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Multithreading on iOS

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Why Multithreading?

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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

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How to do concurrent work?

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How to do concurrent work? • NSThread • NSRunLoop • NSOperation / NSOperationQueue • Grand Central Dispatch Available concepts

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How to do concurrent work? • NSThread • NSRunLoop • NSOperation / NSOperationQueue • Grand Central Dispatch Available concepts

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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

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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

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How to do concurrent work? • Serial and concurrent queues • QoS classes • Main queue, default queues and custom queues Grand Central Dispatch

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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

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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

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Living in threaded environments

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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

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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

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Making use of queues

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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 *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

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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 *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

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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 *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

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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 *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

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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 *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

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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 *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

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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 *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

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Making use of queues @interface Event : NSObject + (NSProgress *)getWithCoordinate:(CLLocationCoordinate2D)coordinate
 completionHandler:(void(^)(
 NSArray *events,
 NSError *error) )completionHandler; @end Data Source

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Being thread safe

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Being thread safe @interface Event : NSObject @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

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Being thread safe @interface Event : NSObject @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

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Being thread safe @interface Event : NSObject @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

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Being thread safe @interface Event : NSObject @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

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Being thread safe @interface Event : NSObject @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

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Being thread safe @interface Event : NSObject @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

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Being thread safe @interface Event : NSObject @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

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Being thread safe @interface FavoriteEventsController : NSObject @property (nonatomic, strong, readonly) NSArray *events; - (void)addEvent:(Event *)event; - (void)removeEvent:(Event *)event; @end Thread safety

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Being thread safe @interface FavoriteEventsController : NSObject @property (nonatomic, strong, readonly) NSArray *events; - (void)addEvent:(Event *)event; - (void)removeEvent:(Event *)event; @end Thread safety

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Being thread safe @interface FavoriteEventsController : NSObject @property (nonatomic, strong, readonly) NSArray *events; - (void)addEvent:(Event *)event; - (void)removeEvent:(Event *)event; @end Thread safety

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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

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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

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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

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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

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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

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Being thread safe - (NSArray *)events { __block NSArray *events; dispatch_sync(self.workerQueue, ^{ events = _events; }); return events; } - (void)setEvents:(NSArray *)events { dispatch_barrier_async(self.workerQueue, ^{ _events = events; }); } Thread safety - FavoriteEventsController

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Being thread safe - (NSArray *)events { __block NSArray *events; dispatch_sync(self.workerQueue, ^{ events = _events; }); return events; } - (void)setEvents:(NSArray *)events { dispatch_barrier_async(self.workerQueue, ^{ _events = events; }); } Thread safety - FavoriteEventsController

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Being thread safe - (NSArray *)events { __block NSArray *events; dispatch_sync(self.workerQueue, ^{ events = _events; }); return events; } - (void)setEvents:(NSArray *)events { dispatch_barrier_async(self.workerQueue, ^{ _events = events; }); } Thread safety - FavoriteEventsController

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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

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Convenience

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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

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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

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Thank you Michael Ochs @_mochs http://ios-coding.com