キャプチャリスト — weak/unownedと値のキャプチャ

キャプチャリストを使う理由

強いキャプチャ = 循環のリスク

class Owner {
    var name = "Owner"
    var onDone: (() -> Void)?

    func start() {
        // Strong capture of self can cause a cycle if onDone is stored:
        onDone = {
            print("Done by", self.name) // self captured strongly
        }
    }
}
var o: Owner? = Owner()
o?.start()
// If nothing nils onDone or o, both can keep each other alive.

弱いキャプチャのパターン

class Owner2 {
    var name = "Owner2"
    var onDone: (() -> Void)?

    func start() {
        onDone = { [weak self] in
            guard let self = self else { return }  // safely unwrap optional self
            print("Done by", self.name)
        }
    }
}
let o2 = Owner2()
o2.start()
o2.onDone?()

unowned（慎重に使用）

class Owner3 {
    var onDone: (() -> Void)?
    func work() {
        onDone = { [unowned self] in
            // self must be alive when this runs; otherwise crash
            print("Owner3 still alive")
        }
    }
}
let o3 = Owner3()
o3.work()
o3.onDone?()

値のスナップショットという考え方

var count = 0
let makeLogger: () -> () -> Void = {
    let snapshot = count     // captured by value at creation
    return {
        print("snapshot =", snapshot, "current =", count)
    }
}
count = 10
let logger = makeLogger()
count = 20
logger()   // snapshot = 10, current = 20

キャプチャリストでの値の束縛

var total = 5
let closure = { [initial = total] in
    print("initial:", initial, " now:", total)
}
total = 9
closure()   // initial uses captured value 5

weak selfを使う理由

まとめ