iOS Concurrency with GCD & Operations

Sep 12 2023 · Swift 5.8, macOS 13, iOS 16, Xcode 14.3

Part 2: Concurrency Problems & Solutions

13. Make Class Thread-safe

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Previous episode: 12. Explore Priority Inversion Next episode: 14. Challenge: Make Number Class Thread-Safe

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Simple example

Build and run the DataRace project in the starter folder. The screen just displays the default message Hello, world! while the two for-loops in raceYou() run until these messages appear in the console:

>>> Main Queue counter = 19
>>> notMain Queue counter = 20

var counter = 0

let queue = DispatchQueue(label: "notMain")
queue.async {
  for _ in 1 ... 10 {
    Thread.sleep(forTimeInterval: Double.random(in: 0.1..<0.5))
    counter += 1
  }
  print(">>> notMain Queue counter = \(counter)")
}

DispatchQueue.main.async {
  for _ in 1 ... 10 {
    Thread.sleep(forTimeInterval: Double.random(in: 0.1..<0.5))
    counter += 1
  }
  print(">>> Main Queue counter = \(counter)")
}

WARNING: ThreadSanitizer: Swift access race (pid=13189)
...
SUMMARY: ThreadSanitizer: Swift access race ... in ContentView.raceYou()
==================
ThreadSanitizer report breakpoint hit.

Thread-safe Class Playground

An object type is thread-safe if concurrent tasks can access its instances without causing any concurrency problems. If a variable or mutable data structure is not thread-safe, you should access it from only one thread at a time.

"Brian Biggles" (in sidebar)

for (idx, name) in nameList.enumerated() {
  workerQueue.async(group: nameChangeGroup) {
    usleep(UInt32(10_000 * idx))
    nameChangingPerson.changeName(firstName: name.0, lastName: name.1)
    print("Current Name: \(nameChangingPerson.name)")
  }
}

nameChangeGroup.notify(queue: DispatchQueue.global()) {
  print("Final name: \(nameChangingPerson.name)")
  PlaygroundPage.current.finishExecution()
}

nameChangeGroup.wait()

Current Name: Freddie Dingle
Current Name: Freddie Evershed
Current Name: Freddie Gregory
Current Name: Freddie Frost
Current Name: Freddie Cheesecake
Final name: Freddie Cheesecake

class ThreadSafePerson: Person {
  
}

let isolationQueue = DispatchQueue(
  label: "com.kodeco.person.isolation", 
  attributes: .concurrent)

override func changeName(firstName: String, lastName: String) {
  isolationQueue.async(flags: .barrier) {
    // barrier task
    super.changeName(firstName: firstName, lastName: lastName)
  }
}

override var name: String {
  isolationQueue.sync {
    super.name
  }
}

print("\n=== Threadsafe ===")

let threadSafeNameGroup = DispatchGroup()

let threadSafePerson = ThreadSafePerson(firstName: "Anna", lastName: "Adams")

for (idx, name) in nameList.enumerated() {
  workerQueue.async(group: threadSafeNameGroup) {
    usleep(UInt32(10_000 * idx))
    threadSafePerson.changeName(firstName: name.0, lastName: name.1)
    print("Current threadsafe name: \(threadSafePerson.name)")
  }
}

threadSafeNameGroup.notify(queue: DispatchQueue.global()) {
  print("Final threadsafe name: \(threadSafePerson.name)")
  sleep(1)
  PlaygroundPage.current.finishExecution()
}

=== Threadsafe ===
Current threadsafe name: Eva Evershed
Current threadsafe name: Eva Evershed
Current threadsafe name: Eva Evershed
Current threadsafe name: Freddie Frost
Current threadsafe name: Gina Gregory
Final threadsafe name: Gina Gregory

Thread-safe Class Project

Now look at an app version of this code, in NameChanger: It has only a placeholder UI, but you need an app to use TSan. As you did with the DataRace project, edit the scheme to enable TSan, then build and run.

// TSan finds race condition errors
// changeNameRace()

// TSan finds no errors
changeNameSafely()

Current threadsafe name: Charlie Cheesecake
Current threadsafe name: Delia Dingle
Current threadsafe name: Eva Evershed
Current threadsafe name: Freddie Frost
Current threadsafe name: Gina Gregory
Final threadsafe name: Gina Gregory