Multithreading on iOS

This talk gives an overview why and when it is useful to consider to do stuff concurrently in an iOS application. It then goes into detail about some of the concepts and useful patterns that exist in Grand Central Dispatch.

Transcript

1. Multithreading on iOS
2. Why Multithreading?
3. Why Multithreading? • Keep the main thread free for UI rendering • Use all available CPU cores • Save energy and reduce battery consumption • Prioritise tasks to enable the system to optimise the workload
4. How to do concurrent work?
5. How to do concurrent work? • NSThread • NSRunLoop • NSOperation / NSOperationQueue • Grand Central Dispatch Available concepts
6. How to do concurrent work? • NSThread • NSRunLoop • NSOperation / NSOperationQueue • Grand Central Dispatch Available concepts
7. How to do concurrent work? • Queues are an abstraction on top of threads • Queues can have parent queues • Multiple queues can run on a single thread • A single queue can run on multiple threads • Multiple queues use a thread pool Threads vs. Queues
8. How to do concurrent work? • Configurable number of parallel executions • Cancellable • Dependencies, priorities, QoS classes • Based on GCD • Main queue and custom queues NSOperation / NSOperationQueue
9. How to do concurrent work? • Serial and concurrent queues • QoS classes • Main queue, default queues and custom queues Grand Central Dispatch
10. How to do concurrent work? • NSOperationQueue can control maximum parallel execution • GCD has default queues • GCD has groups and semaphores NSOperationQueue vs. Grand Central Dispatch
11. How to do concurrent work? • NSOperationQueue is more convenient • GCD has all the fancy shit • GCD is much more than just getting stuff off the main queue NSOperationQueue vs. Grand Central Dispatch
12. Living in threaded environments
13. Living in threaded environments • Make your models immutable • Have a single source of truth • Make your data source thread safe • Never rely on messages arriving on a certain queue unless explicitly documented • Explicitly document if you rely on being run on a certain queue Models
14. Living in threaded environments • Design your APIs for asynchronous use • Ensure completion handler are called on a well defined queue • Never use mutable data types on public APIs • Ensure mutating methods are thread safe API Design
15. Making use of queues
16. Making use of queues __weak typeof(self) weakSelf = self; qos_class qosClass = QOS_CLASS_USER_INITIATED; dispatch_queue_t queue = dispatch_get_global_queue(qosClass, 0); dispatch_async(queue, ^{ NSArray<Location *> *locations = // do some heavy work here dispatch_async(dispatch_get_main_queue(), ^{ typeof(weakSelf) self = weakSelf; [self.mapView addAnnotations:locations]; [self.mapView showAnnotations:locations animated:YES]; }); }); User initiated actions
17. Making use of queues __weak typeof(self) weakSelf = self; qos_class qosClass = QOS_CLASS_USER_INITIATED; dispatch_queue_t queue = dispatch_get_global_queue(qosClass, 0); dispatch_async(queue, ^{ NSArray<Location *> *locations = // do some heavy work here dispatch_async(dispatch_get_main_queue(), ^{ typeof(weakSelf) self = weakSelf; [self.mapView addAnnotations:locations]; [self.mapView showAnnotations:locations animated:YES]; }); }); User initiated actions
18. Making use of queues __weak typeof(self) weakSelf = self; qos_class qosClass = QOS_CLASS_USER_INITIATED; dispatch_queue_t queue = dispatch_get_global_queue(qosClass, 0); dispatch_async(queue, ^{ NSArray<Location *> *locations = // do some heavy work here dispatch_async(dispatch_get_main_queue(), ^{ typeof(weakSelf) self = weakSelf; [self.mapView addAnnotations:locations]; [self.mapView showAnnotations:locations animated:YES]; }); }); User initiated actions
19. Making use of queues __weak typeof(self) weakSelf = self; qos_class qosClass = QOS_CLASS_USER_INITIATED; dispatch_queue_t queue = dispatch_get_global_queue(qosClass, 0); dispatch_async(queue, ^{ NSArray<Location *> *locations = // do some heavy work here dispatch_async(dispatch_get_main_queue(), ^{ typeof(weakSelf) self = weakSelf; [self.mapView addAnnotations:locations]; [self.mapView showAnnotations:locations animated:YES]; }); }); User initiated actions
20. Making use of queues __weak typeof(self) weakSelf = self; qos_class qosClass = QOS_CLASS_USER_INITIATED; dispatch_queue_t queue = dispatch_get_global_queue(qosClass, 0); dispatch_async(queue, ^{ NSArray<Location *> *locations = // do some heavy work here dispatch_async(dispatch_get_main_queue(), ^{ typeof(weakSelf) self = weakSelf; [self.mapView addAnnotations:locations]; [self.mapView showAnnotations:locations animated:YES]; }); }); User initiated actions
21. Making use of queues __weak typeof(self) weakSelf = self; qos_class qosClass = QOS_CLASS_USER_INITIATED; dispatch_queue_t queue = dispatch_get_global_queue(qosClass, 0); dispatch_async(queue, ^{ NSArray<Location *> *locations = // do some heavy work here dispatch_async(dispatch_get_main_queue(), ^{ typeof(weakSelf) self = weakSelf; [self.mapView addAnnotations:locations]; [self.mapView showAnnotations:locations animated:YES]; }); }); User initiated actions
22. Making use of queues __weak typeof(self) weakSelf = self; qos_class qosClass = QOS_CLASS_USER_INITIATED; dispatch_queue_t queue = dispatch_get_global_queue(qosClass, 0); dispatch_async(queue, ^{ NSArray<Location *> *locations = // do some heavy work here dispatch_async(dispatch_get_main_queue(), ^{ typeof(weakSelf) self = weakSelf; [self.mapView addAnnotations:locations]; [self.mapView showAnnotations:locations animated:YES]; }); }); User initiated actions
23. Making use of queues @interface Event : NSObject <NSCopying, NSMutableCopying, NSCoding> + (NSProgress *)getWithCoordinate:(CLLocationCoordinate2D)coordinate
 completionHandler:(void(^)(
 NSArray<Event *> *events,
 NSError *error) )completionHandler; @end Data Source
24. Being thread safe
25. Being thread safe @interface Event : NSObject <NSCopying, NSMutableCopying, NSCoding> @property (nonatomic, strong, readonly) NSNumber *identifier; @property (nonatomic, strong, readonly) NSDate *startDate; @property (nonatomic, strong, readonly) NSString *name; @end @interface MutableEvent : Event @property (nonatomic, strong, readwrite) NSNumber *identifier; @property (nonatomic, strong, readwrite) NSDate *startDate; @property (nonatomic, strong, readwrite) NSString *name; @end Immutability
26. Being thread safe @interface Event : NSObject <NSCopying, NSMutableCopying, NSCoding> @property (nonatomic, strong, readonly) NSNumber *identifier; @property (nonatomic, strong, readonly) NSDate *startDate; @property (nonatomic, strong, readonly) NSString *name; @end @interface MutableEvent : Event @property (nonatomic, strong, readwrite) NSNumber *identifier; @property (nonatomic, strong, readwrite) NSDate *startDate; @property (nonatomic, strong, readwrite) NSString *name; @end Immutability
27. Being thread safe @interface Event : NSObject <NSCopying, NSMutableCopying, NSCoding> @property (nonatomic, strong, readonly) NSNumber *identifier; @property (nonatomic, strong, readonly) NSDate *startDate; @property (nonatomic, strong, readonly) NSString *name; @end @interface MutableEvent : Event @property (nonatomic, strong, readwrite) NSNumber *identifier; @property (nonatomic, strong, readwrite) NSDate *startDate; @property (nonatomic, strong, readwrite) NSString *name; @end Immutability
28. Being thread safe @interface Event : NSObject <NSCopying, NSMutableCopying, NSCoding> @property (nonatomic, strong, readonly) NSNumber *identifier; @property (nonatomic, strong, readonly) NSDate *startDate; @property (nonatomic, strong, readonly) NSString *name; @end @interface MutableEvent : Event @property (nonatomic, strong, readwrite) NSNumber *identifier; @property (nonatomic, strong, readwrite) NSDate *startDate; @property (nonatomic, strong, readwrite) NSString *name; @end Immutability
29. Being thread safe @interface Event : NSObject <NSCopying, NSMutableCopying, NSCoding> @property (nonatomic, strong, readonly) NSNumber *identifier; @property (nonatomic, strong, readonly) NSDate *startDate; @property (nonatomic, strong, readonly) NSString *name; @end @interface MutableEvent : Event @property (nonatomic, strong, readwrite) NSNumber *identifier; @property (nonatomic, strong, readwrite) NSDate *startDate; @property (nonatomic, strong, readwrite) NSString *name; @end Immutability
30. Being thread safe @interface Event : NSObject <NSCopying, NSMutableCopying, NSCoding> @property (nonatomic, strong, readonly) NSNumber *identifier; @property (nonatomic, strong, readonly) NSDate *startDate; @property (nonatomic, strong, readonly) NSString *name; @end @interface MutableEvent : Event @property (nonatomic, strong, readwrite) NSNumber *identifier; @property (nonatomic, strong, readwrite) NSDate *startDate; @property (nonatomic, strong, readwrite) NSString *name; @end Immutability
31. Being thread safe @interface Event : NSObject <NSCopying, NSMutableCopying, NSCoding> @property (nonatomic, strong, readonly) NSNumber *identifier; @property (nonatomic, strong, readonly) NSDate *startDate; @property (nonatomic, strong, readonly) NSString *name; @end @interface MutableEvent : Event @property (nonatomic, strong, readwrite) NSNumber *identifier; @property (nonatomic, strong, readwrite) NSDate *startDate; @property (nonatomic, strong, readwrite) NSString *name; @end Immutability
32. Being thread safe @interface FavoriteEventsController : NSObject @property (nonatomic, strong, readonly) NSArray<Event *> *events; - (void)addEvent:(Event *)event; - (void)removeEvent:(Event *)event; @end Thread safety
33. Being thread safe @interface FavoriteEventsController : NSObject @property (nonatomic, strong, readonly) NSArray<Event *> *events; - (void)addEvent:(Event *)event; - (void)removeEvent:(Event *)event; @end Thread safety
34. Being thread safe @interface FavoriteEventsController : NSObject @property (nonatomic, strong, readonly) NSArray<Event *> *events; - (void)addEvent:(Event *)event; - (void)removeEvent:(Event *)event; @end Thread safety
35. Being thread safe - (instancetype)init { self = [super init]; if (self) { dispatch_queue_attr_t attributes = DISPATCH_QUEUE_CONCURRENT; if (&dispatch_queue_attr_make_with_qos_class != NULL) { attr = dispatch_queue_attr_make_with_qos_class(attributes,
 QOS_CLASS_BACKGROUND, -1); } dispatch_queue_t workerQueue = dispatch_queue_create("fav", attr); _workerQueue = workerQueue; _events = @[]; } return self; } Thread safety - FavoriteEventsController
36. Being thread safe - (instancetype)init { self = [super init]; if (self) { dispatch_queue_attr_t attributes = DISPATCH_QUEUE_CONCURRENT; if (&dispatch_queue_attr_make_with_qos_class != NULL) { attr = dispatch_queue_attr_make_with_qos_class(attributes,
 QOS_CLASS_BACKGROUND, -1); } dispatch_queue_t workerQueue = dispatch_queue_create("fav", attr); _workerQueue = workerQueue; _events = @[]; } return self; } Thread safety - FavoriteEventsController
37. Being thread safe - (instancetype)init { self = [super init]; if (self) { dispatch_queue_attr_t attributes = DISPATCH_QUEUE_CONCURRENT; if (&dispatch_queue_attr_make_with_qos_class != NULL) { attr = dispatch_queue_attr_make_with_qos_class(attributes,
 QOS_CLASS_BACKGROUND, -1); } dispatch_queue_t workerQueue = dispatch_queue_create("fav", attr); _workerQueue = workerQueue; _events = @[]; } return self; } Thread safety - FavoriteEventsController
38. Being thread safe - (instancetype)init { self = [super init]; if (self) { dispatch_queue_attr_t attributes = DISPATCH_QUEUE_CONCURRENT; if (&dispatch_queue_attr_make_with_qos_class != NULL) { attr = dispatch_queue_attr_make_with_qos_class(attributes,
 QOS_CLASS_BACKGROUND, -1); } dispatch_queue_t workerQueue = dispatch_queue_create("fav", attr); _workerQueue = workerQueue; _events = @[]; } return self; } Thread safety - FavoriteEventsController
39. Being thread safe - (instancetype)init { self = [super init]; if (self) { dispatch_queue_attr_t attributes = DISPATCH_QUEUE_CONCURRENT; if (&dispatch_queue_attr_make_with_qos_class != NULL) { attr = dispatch_queue_attr_make_with_qos_class(attributes,
 QOS_CLASS_BACKGROUND, -1); } dispatch_queue_t workerQueue = dispatch_queue_create("fav", attr); _workerQueue = workerQueue; _events = @[]; } return self; } Thread safety - FavoriteEventsController
40. Being thread safe - (NSArray<Event *> *)events { __block NSArray *events; dispatch_sync(self.workerQueue, ^{ events = _events; }); return events; } - (void)setEvents:(NSArray<Event *> *)events { dispatch_barrier_async(self.workerQueue, ^{ _events = events; }); } Thread safety - FavoriteEventsController
41. Being thread safe - (NSArray<Event *> *)events { __block NSArray *events; dispatch_sync(self.workerQueue, ^{ events = _events; }); return events; } - (void)setEvents:(NSArray<Event *> *)events { dispatch_barrier_async(self.workerQueue, ^{ _events = events; }); } Thread safety - FavoriteEventsController
42. Being thread safe - (NSArray<Event *> *)events { __block NSArray *events; dispatch_sync(self.workerQueue, ^{ events = _events; }); return events; } - (void)setEvents:(NSArray<Event *> *)events { dispatch_barrier_async(self.workerQueue, ^{ _events = events; }); } Thread safety - FavoriteEventsController
43. Being thread safe - (void)addEvent:(Event *)event { dispatch_barrier_async(self.workerQueue, ^{ _events = [_events arrayByAddingObject:event]; }); } - (void)removeEvent:(Event *)event { dispatch_barrier_async(self.workerQueue, ^{ NSPredicate *p = [NSPredicate
 predicateWithFormat:@"NOT (SELF == %@)", event]; _events = [_events filteredArrayUsingPredicate:p]; }); } Thread safety - FavoriteEventsController
44. Convenience
45. Convenience dispatch_group_t fetchGroup = dispatch_group_create(); for (int i = 0; i < 10; i++) { dispatch_group_enter(fetchGroup); [Fetcher doWithCompletionHandler:^(BOOL success) { // Handle completion dispatch_group_leave(fetchGroup); }]; } dispatch_group_wait(fetchGroup, DISPATCH_TIME_FOREVER); // Do stuff after all completions finished Dispatch groups - Synchronously
46. Convenience dispatch_group_t fetchGroup = dispatch_group_create(); for (int i = 0; i < 10; i++) { dispatch_group_enter(fetchGroup); [Fetcher doWithCompletionHandler:^(BOOL success) { // Handle completion dispatch_group_leave(fetchGroup); }]; } dispatch_group_notify(fetchGroup, dispatch_get_main_queue(), ^{ // Do stuff after all completions finished }); Dispatch groups - Asynchronously
47. Thank you Michael Ochs @_mochs http://ios-coding.com