This document discusses concurrency programming and Grand Central Dispatch (GCD) in iOS. It covers threads and multi-threading, NSThread, NSObject, NSOperation, run loops, and GCD. GCD uses dispatch queues and blocks to simplify concurrent programming compared to directly using threads. It manages pools of threads to dispatch tasks asynchronously or synchronously. Serial dispatch queues are thread-safe. Common GCD functions include dispatch_async, dispatch_sync, dispatch_once, and dispatch_group_notify.

1. GCD and Concurrency
Programming
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9.1.13

3. GCD
• Sync vs Async

4. Concurrency
Programming
• NSThread
• NSObject
• NSOperation
• Run Loop
• GCD

5. Thread
• What is thread
• What is multi-thread

6. Why Multithread on a
Single Core?
•
Keep UI responsive.

7. NSThread

8. NSThread
• Main thread and other threads
• detachNewThreadSelector:toTarget:with
Object:
• Pro: light-weight (for code), directly control
• Cons: Life cycle issue, race condition issue (lock,
retain cycle)

9. NSObject
• performSelectorInBackground: withObject:

10. What to use Thread?
• Not in most cases

11. NSThread

12. NSOperation
• Pros:
• Add dependency
• setMaxConcurrentOperationCount
• re-use
• Cancel, Suspend
• Priority
• suspend
• compatible with KVO

13. Grand Central Dispatch

14. GCD
• Less memory penalty
• Less configure
• Less manage
• Simplifies the code

15. GCD
• GCD的核⼼心是dispatch queues
• 它管理pools of threads
• 管理queues，⽽而⾮非直接管理threads
It was first released with Mac OS X 10.6, and is also
available with iOS 4 and above.

16. Thread pools
• Thread pools.
• Grab a thread from the pool and dispatch
the task to it
• Queue the task and wait for a thread to
become available.

17. Blocks
void (^aBlock)(int) = ^(int z) {!
!
NSLog(@“log”);!
!
};!
!
• Like a function pointer, except it also stores
the context the block was created in.
• Beware of retain cycles

18. GCD
• Use queues of blocks rather than threads

19. Dispatch Queue
• Main Queue
• Global Queue
• Serial Queue

20. Dispatch async, sync
• dispatch_async
• dispatch_sync

21. Singleton
(for initialisation)
• dispatch_once
static MyObject *myObject = nil;
+(MyObject *)sharedInstance
{
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
if(!myObject){
myObject = [[myObject alloc] init];
}
return myObject;
}

22. Lockless Exclusion
• Serial queue are thread safe inside
-(void)accessSharedVariableAsync:(void(^)(id sharedVariable))block{
// myQueue must be a serial queue
dispatch_sync(myQueue, ^{
block([self sharedVariable]);
});
}

23. dispatch_group
• dispatch_group_async
• dispatch_group_notify

24. Dispatch
• dispatch_apply
• dispatch_barrier_async
• dispatch_source_create

25. Deadlock

27. Q &A

28. Best Sources
• How To Use NSOperations and
NSOperationQueues http://
www.raywenderlich.com/19788/how-touse-nsoperations-and-nsoperationqueues

29. • NSOperation vs Grand Central Dispatch
• http://stackoverflow.com/questions/10373331/
nsoperation-vs-grand-central-dispatch
• Grand Central Dispatch Design Patterns by
Robert Brown
• http://www.slideshare.net/robby_brown/grandcentral-dispatch-design-patterns