An example of a retain cycle using swift blocks.

RetainCycle.swift

//
//  ViewController.swift
//  RetainCylceTests
//
//  Created by Desmond McNamee on 15/09/16.
//  Copyright © 2016 Teamplace. All rights reserved.
//

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        
        var counter: Counter? = Counter()
        counter?.incrementCounter()
        
        print("counter: \(counter!.count)")
        
        counter = nil
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


}

class Do {
    func something(closure: ()->Void) {
        closure()
    }
}

class Counter {
    var count: Int = 0
    var closure: (()->Void)?
    
    func incrementCounter() {
        let doo = Do()
        self.closure = {
            // THIS CAUSES A RETAIN CYCLE!
            self.count += 1
        }
        doo.something(self.closure!)
    }
    
    deinit {
        print("Yay!")
    }
}