1. GCD and Concurrency
Programming
allenlinli
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