iOS Concurrency

Concurrency

Grand Central Dispatch

Serial and Concurrent Queues

The following example is an app with the following UI elements:

• A button to add a job to a serial queue.
• A button to add a job to the default global concurrent queue.
• A text field to display results when a job has been completed.

import "MyViewController.h"

@interface MyViewController : UIViewController

@property (weak, nonatomic) IBOutlet UITextView *myTextView;
@property (weak, nonatomic) IBOutlet UIButton *serialButton;
@property (weak, nonatomic) IBOutlet UIButton *concurrentButton;

// Must be publically available for it to be called from another thread.
-(void)delayAndDisplay:(NSString*) queueType;
@end

@implementation MyViewController

- (void)sendJobToSerialQueue {
    // Creates a serial queue with a specified identifier.
    dispatch_queue_t serialQueue =
        dispatch_queue_create("mySerialQueue", DISPATCH_QUEUE_SERIAL);

    dispatch_async(serialQueue, ^{
        [self delayAndDisplay:@"Serial"];
    });
}

-(void)sendJobToConcurrentQueue {
    /**
        Gets a reference to one of the global concurrent queues.

        Global queues ranked by highest priority first:
        DISPATCH_QUEUE_PRIORITY_HIGH
        DISPATCH_QUEUE_PRIORITY_DEFAULT
        DISPATCH_QUEUE_PRIORITY_LOW
        DISPATCH_QUEUE_PRIORITY_BACKGROUND
    */
    dispatch_queue_t concurrentQueue =
        dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    dispatch_async(concurrentQueue, ^{
        [self delayAndDisplay:@"Concurrent"];
    });
}

-(void)delayAndDisplay:(NSString *)queueType {
    // Sleep
    [NSThread sleepForTimeInterval: 3.0];

    /**
        All UI updates must be done on the main thread, which
        can be referenced via `dispatch_get_main_queue`.
    */
    dispatch_async(dispatch_get_main_queue(), ^{
        NSString *text = [[self myTextView] text];

        [[self myTextView] setText:[NSString stringWithFormat:@"%@\n%@", text, queueType]];
    });
}

- (IBAction)onButtonTap:(id)sender {
    if(sender == _serialButton) {
        [self sendJobToSerialQueue];
    }
    else {
        [self sendJobToConcurrentQueue];
    }
}
@end

References

• Apple Developer
  • GCD Reference
• Creating dispatch queues in Swift 3 - Stack Overflow
• Grand Central Dispatch - Ray Wenderlich
• GCD 101 - Hacking With Swift

NSOperationQueue

The example before has a single button and a text field that logs are printed to.

#import "MyViewController.h"

@interface MyViewController : UIViewController

@property (weak, nonatomic) IBOutlet UITextView *myTextView;
@property (weak, nonatomic) IBOutlet UIButton *performJobButton;

-(void)display:(NSString*) queueType withDelay:(NSTimeInterval) delayInSeconds;
@end

@implementation MyViewController

- (IBAction)onButtonTap:(id)sender {
    // Create an operation queue
    NSOperationQueue *queue = [NSOperationQueue new];

    // Convenience method to add a block operation to the queue
    [queue addOperationWithBlock:^{
        [self display:@"Simple job done" withDelay:1.0];
    }];

    /**
        For granular control over operations, NSOperation instances must
        be created.

        In this example, secondPart is dependent on firstPart and will
        not be executed until it's dependency is complete.
     */
    NSOperation *firstPart = [NSBlockOperation blockOperationWithBlock:^{
        [self display:@"First part done" withDelay:1.0];
    }];

    // Add a block to be executed on operation completion.
    firstPart.completionBlock = ^{
        [self display:@"boo" withDelay:1.0];
    };

    NSOperation *secondPart = [NSBlockOperation blockOperationWithBlock:^{
        [self display:@"Second part done" withDelay:2.0];
    }];

    // Make the second part a dependency of the first part.
    [secondPart addDependency:firstPart];

    // Can be done in any order, since a dependency between
    // the two operations has been established.
    [queue addOperation:secondPart];
    [queue addOperation:firstPart];
}

-(void)display:(NSString *) message withDelay:(NSTimeInterval) delayInSeconds {

    [NSThread sleepForTimeInterval: delayInSeconds];

    NSString *dateString = [[NSDate date] description];

    dispatch_async(dispatch_get_main_queue(), ^{
        NSString *text = [[self myTextView] text];
        [[self myTextView] setText:[NSString stringWithFormat:@"%@\n%@\t%@", text, message, dateString]];
    });
}
@end

References

• Apple Developers
  • UIApplicationDelegate
  • Background execution
• applicationWillEnterForeground vs applicationDidBecomeActive - Stack Overflow
• Multithreading in iOS - Codepath