Gcd and concurrency programming
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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.
![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;
}](https://image.slidesharecdn.com/gcdandconcurrencyprogramming-140110122735-phpapp02/85/Gcd-and-concurrency-programming-21-320.jpg)
![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]);
});
}](https://image.slidesharecdn.com/gcdandconcurrencyprogramming-140110122735-phpapp02/85/Gcd-and-concurrency-programming-22-320.jpg)
Gcd and concurrency programming
- 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)
- 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
- 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]); }); }
- 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