Dismissal of UIAlertController (best practice) - ios

Question

When using UIAlertController like this:
var alert = UIAlertController(title: "Core Location",
message: "Location Services Disabled!",
preferredStyle: UIAlertControllerStyle.Alert)
alert.addAction(UIAlertAction(title: "OK", style: UIAlertActionStyle.Default,
handler: nil))
self.navigationController.presentViewController(alert, animated: true,
completion: nil)
I noticed that the dismissal of the alert view is seemingly done automatically.
Shouldn't the dismissal of a presented ViewController be done by the presenting ViewController via a delegate call?

Answer 1

The dismissal is "included" in the presentViewController call. You do not need a delegate because you have the completion block. In this block you put what you would normally put into the delegate callback, except the call to dismiss the alert.
As far as "best practice" is concerned, I noted that in many APIs, Apple replaced delegate callbacks with completion blocks. Apple typically recommends using the block syntax. I surmise this could be partly because it helps keeping the related code sections together.

Answer 2

Is some Cases you may like to use this:
class MyAlertController : UIAlertController {
var dismissHandler : (()->())?
override func viewDidDisappear(_ animated: Bool) {
dismissHandler?()
super.viewDidDisappear(animated)
}
}
Usage:
let alert = MyAlertController(title: ...
let cancelButton = UIAlertAction(titl
alert.dismissHandler = { /*...do something */ }
alert.addAction(cancelButton)
...

Answer 3

There is an elegant way! Just write the action or function inside the alert controller's cancel action. (here the action style should be .cancel)
Code for Swift 3:
let Alert: UIAlertController = UIAlertController(title: nil, message: nil, preferredStyle: UIAlertControllerStyle.actionSheet)
let OkAction: UIAlertAction = UIAlertAction(title: “Ok”, style: .default) { ACTION in
//Will be called when tapping Ok
}
let CancelAction: UIAlertAction = UIAlertAction(title: "Cancel", style: .cancel) {ACTION in
// Will be called when cancel tapped or when alert dismissed.
// Write your action/function here if you want to do something after alert got dismissed”
}
Alert.addAction(OkAction)
Alert.addAction(CancelAction)
present(Alert, animated: true, completion: nil)