Answer.

In Swift, closures can "capture" values and references from the surrounding context. If the captured value is a value type (struct, enum), the closure copies the data. If a reference is captured (e.g., a class), the closure holds a reference to the class instance, which can lead to retain cycles if [weak self] or [unowned self] are not used.

The capture mechanism allows for asynchronous tasks and callbacks, but improper usage hinders memory management.

Example:

class MyClass {
    var value = 10

    func doSomething() {
        let closure = { [weak self] in
            print(self?.value ?? "nil")
        }
        closure()
    }
}

In this example, [weak self] is used to avoid strong reference cycles as asynchronous work is in effect.

Trick question.

Question: "Does the use of [weak self] always guarantee no memory leaks when capturing self in a closure?"

Answer: No, if a strong reference appears inside the closure (e.g., through intermediate variables), even with [weak self], a retain cycle can occur. For example:

let closure = { [weak self] in
    let strongSelf = self
    strongSelf?.doSomething()
}

If the closure is tied to a long-living object (e.g., NotificationCenter), a retain cycle can occur if there are other references to self.

Examples of real mistakes due to ignorance of the nuances of the topic.

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Story 1

In an iOS application project, a developer forgot to specify [weak self] when passing a closure to a network request. As a result, while the request was executing, the view controller object was not released, even if the user closed the screen. This caused memory leaks during intensive use of network operations.

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Story 2

In a complex timer subscription system, an object that subscribed via a closure without using weak continued executing its actions even after the screen was removed from memory. This led to repeated calls to UI elements that were no longer in the hierarchy, causing application crashes.

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Story 3

When implementing data caching, the closure captured a reference to self without the [unowned self] annotation in a long-living cache object. After the original object was destroyed, attempts to access self from the closure resulted in accessing already freed memory and crashing the application.