Keeping Your Interface Responsive
As we’ve mentioned a few times in this book, if you try to do too much at one time in an action or delegate method, or in a method called from one of those methods, your application’s interface can skip or even freeze while the long-running method does its job. As a general rule, you do not want your application’s user interface to ever become unresponsive. Your user will expect to be able to interact with your application at all times, or at the very least will expect to be kept updated by your user interface when they aren’t allowed to interact with it.
In computer programming, the ability to have multiple sets of operations happening at the same time is referred to, generally, as concurrency ...