weak var traveler: Traveler?
var points: Int
}
func test() {
let traveler = Traveler(name: "Lily")
let account = Account(traveler: traveler, points: 1000)
traveler.account = account
traveler.printSummary()
}Safe techniques for handling weak references - Redesign to avoid weak/unowned reference
class PersonalInfo {
var name: String
}
class Traveler {
var info: PersonalInfo
var account: Account?
}
class Account {
var info: PersonalInfo
var points: Int
}Deinitializer Example
class Traveler {
var name: String
var destination: String?
deinit {
print("\(name) is deinitializing")
}
}
func test() {
let traveler1 = Traveler(name: "Lily")
let traveler2 = traveler1
traveler2.destination = "Big Sur"
print("Done traveling")
}Sequencing deinitializer side-effects with external program effects
class Traveler {
var name: String
var id: UInt
var destination: String?
var travelMetrics: TravelMetrics
// Update destination and record travelMetrics
func updateDestination(_ destination: String) {
self.destination = destination
travelMetrics.destinations.append(self.destination)
}
// Publish computed metrics
deinit {
travelMetrics.publish()
}
}
class TravelMetrics {
let id: UInt
var destinations = [String]()
var category: String?
// Finds the most interested travel category based on recorded destinations
func computeTravelInterest()
// Publishes id, destinations.count and travel interest category
func publish()
}
func test() {
let traveler = Traveler(name: "Lily", id: 1)
let metrics = traveler.travelMetrics
...
traveler.updateDestination("Big Sur")
...
traveler.updateDestination("Catalina")
metrics.computeTravelInterest()
}
verifyGlobalTravelMetrics()Safe techniques for handing deinitalizer side effects - withExtendedLifetime()
func test() {
let traveler = Traveler(name: "Lily", id: 1)
let metrics = traveler.travelMetrics
...
traveler.updateDestination("Big Sur")
...
traveler.updateDestination("Catalina")
withExtendedLifetime(traveler) {
metrics.computeTravelInterest()
}
}Safe techniques for handing deinitalizer side effects - Redesign to limit visibility of internal class details
class Traveler {
...
private var travelMetrics: TravelMetrics
deinit {
travelMetrics.computeTravelInterest()
travelMetrics.publish()
}
}
func test() {
let traveler = Traveler(name: "Lily", id: 1)
...
traveler.updateDestination("Big Sur")
...
traveler.updateDestination("Catalina")
}Safe techniques for handling deinitalizer side effects - Redesign to avoid deinitializer side-effects
class Traveler {
...
private var travelMetrics: TravelMetrics
func publishAllMetrics() {
travelMetrics.computeTravelInterest()
travelMetrics.publish()
}
deinit {
assert(travelMetrics.published)
}
}
class TravelMetrics {
...
var published: Bool
...
}
func test() {
let traveler = Traveler(name: "Lily", id: 1)
defer { traveler.publishAllMetrics() }
...
traveler.updateDestination("Big Sur")
...
traveler.updateDestination("Catalina")
}