Illustrates a specific type of retain cycle

ViewControllerMemoryLeak.swift

/// Schedules a timer on a queue when the view is loaded.
/// Also logs when deinit is invoked
class ViewControllerBase: UIViewController {
    let timer:DispatchSourceTimer = DispatchSource.makeTimerSource(flags: [], queue:  DispatchQueue(label: "q.q"))
    
    deinit {
        NSLog("deinit of \(NSStringFromClass(type(of: self)))")
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        timer.scheduleRepeating(deadline: .now(), interval: .seconds(1))
    }
}

/// SUBTLE MEMORY LEAK ILLUSTRATED
/// Which of the following `ViewController`s:
/// - Leaks memory?
/// - Compiles with an error?
/// - Is correct?

class ViewControllerA: ViewControllerBase {
    override func viewDidLoad() {
        super.viewDidLoad()
        
        timer.setEventHandler {
            UIView.animate(withDuration: 0.2) {
                self.view.backgroundColor = UIColor.green
            }
        }
    }
}

class ViewControllerB: ViewControllerBase {
    override func viewDidLoad() {
        super.viewDidLoad()

        timer.setEventHandler {
            UIView.animate(withDuration: 0.2) {
                NSLog("Hello")
            }
        }
    }
}

class ViewControllerC: ViewControllerBase {
    override func viewDidLoad() {
        super.viewDidLoad()
        
        timer.setEventHandler {
            UIView.animate(withDuration: 0.2) { [weak self] in
                NSLog("Hello")
            }
        }
    }
}

class ViewControllerD: ViewControllerBase {
    override func viewDidLoad() {
        super.viewDidLoad()
        
        timer.setEventHandler { [weak self] in
            UIView.animate(withDuration: 0.2) {
                self?.view.backgroundColor = UIColor.green
            }
        }
    }
}

class ViewControllerE: ViewControllerBase {
    override func viewDidLoad() {
        super.viewDidLoad()
        
        timer.setEventHandler {
            UIView.animate(withDuration: 0.2) { [weak self] in
                self?.view.backgroundColor = UIColor.green
            }
        }
    }
}