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
   

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2. Why Multithreading?
   

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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
   

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4. How to do concurrent work?
   

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

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

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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
   

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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
   

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9. 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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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
    

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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
    

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12. Living in threaded environments
    

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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
    

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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
    

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15. Making use of queues
    

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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 *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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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 *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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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 *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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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 *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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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 *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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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 *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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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 *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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23. 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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24. Being thread safe
    

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25. 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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26. 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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27. 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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28. 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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29. 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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30. 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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31. 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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32. 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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33. 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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34. 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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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
    

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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
    

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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
    

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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
    

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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
    

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40. 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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41. 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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42. 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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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
    

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44. Convenience
    

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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
    

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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
    

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

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