Programming iOS 5, 2nd Edition

Chapter 38. Threads

A thread is, simply put, a subprocess of your app that can execute even while other subprocesses are also executing. Such simultaneous execution is called concurrency. The iOS frameworks use threads all the time; if they didn’t, your app would be less responsive to the user — perhaps even completely unresponsive. The genius of the frameworks, though, is that they use threads precisely so that you don’t have to.

For example, suppose your app is downloading something from the network (Chapter 37). This download doesn’t happen all by itself; somewhere, someone is running code that interacts with the network and obtains data. Similarly, how does Core Motion work (Chapter 35)? The data from the sensors is being gathered and processed constantly, with extra calculations to separate gravity from user-induced acceleration and to account for bias and scaling in the gyroscope. Yet neither a download nor sensor updating prevents your code from running, nor do they prevent the user from tapping and swiping things in your interface. That’s concurrency in action.

It is a testament to the ingenuity of the iOS frameworks that this book has proceeded so far with so little mention of threads. Indeed, it would have been nice to avoid the topic altogether. Threads are difficult and dangerous, and if at all possible you should avoid them. But sometimes that isn’t possible. So this chapter introduces threads, along with a warning: threads entail complications and subtle pitfalls, and ...