When I first started learning Objective-C, I already had a good understanding of concurrency and multitasking with my background in other languages, such as C and Java. This background made it very easy for me to create multithreaded applications using threads. Then, Apple changed everything when they released Grand Central Dispatch (GCD) with OS X 10.6 and iOS 4. At first, I went into denial; there was no way GCD could manage my application's threads better than I could. Then I entered the anger phase; GCD was hard to use and understand. Next was the bargaining phase; maybe I can use GCD with my threading code, so I could still control how the threading worked. Then there was the depression phase; maybe GCD does handle threading better than I could. Finally, I entered the wow phase; this GCD thing is really easy to use and works amazingly well. After using GCD and operation queues with Objective-C, I do not see a reason for using manual threads with Swift.

In this chapter, we will learn the following topics:

• What are basics of concurrency and parallelism?
• How to use GCD to create and manage concurrent dispatch queues?
• How to use GCD to create and manage serial dispatch queues?
• How to use various GCD functions to add tasks to the dispatch queues?
• How to use Operation and OperationQueues to add concurrency to our applications?