循環参照とweak/unownedによる解消

final class Owner {
    var gadget: Gadget?
    deinit { print("Owner deinit") }
}
final class Gadget {
    var owner: Owner?
    deinit { print("Gadget deinit") }
}
// Strong ⇄ strong (cycle)
var o: Owner? = Owner()
var g: Gadget? = Gadget()
o!.gadget = g
g!.owner = o
o = nil      // no deinit
g = nil      // still no deinit (leak)

final class Owner2 {
    var gadget: Gadget2?
    deinit { print("Owner2 deinit") }
}
final class Gadget2 {
    weak var owner: Owner2?   // weak breaks cycle; becomes nil automatically
    deinit { print("Gadget2 deinit") }
}
var o2: Owner2? = Owner2()
var g2: Gadget2? = Gadget2()
o2!.gadget = g2
g2!.owner = o2
o2 = nil     // Owner2 deinit
g2 = nil     // Gadget2 deinit

final class TimerClient {
    var tick: (() -> Void)?
    deinit { print("TimerClient deinit") }
    func start() {
        // BAD: closure strongly captures self; self keeps tick strongly
        tick = {
            print("tick from", self) // strong capture of self
        }
    }
}
var c: TimerClient? = TimerClient()
c!.start()
c = nil   // no deinit — closure holds self strongly

final class TimerClient2 {
    var tick: (() -> Void)?
    deinit { print("TimerClient2 deinit") }
    func start() {
        // Use [weak self] when self can go away before tick runs
        tick = { [weak self] in
            guard let self = self else { return } // safe unwrap
            print("tick from", self)
        }
        // Alternative when lifetime is guaranteed: [unowned self]
        // tick = { [unowned self] in print("tick from", self) }
    }
}
var c2: TimerClient2? = TimerClient2()
c2!.start()
c2 = nil   // deinit now prints — no cycle