Programming iOS 6, 3rd Edition

The Main Thread

You are always using some thread. All your code must run somewhere; “somewhere” means a thread. When code calls a method, that method normally runs on the same thread as the code that called it. Your code is called through events (Chapter 11); those events normally call your code on the main thread. The main thread has certain special properties:

The main thread automatically has a run loop.

A run loop is a recipient of events. It is how your code is notified that something is happening; without a run loop, a thread can’t receive events. Cocoa events normally arrive on the main thread’s run loop; that’s why your code, called by those events, executes on the main thread.

The main thread is the interface thread.

When the user interacts with the interface, those interactions are reported as events — on the main thread. When your code interacts with the interface, it too must do so on the main thread. Of course that will normally happen automatically, because your code normally runs on the main thread.

What Is Concurrency, Really?

Part of the power of threads is that they implement concurrency without your having to worry about precisely how they do it. On a Mac Pro with eight cores, you could theoretically run eight threads truly simultaneously, one on each core. A recent model iPhone or iPad has two cores; but earlier iOS devices have just one core. Nevertheless, an iOS app is multithreaded. How can this be? Basically, the processor performs a little code from one ...