Memory Management

Note

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Memory Management

Use this when:

• A task or async sequence is keeping objects alive longer than expected.
• You suspect a retain cycle between a task and its owner.
• You need to verify deallocation behavior or use isolated deinit.

Skip this file if:

• You mainly need to protect mutable state from races. Use actors.md.
• You are debugging slow async code. Use performance.md.

Jump to:

• Core Concepts (Task Capture)
• Retain Cycles
• One-Way Retention
• Async Sequences and Retention
• Isolated Deinit (Swift 6.2+)
• Detection and Testing
• Common Patterns

Core Concepts

Tasks capture like closures

Tasks capture variables and references just like regular closures. Swift doesn’t automatically prevent retain cycles in concurrent code.

Task {
    self.doWork() // ⚠️ Strong capture of self
}

Why concurrency hides memory issues

• Tasks may live longer than expected
• Async operations delay execution
• Harder to track when memory should be released
• Long-running tasks can hold references indefinitely

Retain Cycles

What is a retain cycle?

Two or more objects hold strong references to each other, preventing deallocation.

class A {
    var b: B?
}

class B {
    var a: A?
}

let a = A()
let b = B()
a.b = b
b.a = a // Retain cycle - neither can be deallocated

Retain cycles with Tasks

When task captures self strongly and self owns the task:

@MainActor
final class ImageLoader {
    var task: Task<Void, Never>?

    func startPolling() {
        task = Task {
            while true {
                self.pollImages() // ⚠️ Strong capture
                try? await Task.sleep(for: .seconds(1))
            }
        }
    }
}

var loader: ImageLoader? = .init()
loader?.startPolling()
loader = nil // ⚠️ Loader never deallocated - retain cycle!

Problem: Task holds self, self holds task → neither released.

Breaking Retain Cycles

Use weak self

func startPolling() {
    task = Task { [weak self] in
        while let self = self {
            self.pollImages()
            try? await Task.sleep(for: .seconds(1))
        }
    }
}

var loader: ImageLoader? = .init()
loader?.startPolling()
loader = nil // ✅ Loader deallocated, task stops

Pattern for long-running tasks

task = Task { [weak self] in
    while let self = self {
        await self.doWork()
        try? await Task.sleep(for: interval)
    }
}

Loop exits when self becomes nil.

One-Way Retention

Task retains self, but self doesn’t retain task. Object stays alive until task completes.

@MainActor
final class ViewModel {
    func fetchData() {
        Task {
            await performRequest()
            updateUI() // ⚠️ Strong capture
        }
    }
}

var viewModel: ViewModel? = .init()
viewModel?.fetchData()
viewModel = nil // ViewModel stays alive until task completes

Execution order:

1. Task starts
2. viewModel = nil (but object not deallocated)
3. Task completes
4. ViewModel finally deallocated

When one-way retention is acceptable

Short-lived tasks that complete quickly:

func saveData() {
    Task {
        await database.save(self.data) // OK - completes quickly
    }
}

When to use weak self

Long-running or indefinite tasks:

func startMonitoring() {
    Task { [weak self] in
        for await event in eventStream {
            self?.handle(event)
        }
    }
}

Async Sequences and Retention

Problem: Infinite sequences

@MainActor
final class AppLifecycleViewModel {
    private(set) var isActive = false
    private var task: Task<Void, Never>?

    func startObserving() {
        task = Task {
            for await _ in NotificationCenter.default.notifications(
                named: .didBecomeActive
            ) {
                isActive = true // ⚠️ Strong capture, never ends
            }
        }
    }
}

var viewModel: AppLifecycleViewModel? = .init()
viewModel?.startObserving()
viewModel = nil // ⚠️ Never deallocated - sequence continues

Problem: Async sequence never finishes, task holds self indefinitely.

Solution 1: Manual cancellation

func startObserving() {
    task = Task {
        for await _ in NotificationCenter.default.notifications(
            named: .didBecomeActive
        ) {
            isActive = true
        }
    }
}

func stopObserving() {
    task?.cancel()
}

// Usage
viewModel?.startObserving()
viewModel?.stopObserving() // Must call before release
viewModel = nil

Solution 2: Weak self with guard

func startObserving() {
    task = Task { [weak self] in
        for await _ in NotificationCenter.default.notifications(
            named: .didBecomeActive
        ) {
            guard let self = self else { return }
            self.isActive = true
        }
    }
}

Task exits when self deallocates.

Isolated deinit (Swift 6.2+)

Clean up actor-isolated state in deinit:

@MainActor
final class ViewModel {
    private var task: Task<Void, Never>?

    isolated deinit {
        task?.cancel()
    }
}

Limitation: Won’t break retain cycles (deinit never called if cycle exists).

Use for: Cleanup when object is being deallocated normally.

Common Patterns

Short-lived task (strong capture OK)

func saveData() {
    Task {
        await database.save(self.data)
        self.updateUI()
    }
}

When safe: Task completes quickly, acceptable for object to live until done.

Long-running task (weak self required)

func startPolling() {
    task = Task { [weak self] in
        while let self = self {
            await self.fetchUpdates()
            try? await Task.sleep(for: .seconds(5))
        }
    }
}

Async sequence monitoring (weak self + guard)

func startMonitoring() {
    task = Task { [weak self] in
        for await event in eventStream {
            guard let self = self else { return }
            self.handle(event)
        }
    }
}

Cancellable work with cleanup

func startWork() {
    task = Task { [weak self] in
        defer { self?.cleanup() }

        while let self = self {
            await self.doWork()
            try? await Task.sleep(for: .seconds(1))
        }
    }
}

Detection Strategies

Add deinit logging

deinit {
    print("✅ \(type(of: self)) deallocated")
}

If deinit never prints → likely retain cycle.

Memory graph debugger

1. Run app in Xcode
2. Debug → Debug Memory Graph
3. Look for cycles in object graph

Instruments

Use Leaks instrument to detect retain cycles at runtime.

Decision Tree

Task captures self?
├─ Task completes quickly?
│  └─ Strong capture OK
│
├─ Long-running or infinite?
│  ├─ Can use weak self? → Use [weak self]
│  ├─ Need manual control? → Store task, cancel explicitly
│  └─ Async sequence? → [weak self] + guard
│
└─ Self owns task?
   ├─ Yes → High risk of retain cycle
   └─ No → Lower risk, but check lifetime

Best Practices

1. Default to weak self for long-running tasks
2. Use guard let self in async sequences
3. Cancel tasks explicitly when possible
4. Add deinit logging during development
5. Test object deallocation in unit tests
6. Use Memory Graph to verify no cycles
7. Document lifetime expectations in comments
8. Prefer cancellation over weak self when possible
9. Avoid nested strong captures in task closures
10. Use isolated deinit for cleanup (Swift 6.2+)

Testing for Leaks

Unit test pattern

func testViewModelDeallocates() async {
    var viewModel: ViewModel? = ViewModel()
    weak var weakViewModel = viewModel

    viewModel?.startWork()
    viewModel = nil

    // Give tasks time to complete
    try? await Task.sleep(for: .milliseconds(100))

    XCTAssertNil(weakViewModel, "ViewModel should be deallocated")
}

SwiftUI view test

func testViewDeallocates() {
    var view: MyView? = MyView()
    weak var weakView = view

    view = nil

    XCTAssertNil(weakView)
}

Common Mistakes

❌ Forgetting weak self in loops

Task {
    while true {
        self.poll() // Retain cycle
        try? await Task.sleep(for: .seconds(1))
    }
}

❌ Strong capture in async sequences

Task {
    for await item in stream {
        self.process(item) // May never release
    }
}

❌ Not canceling stored tasks

class Manager {
    var task: Task<Void, Never>?

    func start() {
        task = Task {
            await self.work() // Retain cycle
        }
    }

    // Missing: deinit { task?.cancel() }
}

❌ Assuming deinit breaks cycles

deinit {
    task?.cancel() // Never called if retain cycle exists
}

Examples by Use Case

Polling service

final class PollingService {
    private var task: Task<Void, Never>?

    func start() {
        task = Task { [weak self] in
            while let self = self {
                await self.poll()
                try? await Task.sleep(for: .seconds(5))
            }
        }
    }

    func stop() {
        task?.cancel()
    }
}

Notification observer

@MainActor
final class NotificationObserver {
    private var task: Task<Void, Never>?

    func startObserving() {
        task = Task { [weak self] in
            for await notification in NotificationCenter.default.notifications(
                named: .someNotification
            ) {
                guard let self = self else { return }
                self.handle(notification)
            }
        }
    }

    isolated deinit {
        task?.cancel()
    }
}

Download manager

final class DownloadManager {
    private var tasks: [URL: Task<Data, Error>] = [:]

    func download(_ url: URL) async throws -> Data {
        let task = Task { [weak self] in
            defer { self?.tasks.removeValue(forKey: url) }
            return try await URLSession.shared.data(from: url).0
        }

        tasks[url] = task
        return try await task.value
    }

    func cancelAll() {
        tasks.values.forEach { $0.cancel() }
        tasks.removeAll()
    }
}

Timer

actor Timer {
    private var task: Task<Void, Never>?

    func start(interval: Duration, action: @Sendable () async -> Void) {
        task = Task {
            while !Task.isCancelled {
                await action()
                try? await Task.sleep(for: interval)
            }
        }
    }

    func stop() {
        task?.cancel()
    }
}

Common Mistakes Agents Make

• Forgetting [weak self] in stored tasks: When self owns the task and the task captures self, a retain cycle prevents deallocation.
• Strong capture in infinite AsyncSequence loops: for await over an infinite sequence with a strong self capture keeps the object alive forever.
• Not cancelling stored tasks on cleanup: If the task outlives its owner, it retains captured objects indefinitely.
• Assuming isolated deinit breaks retain cycles: isolated deinit runs cleanup on the correct actor, but if a cycle prevents deinit from being called at all, the cleanup never executes.
• Using try? in loops with Task.sleep: try? can swallow CancellationError, causing the loop to continue running after cancellation. Always check Task.isCancelled explicitly.

Debugging Checklist

When object won’t deallocate:

• Check for strong self captures in tasks
• Verify tasks are canceled or complete
• Look for infinite loops or sequences
• Check if self owns the task
• Use Memory Graph to find cycles
• Add deinit logging to verify
• Test with weak references
• Review async sequence usage
• Check nested task captures
• Verify cleanup in deinit