Confusion Regarding How to Use Capture Lists to Avoid a Reference Cycle

Question

My custom UIViewController subclass has a stored closure property. The closure signature is defined as to take a single argument of the same type of the class:

class MyViewController {

    var completionHandler : ((MyViewController)->(Void))?

    // ...
}

...the idea being, the object passing itself back as an argument of the handler, a bit like the UIAlertAction initializer.

In addition, and for convenience, I have a factory(-ish) class method:

class func presentInstance(withCompletionHandler handler:((MyViewController)->(Void)))
{
   // ...
}

...that performs the following actions:

1. Creates an instance of the view controller,
2. Assigns the completion handler to the property,
3. Presents it modally from whatever happens to be the top/root view controller at the time of the call.

My view controller is definitely leaking: I set up a breakpoint on deinit() but execution never hits it, even way after I'm done with my view controller and it is dismissed.

I am not sure of how or where I should specify a capture list in order to avoid the cycle. Every example I have come across seems to place it where the closure body is defined, but I can't get my code to compile.

1. Where I declare the closure property? (how?)

   var completionHandler : ((MyViewController)->(Void))?
   // If so, where does it go?
   

2. Where I declare the closure parameter?

   class func presentInstance(withCompletionHandler handler:((MyViewController)->(Void)))
   {
   // Again, where could it go?
   

3. Where I call the above function and pass the closure body?

   MyViewController.presentInstance(withCompletionHandler:{
       [unowned viewController] viewController in 
   
       // ...
   })
   // ^ Use of unresolved identifier viewController
   // ^ Definition conflicts with previous value
   

4. Where I actually call the closure, towards self? None of these will compile:

   self.completionHandler?(unowned self)
   self.completionHandler?([unowned self] self)
   self.completionHandler?([unowned self], self)
   

Answer 1

Well, it turns out my view controller was being retained by a block, but not the one I was thinking:

class MyViewController
{
    deinit(){
        print("Deinit...")
    }

    // ...

    @IBAction func cancel(sender:UIButton)
    {
        completionHandler(self) 
        // View controller is dismissed, AND
        // deinit() gets called.     
    }

    @IBAction func punchIt(sender: UIButton)
    {
        MyWebService.sharedInstance.sendRequest(    
            completion:{ error:NSError? in

                 self.completionHandler(self)  
                 // View controller is dismissed, BUT
                 // deinit() does NOT get called. 
            }
        )
    }

...so it is the closure passed to MyWebService.sharedInstance.sendRequest() that was keeoing my view controller alive. I fixed it by adding a capture list like this:

MyWebService.sharedInstance.sendRequest(    
            completion:{ [unowned self] error:NSError? in

However, I still don't quite understand why the short-lived completion handler, passed to the web service class, executed once, and disposed, was keeping my view controller alive. That closure, not stored anywhere as a property, should be deallocated as soon as it is exited, right?

I must be missing something. I guess I'm still not fully thinking in portals closures.