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

Answer 2

Approach 1

Create a custom dispatch_async method that returns a "cancelable" block.

// The dispatch_cancel_block_t takes as parameter the "cancel" directive to suspend the block execution or not whenever the block to execute is dispatched. 
// The return value is a boolean indicating if the block has already been executed or not.
typedef BOOL (^dispatch_cancel_block_t)(BOOL cancelBlock);

dispatch_cancel_block_t dispatch_async_with_cancel_block(dispatch_queue_t queue, void (^block)())
{
    __block BOOL execute = YES;
    __block BOOL executed = NO;

    dispatch_cancel_block_t cancelBlock = ^BOOL (BOOL cancelled) {
        execute = !cancelled;
        return executed == NO;
    };

    dispatch_async(queue, ^{
        if (execute)
            block();
        executed = YES;
    });

    return cancelBlock;
}

- (void)testCancelableBlock
{
    dispatch_cancel_block_t cancelBlock = dispatch_async_with_cancel_block(dispatch_get_main_queue(), ^{
        NSLog(@"Block 1 executed");
    });

    // Canceling the block execution
    BOOL success1 = cancelBlock(YES);
    NSLog(@"Block is cancelled successfully: %@", success1?@"YES":@"NO");

    // Resuming the block execution
    // BOOL success2 = cancelBlock(NO);
    // NSLog(@"Block is resumed successfully: %@", success2?@"YES":@"NO");
}

Approach 2

Defining a macro for executing a block asynchronously if a condition is validated:

#define dispatch_async_if(queue,condition,block) \
dispatch_async(queue, ^{\
    if (condition == YES)\
        block();\
});

- (void)testConditionBlock
{
    // Creating condition variable
    __block BOOL condition = YES;

    dispatch_async_if(dispatch_get_main_queue(), condition, ^{
        NSLog(@"Block 2 executed");
    });

    // Canceling the block execution
    condition = NO;

    // Also, we could use a method to test the condition status
    dispatch_async_if(dispatch_get_main_queue(), ![self mustCancelBlockExecution], ^{
        NSLog(@"Block 3 executed");
    });
}

Answer 3

Try to apply the following code sample to your situation:

__block UIView * tempView = [[UIView alloc] initWithFrame:CGRectMake(50, 100, 220, 30)];
[tempView setBackgroundColor:[UIColor grayColor]];
[self.view addSubview:tempView];
[tempView release];

__block BOOL cancel = NO;
//点击之后就会开始执行这个方法
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
    int i = 0;
    while (i < 1000000000 && cancel == NO) {
        i++;
    }
    NSLog(@"Task end: i = %d", i);
    //这个不会执行，因为在之前，gcd task已经结束
    [tempView removeFromSuperview];
});

//1s 之后执行这个方法
double delayInSeconds = 1.0;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
    NSLog(@"A GCD Task Start");
    cancel = YES;
    [tempView setBackgroundColor:[UIColor blackColor]];
});