Pragma Mark: Simple Concurrent Programming

1. Simple Concurrent Programming Chris Eidhof
2. First, a story.
3. [self performSelector:@selector(update) withObject:nil afterDelay:0]
4. dispatch_async(dispatch_get_main_queue(), ^{ [self update]; });
5. double delayInSeconds = 0.1; dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC)); dispatch_after(popTime, dispatch_get_main_queue(), ^(void){ // make sure this gets triggered after a layout pass. }); Rethink your code.
6. • Your scrolling is too slow • When doing network requests, your app gets unresponsive • When importing data, your app gets unresponsive
7. • GPU • CPU • Areas in memory (objects, numbers, ...) • Files It's all about shared resources
8. Why is concurrency hard?
9. 1. Write code
10. 2. ⌘-R
11. 3. Test on your device: everything works
12. Better: TDD
13. 1. Write Unit Test
14. 2. Write Code
15. 3. ⌘-U: All tests succeeded
16. Problem You might still have bugs.
17. It's hard to test asynchronous apps, because it's non- deterministic Why is concurrency hard?
18. Lots of tricky problems (more on that later) Why is concurrency hard?
19. Before you even consider redesigning your code to support concurrency, you should ask yourself whether doing so is necessary. — Concurrency Programming Guide
20. 1. Determine what is slow: analyze and measure 2. If neccessary: factor out units of work 3. Isolate the units in a separate queue 4. Measure again Recipe Improving the responsiveness of your app
21. Problems and solutions
22. Problem Your scrolling is too slow
23. Draw Complicated Drawing Core data URL Request Draw (The main thread) What's happening?
24. What's happening? Shared resource: CPU
25. We should free up the main thread so that drawing and user input gets handled immediately How to solve this?
26. UIKit is not thread-safe.
27. 1. Take drawRect: code and isolate it into an operation 2. Replace original view by UIImageView 3. Update UIImageView on main thread Async drawing
28. - (void)drawRect:(CGRect)rect { CGContextRef ctx = UIGraphicsGetCurrentContext(); // expensive // drawing // code } Before:
29. [queue addOperationWithBlock:^{ UIGraphicsBeginImageContextWithOptions(size, NO, 0); // expensive // drawing // code UIImage *i = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext(); [[NSOperationQueue mainQueue] addOperationWithBlock:^{ self.imageView.image = i; }]; }]; After:
30. Problem Loading an image over the network
31. dispatch_async(backgroundQueue, ^{ NSData* imageData = [NSData dataWithContentsOfURL:url]; UIImage* image = [[UIImage alloc] initWithData:imageData]; dispatch_async(dispatch_get_main_queue(), ^{ self.imageView = image; }); }); Warning: please don't use this code Problem Loading an image over the network
32. dispatch_async(backgroundQueue, ^{ NSData* imageData = [NSData dataWithContentsOfURL:url]; UIImage* image = [[UIImage alloc] initWithData:imageData]; dispatch_async(dispatch_get_main_queue(), ^{ self.imageView = image; }); }); How to cancel this? Problem Loading an image over the network
33. dispatch_async(backgroundQueue, ^{ NSData* imageData = [NSData dataWithContentsOfURL:url]; UIImage* image = [[UIImage alloc] initWithData:imageData]; dispatch_async(dispatch_get_main_queue(), ^{ self.imageView = image; }); }); What happens if the request times out? Problem Loading an image over the network
34. GCD Thread Pool Main Thread High Priority Queue Serial Queue Parallel Queue Serial Queue Main Queue Serial Queue Concurrent Queue Serial Queue Default Priority Queue Low Priority Queue Background Priority Queue Custom Queues GCD Queues Threads Aside GCD
35. What if the request is blocking the queue?
36. GCD Thread Pool Main Thread High Priority Queue Serial Queue Parallel Queue Serial Queue Main Queue Serial Queue Concurrent Queue Serial Queue Default Priority Queue Low Priority Queue Background Priority Queue Custom Queues GCD Queues Threads GCD adds more threads. This is expensive.
37. Built on top of Grand Central Dispatch Operation Queues
38. NSOperationQueue* downloadQueue; [downloadQueue addOperationWithBlock:^{ NSData* imageData = [NSData dataWithContentsOfURL:url]; UIImage* image = [UIImage imageWithData:imageData]; [[NSOperationQueue mainQueue] addOperationWithBlock:^{ self.imageView.image = image; }]; }]; How to cancel this? Step 1: Use NSOperation
39. NSBlockOperation* downloadOperation = [NSBlockOperation blockOperationWithBlock:^{ NSData* imageData = [NSData dataWithContentsOfURL:url]; UIImage* image = [UIImage imageWithData:imageData]; [[NSOperationQueue mainQueue] addOperationWithBlock:^{ self.imageView.image = image; }]; }]; [downloadQueue addOperation:downloadOperation]; // Sometime later [downloadOperation cancel]; Step 2: Use NSBlockOperation
40. NSOperation* downloadOperation = [NSBlockOperation blockOperationWithBlock:^{ NSData* imageData = [NSData dataWithContentsOfURL:url]; self.downloadedImage = [UIImage imageWithData:imageData]; }]; downloadOperation.completionBlock = ^{ [[NSOperationQueue mainQueue] addOperationWithBlock:^{ self.imageView.image = self.downloadedImage; }]; }; Step 3: Pull out the completion handler
41. NSOperation* downloadOperation = [DownloadOperation downloadOperationWithURL:url]; downloadOperation.completionBlock = ^{ [[NSOperationQueue mainQueue] addOperationWithBlock:^{ self.imageView.image = image; }]; }; Step 4: Custom NSOperation subclass
42. NSURLSession* session = [NSURLSession sharedSession]; [session downloadTaskWithURL:url completionHandler: ^(NSURL *location, NSURLResponse *response, NSError *err) { // Process downloaded data. }]; (or use NSURLSession)
43. Problem You want to import a (large) data set
44. Normal Core Data Stack
45. NSManagedObjectContext NSManagedObjectContext NSManagedObjectContext SQLite NSPersistentStore NSPersistentStoreCoordinator Double MOC Stack
46. NSManagedObjectContext* context = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType]; context.persistentStoreCoordinator = self.ptStoreCoordinator; [self.context [[performBlock:^{]] [self import]; }]; Small imports
47. [[NSNotificationCenter defaultCenter] addObserverForName:NSManagedObjectContextDidSaveNotification object:nil queue:nil usingBlock:^(NSNotification* note) { NSManagedObjectContext *moc = self.mainMOC; if (note.object != moc) { [moc performBlock:^(){ [moc mergeChangesFromContextDidSaveNotification:note]; }]; }; }];
48. NSManagedObjectContext NSManagedObjectContext NSManagedObjectContext ! SQLite NSPersistentStore NSPersistentStoreCoordinator ! ! ! Double MOC Stack
49. You might want to consider using a different concurrency style, and this time you have two persistent store coordinators, two almost completely separate Core Data stacks. Source: http://asciiwwdc.com/2013/sessions/211
50. SQLite ! NSPersistentStore NSPersistentStoreCoordinator NSManagedObjectContext NSPersistentStore NSPersistentStoreCoordinator NSManagedObjectContext ! ! ! NSPersistentStore NSPersistentStoreCoordinator NSManagedObjectContext NSPersistentStore NSPersistentStoreCoordinator NSManagedObjectContext ! ! !
51. What about fetched results controllers?
53. General concurrency problems Locking
54. @interface Account : NSObject @property (nonatomic) double balance; - (void)transfer:(double)euros to:(Account*)other; @end
55. - (void)transfer:(double)euros to:(Account*)other { self.balance = self.balance - euros; other.balance = other.balance + euros; } What happens if two methods call this method at the same time? From different threads?
56. - (void)transfer:(double)euros to:(Account*)other { double currentBalance = self.balance; self.balance = currentBalance - euros; double otherBalance = other.balance; other.balance = otherBalance + euros; } The same code, how the compiler sees it
57. a b [a.transfer:20 to:b] [a.transfer:30 to:b] 100 0 currentBalance = 100 currentBalance = 100 100 0 a.balance = 100 - 20 80 0 b.balance = b.balance + 20 80 20 a.balance = 100 - 30 70 20
58. a b [a.transfer:20 to:b] [a.transfer:30 to:b] 100 0 currentBalance = 100 currentBalance = 100 100 0 a.balance = 100 - 20 80 0 b.balance = b.balance + 20 80 20 a.balance = 100 - 30 70 20
59. - (void)transfer:(double)euros to:(Account*)other { @synchronized(self) { self.balance = self.balance - euros; other.balance = other.balance + euros; } }
60. - (void)transfer:(double)euros to:(Account*)other { @synchronized(self) { @synchronized(other) { self.balance = self.balance - euros; other.balance = other.balance + euros; } } } Problem: deadlock.
61. - (void)transfer:(double)euros to:(Account*)other { @synchronized(self.class) { self.balance = self.balance - euros; other.balance = other.balance + euros; } } Working, but possibly slow
62. - (void)transfer:(double)euros to:(Account*)other { objc_sync_enter(self.class) objc_exception_try_enter setjmp objc_exception_extract self.balance = self.balance - euros; other.balance = other.balance + euros; objc_exception_try_exit objc_sync_exit(self.class) ... objc_exception_throw ... } }
63. Account* account = [Account new]; Account* other = [Account new]; dispatch_queue_t accountOperations = dispatch_queue_create("accounting", DISPATCH_QUEUE_SERIAL); dispatch_async(accountOperations, ^{ [account transfer:200 to:other]; }); dispatch_async will never block. Do it the GCD way
64. Realize: at which level in your app do you want to be concurrent?
65. + (id)sharedInstance { static id sharedInstance = nil; @synchronized(self) { if (sharedInstance == nil) { sharedInstance = [[self alloc] init]; } } return sharedInstance; } Singletons
66. + (id)sharedInstance { static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ sharedInstance = [[self alloc] init]; }); } Faster than synchronized, but still blocking.
67. • Lock contention • Starvation • Priority Inversion Other Problems
68. • Lots of subtle problems that don't show up during testing. • Keep it very simple • Use the main thread when possible Conclusion
69. ... one more thing
70. Pasta Theory of Software
71. Spaghetti Code
72. Lasagna Code
73. The ideal software structure is one having components that are small and loosely coupled; this ideal structure is called ravioli code. In ravioli code, each of the components, or objects, is a package containing some meat or other nourishment for the system; any component can be modified or replaced without significantly affecting other components. — http://www.gnu.org/fun/jokes/pasta.code.html Ravioli Code
74. • chris@eidhof.nl • @chriseidhof • http://www.objc.io • http://www.uikonf.com Thanks
75. Resources
76. • Concurrency Programming Guide • http://googlemac.blogspot.de/2006/10/ synchronized-swimming.html • NSOperationQueue class reference • http://www.objc.io/issue-2/ • http://www.gnu.org/fun/jokes/pasta.code.html • https://developer.apple.com/library/ios/ documentation/Cocoa/Conceptual/Multithreading/ ThreadSafetySummary/ • http://www.opensource.apple.com/source/objc4/ objc4-551.1/runtime/objc-sync.mm
77. • WWDC12 #211: Concurrent User Interfaces on iOS