Identifying Common Memory Leak Patterns in Swift

Understanding Memory Leaks in Swift

Memory leaks occur when allocated memory is not released, causing your application to consume more memory over time. In Swift, this can happen due to strong reference cycles, improper use of closures, and certain design patterns. Below, we explore some common patterns that lead to memory leaks.

1. Strong Reference Cycles with Closures

Example:

class ViewController: UIViewController {
    var closure: (() -> Void)?

    override func viewDidLoad() {
        super.viewDidLoad()
        closure = { [self] in
            // Some operation
            print(self)
        }
    }
}

Explanation: In this example, self is captured strongly within the closure. If ViewController holds a reference to the closure, and the closure holds a reference back to ViewController, a strong reference cycle is formed, leading to a memory leak. To fix this, use a weak reference to self:

closure = { [weak self] in
    guard let self = self else { return }
    // Some operation
    print(self)
}

2. Deallocated Objects Still Held by Closures

Example:

class NetworkManager {
    var completionHandler: (() -> Void)?

    func fetchData() {
        // Simulating a network call
        DispatchQueue.global().async {
            self.completionHandler?()
        }
    }
}

class DataController {
    var networkManager = NetworkManager()

    func loadData() {
        networkManager.completionHandler = {
            // Process data
        }
        networkManager.fetchData()
    }
}

Explanation: If DataController is deallocated but the completionHandler is still referenced by NetworkManager, this will lead to a memory leak. Make sure to set the closure to nil when it is no longer needed:

networkManager.completionHandler = nil

3. Using Delegates Without Weak References

Example:

protocol TaskDelegate: AnyObject {
    func taskDidComplete()
}

class Task {
    var delegate: TaskDelegate?

    func start() {
        // Task execution logic
        delegate?.taskDidComplete()
    }
}

class ViewController: UIViewController, TaskDelegate {
    var task = Task()

    override func viewDidLoad() {
        super.viewDidLoad()
        task.delegate = self
    }
}

Explanation: The Task class holds a strong reference to its delegate, which is ViewController. If ViewController is deallocated, Task will still reference it, leading to a memory leak. To avoid this, use a weak reference for the delegate:

class Task {
    weak var delegate: TaskDelegate?
}

4. Retaining Notifications

Example:

class Notifier {
    func addObserver() {
        NotificationCenter.default.addObserver(self, selector: #selector(handleNotification), name: .someNotification, object: nil)
    }

    @objc func handleNotification() {
        // Handle notification
    }
}

Explanation: The Notifier class registers itself as an observer without removing the observer when it is deallocated. This leads to a memory leak. To fix this, remove the observer in deinit:

deinit {
    NotificationCenter.default.removeObserver(self)
}

Conclusion

By understanding these common patterns that lead to memory leaks in Swift, developers can write more efficient and reliable code. Always remember to use weak references in closures, manage delegate references carefully, and clean up observers to ensure your applications run smoothly.