Capture lists ni nini, na unaepukaje retain cycles ndani ya closures?

Question

IT Interview Practice · Accepted Answer

Closure **hukamata** (captures) variables inazorejelea, ikiziweka hai. Wakati closure inakamata `self` na `self` pia inashikilia closure (kama stored property), hushikiliana hai milele — **retain cycle** inayovujisha kumbukumbu. **Capture list** kama `[weak self]` huvunja mzunguko huo.

## Jinsi inavyofanya kazi

ARC huachilia object wakati reference count yake inafikia sifuri. Strong capture huongeza hesabu hiyo.

```swift
final class Downloader {
    var onDone: (() -> Void)?          // stored closure -> strong

func start() {
        // BAD: closure captures self strongly, self holds closure -> cycle
        onDone = { self.cleanup() }

// GOOD: weak capture, self can deallocate
        onDone = { [weak self] in
            guard let self else { return }   // safely unwrap; bail if gone
            self.cleanup()
        }
    }
    func cleanup() {}
}
```

## weak dhidi ya unowned katika capture lists

- **`[weak self]`** — `self` huwa optional; ni `nil` iwapo object iliachiliwa. Chaguo salama la default wakati object inaweza kuishi zaidi ya closure.
- **`[unowned self]`** — si optional, hakuna gharama ya ARC, lakini **hu-crash** iwapo `self` tayari imeenda. Tumia tu wakati `self` imehakikishwa kuishi zaidi ya closure.

```swift
Task { [weak self] in
    let data = await self?.fetch()       // self? short-circuits if deallocated
}
```

## Wakati HUHITAJI capture list

Non-escaping closures (mfano `map`, `filter`, `forEach`) huendeshwa kwa mfululizo (synchronously) na huachilia captures mara moja — hakuna cycle, hakuna haja ya `[weak self]`. Hatari iko na closures za **escaping, zilizohifadhiwa**: completion handlers, `Timer`, `NotificationCenter`, Combine sinks, na delegate blocks.

## Kwa nini ni muhimu

Retain cycles ni uvujaji wa kumbukumbu wa kawaida zaidi katika iOS, na hazionekani hadi Instruments zionyeshe kumbukumbu inayoongezeka. Kujua hasa *lini* closure inafanya escape na *kwa nini* `[weak self]` inahitajika — dhidi ya kuitumia kila mahali kiharakaharaka — hutofautisha engineers wanaoelewa ARC na wale wanaoifuata bila kuelewa (cargo-cult).

Jinsi inavyofanya kazi

weak dhidi ya unowned katika capture lists

Wakati HUHITAJI capture list

Kwa nini ni muhimu

Maswali yanayohusiana