The saying "All the UI operations should be performed inside the main queue" becomes a little confusing when it comes to alpha. I have doubt do we really need to perform a simple alpha change operation inside the main Queue?
self.presentingViewController?.view.alpha = 0.6 // such operations
>Solution :
do we really need to perform a simple alpha change operation inside the main Queue?
Yes we do. Basically you don’t want to do anything to a view off the main thread. And do not try to cheat by shifting the burden to the view’s layer.