Checking the current view state after block/closure completion - ios

Question

Within a asynchronously executed block/closure, I want to get a check on my current state before I executed anything within that block.
A common example of where this presents itself is segueing to the next View Controller after a NSURLsession request.
Here's an example:
#IBAction func tappedButton(sender: UIButton) {
//This closure named fetchHistorical goes to the internet and fetches an array
//The response is then sent to the next view controller along with a segue
Order.fetchHistorical(orderID, completionHandler: { (resultEnum) -> () in
switch resultEnum {
case .Success(let result):
let orderItemsArray = result.orderItems!.allObjects
self.performSegueWithIdentifier("showExchanges", sender: orderItemsArray)
default:
let _ = errorModal(title: "Error", message: "Failed!")
}
})
}
Assume that the user has been impatient and tapped this button 3 times.
That would mean this function will be called three times and each time it would attempt to segue to the next view controller (iOS nicely blocks this issue with "Warning: Attempt to present on whose view is not in the window hierarchy!")
I wanted to know how do you folks tackle this problem? Is it something like ... within the closure, check if you are still in the present viewcontroller ... if you are, then segueing is valid. If not, you probably have already segued and don't execute the segue again.
***More generally, how are you checking the current state within the closure because the closure is executed asynchronously?

Answer 1

Since the closure isn't executing on the main thread the state would be in accurate if you check it here (as you stated). You can use GCD to go to the main thread and check the state there. There are a couple of ways you can keep this code from running multiple times. If it will take some time to perform the calculations you can use an acitivity indicator to let the user know the app is busy at the moment. If you want the user to still have the option of pressing the button you can put a tag like:
var buttonWasTapped:Bool = false //class property
#IBAction func tappedButton(sender: UIButton) {
if !self.buttonWasTapped{
self.buttonWasTapped = true
}
}
Then change it back to false on viewDidAppear so they can press once every time that page is shown.

Answer 2

When starting some task that will take some time to complete I would do two things;
Show some sort of activity indicator so that the user knows something is happening
Disable the button so that there is further indication that the request has been received and to prevent the user from tapping multiple times.
It is important that you consider not only the correct operation of your app but also providing a good user experience.