Example showing a retain cycle only if the completionHandler of an object is explicitly stored

Retain Cycle

import UIKit

class ViewController{
    var networking: NetworkManager? = NetworkManager() // Unlike storedHandler this doesn't cause a leak unless it stores the completion
    var isLoading = false
    
    func loadImages() {
        networking?.downloadImages { isSuccess in
            self.isLoading = isSuccess
            print("Images download completed")
        }
    }
    
    deinit {
        print("ViewController Deallocated")
    }
}

class NetworkManager{
    var storedHandler: ((Bool) -> ())?
    
    func downloadImages(completionHandler: @escaping (Bool)->()) {
        //self.storedHandler = completionHandler // Add this line to create retain cycle
        print("downloading images")
        completionHandler(true)
    }
}

var viewController: ViewController? = ViewController()
viewController?.loadImages()
viewController = nil


// With networkManager.storedHandler: Retain cycle
// completionHandler -> viewController -> networking -> completionHandler

// Without networkManager.storedHandler : No retain cycle
// completionHandler -> viewController -> networking

// With weak self: No retain cycle
// completionHandler ~> viewController -> networking -> completionHandler