Swift Academy · Lesson

Retain Cycles: Causes and Detection

Identifying retain cycles between objects and closures with Instruments.

Retain Cycles: Causes and Detection is a free Swift Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Swift Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Welcome

A retain cycle occurs when two or more objects hold strong references to each other, preventing ARC from ever reaching a zero count — causing a memory leak.

Simple Two-Object Cycle

```swift class Owner { var pet: Pet? } class Pet { var owner: Owner? } var o: Owner? = Owner() var p: Pet? = Pet() o?.pet = p // Owner → Pet (strong) p?.owner = o // Pet → Owner (strong) o = nil; p = nil // Neither deinit called — cycle! ```

Parent-Child Cycle

```swift class Node { var children: [Node] = [] var parent: Node? // strong → cycle with parent } let root = Node() let child = Node() root.children.append(child) child.parent = root // cycle ```

Closure Capture Cycle

```swift class ViewController { var onComplete: (() -> Void)? func setup() { onComplete = { // closure strongly captures self self.doWork() // ← cycle! } } } ```

Delegate Pattern Cycle

```swift protocol TaskDelegate: AnyObject { func didFinish() } class Task { var delegate: TaskDelegate? // if strong → potential cycle } class Manager: TaskDelegate { var task = Task() init() { task.delegate = self } // Manager → Task → Manager } ```

Detecting with Instruments

Use Xcode Instruments → Leaks: 1. Run app in Instruments 2. Click Leaks instrument 3. Red marks indicate leaked objects 4. Drill down to see reference graph Alternatively: set `deinit { print(...) }` and check if it fires.

Detecting with Xcode Memory Graph

In Xcode debugger: Debug > Memory Graph Debugger. Click on a leaked object to see its reference chain — the cycle will be visible as a closed loop in the graph.

Common Cycle Patterns

• Delegate storing delegate strongly • Closure capturing `self` strongly • Parent/child bidirectional strong references • NotificationCenter observer holding closure that captures self • Timer's target is `self` (strong)

Timer Retain Cycle

```swift class Poller { var timer: Timer? func start() { timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { [weak self] _ in self?.poll() // weak self breaks cycle } } deinit { timer?.invalidate() } } ```

NotificationCenter Cycle

```swift class Listener { var token: NSObjectProtocol? init() { token = NotificationCenter.default.addObserver(forName: .UIApplicationDidBecomeActive, object: nil, queue: .main) { [weak self] _ in self?.refresh() } } deinit { NotificationCenter.default.removeObserver(token!) } } ```

Quick Check

What is a retain cycle in ARC?

Recap

Key takeaways: • Retain cycles: mutual strong references → neither deallocates • Common in: delegates, closures, parent-child bidirectional links • Detect with Instruments Leaks or Xcode Memory Graph Debugger • Fix with `weak` or `unowned` references • Timers and NotificationCenter also cause cycles — break with `[weak self]` Next: weak and unowned references.

Frequently asked questions

Is the “Retain Cycles: Causes and Detection” lesson free?

Yes — the full text of “Retain Cycles: Causes and Detection” is free to read here on the web, and the Swift Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Swift Academy course, upgrade to CoddyKit PRO.

What will I learn in “Retain Cycles: Causes and Detection”?

Identifying retain cycles between objects and closures with Instruments. You practise Swift Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Swift Academy?

No prior experience is required. Swift Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Retain Cycles: Causes and Detection” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Swift Academy lesson?

Yes. Every Swift Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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