Objective-C, cancel a dispatch queue using UI event

Question

Scenario:

• User taps a button asking for some kind of modification on address book.
• A method is called to start this modification and an alert view is

Answer 1

If you declare your BOOL using __block, then it can be changed outside of the block execution, and the block will see the new value. See the documentation for more details.

An example:

@interface SNViewController ()
{
    BOOL*   cancelledPtr;
}

@end

@implementation SNViewController

- (IBAction)start:(id)sender
{
    __block BOOL cancelled = NO;

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        while (!cancelled) {
            NSLog(@"running");
            sleep(1);
        }        
        NSLog(@"stopped");
    });

    cancelledPtr = &cancelled;
}

- (IBAction)stop:(id)sender
{
    if (cancelledPtr)
    {
        NSLog(@"stopping");

        *cancelledPtr = YES;
    }
}

@end

Alternatively, use an ivar in your class to store the BOOL. The block will implicitly make a copy of self and will access the ivar via that. No need for __block.

@interface SNViewController ()
{
    BOOL   cancelled;
}

@end

@implementation SNViewController

- (IBAction)start:(id)sender
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        while (!cancelled) {
            NSLog(@"running");
            sleep(1);
        }        
        NSLog(@"stopped");
    });
}

- (IBAction)stop:(id)sender
{
    NSLog(@"stopping");
    cancelled = YES;
}

@end