main.swift

class Toster {
    var callback: (() -> ())?

    init(){
        print("Toster -> init")
    }

    func execute() {
        self.callback?()
    }

    func execute(_ closure: () -> ()) {
        closure()
    }

    deinit {
        print("Toster -> deinit")
    }
}

class Runner {
    public weak var toster: Toster?

    init(){
        print("Runner -> init")
    }

    func run() {
        print("Runner -> setup callback to toster: \(self.toster)")
        self.toster?.callback = {
            self.kaboom()
        }
//        self.toster?.callback = {
//            [weak self] in
//            self?.kaboom()
//        }
        self.toster?.execute()
//        self.toster?.execute {
//            self.kaboom()
//        }
        print("Runner -> Runned")
    }

    func kaboom() {
        print("Runner -> Kaboom!")
    }

    deinit {
        print("Runner -> deinit")
    }

}

do {
    let toster: Toster = Toster()
    let runner: Runner = Runner()
    runner.toster = toster
    runner.run()
}