Grand Central Dispatch

Grand Central Dispatch (GCD) and blocks are a great tool that makes multi-threading easy, efficient, and enjoyable.

1. Blocks and Grand Central Dispatch By Robert Brown @robby_brown
2. Blocks
3. Blocks Blocks are a proposed addition to C. Like a function pointer, except it also stores the context the block was created in. Similar to closures, lambdas, and anonymous functions in other languages.
4. Blocks Declaration Syntax: returnType (^blockName) (Arguments) Blocks may be anonymous. Definition Syntax: ^ returnType (Arguments) { code; } The return type and arguments are optional. GCD provides function pointer variants to the block APIs.
5. Blocks Blocks can modify local variables outside of their scope if the variables have the new __block keyword. Global and static variables can be modified without the __block keyword. Blocks automatically retain Objective-C objects. C objects must be manually retained.
6. GCD
7. What is GCD? GCD is a lightweight multithreading engine. Uses a thread pool. Developers create queues of blocks rather than threads. Uses lock-less exclusion rather than mutual exclusion. Replaces blocking and polling APIs.
8. Why Multithread on a Single Core? Keeps the UI responsive. UI code runs on the main thread. Everything else runs on a background thread. Prevents the main thread from blocking or waiting.
9. Frequently Used APIs dispatch_async(queue, block); dispatch_queue_create(name, NULL); dispatch_set_target_queue(object, queue); dispatch_get_global_queue(priority, flags); dispatch_retain(object); dispatch_release(object);
10. Global Queues Four global queues: Main, Low, Default, and High. Only the main thread services the main queue. The three other queues determine the priority of background tasks. Enqueuing is thread safe.
11. Advanced GCD
12. User Queues Developers can create their own queues. Create a queue for each task, subsystem, or resource. Queues should use reverse-DNS naming. Example: edu.byu.cocoaheads.networkqueue Jedi Level
13. User Queues All queues must eventually drain to a global queue. Queues can be organized into arbitrary hierarchies. Don’t create loops! Jedi Level
14. User Queues Queue 2 Queue 1 Queue 3 Default Priority Queue 1 Queue 2 Queue 3 Default Jedi Level ≠
15. Dispatch Sources Queues can respond to low-level events. Decouples monitoring and event handling. Avoids polling for an event. Example: Polling for a directory change or socket available. Dispatch sources can be suspended, resumed, and cancelled. Jedi Level
16. Semaphores Semaphores are used to create locks. Available if you need them. Usually block the running thread. The availability of the semaphore can be checked without blocking (i.e. Test and Set). Sith Level
17. Demo
18. Want to Learn More? Apple Documentation WWDC 2010 Session 206 Session 211 WWDC 2008 Session 382
19. Questions?