Swift retrain cycle passing method to closure argument. Throw this in a playground to play around with it.

SwiftMethodRetainCycle.swift

class Controller {
    var changed: (() -> Void)?

    init() {
        print("Allocate 'Controller'")
    }

    deinit {
        print("Deallocate 'Controller'")
    }
}

class ViewController {
    let controller = Controller()

    init() {
        print("Allocate 'ViewController'")

//        controller.changed = foo // retain-cycle
        controller.changed = { [weak self] in self?.foo() } // no retain-cycle
    }

    deinit {
        print("Deallocate 'ViewController'")
    }

    func foo() { }
}

var vc: ViewController? = ViewController()
vc = nil