iOS developers have a "dispatch_after(when, queue, block)" Grand Central Dispatch (GCD) function they can utilize but once it's set up, these calls can not be easily cancelled.

Describe how you might implement a more convenient version of this named "cancellable_dispatch_after"

Question

iOS developers have a "dispatch_after(when, queue, block)" Grand Central Dispatch (GCD) function they can utilize but once it's set up, these calls can not be easily cancelled.

Describe how you might implement a more convenient version of this named "cancellable_dispatch_after"

Sigiloso · Accepted Answer

If we create an object called BoolNumber, e.g.:

class BoolNumber : NSObject {
  @property (readwrite) Bool boolValue;
}

We can return it from our cancellable_dispatch_after method like so:

- (BoolNumber *) cancellable_dispatch_after(when, queue, block) {
    BoolNumber *cancelState = [[BoolNumber alloc] init];
   // the newBlock only runs after "when" and only if the cancelState bool value is set to false
   newBlock = ^{
	if ( cancelState.boolValue == NO)
             block()
   };
   dispatch_after(when, queue, newBlock);
  return cancelState;
}

So anybody calling this API will have a BoolNumber object which can cancel the block from running anytime during the "when" waiting period.

Sigiloso · Answer

Wrapping the intended block is the way to go:

@interface CancellationToken : NSObject
-(void)cancel;
@end

@interface CancellationToken ()
@property BOOL cancelled;
@end

@implementation CancellationToken
-(void)cancel {
    self.cancelled = @YES;
}

@end

CancellationToken *cancellable_dispatch_after(dispatch_time_t time, dispatch_queue_t queue, dispatch_block_t block) {
    __block CancellationToken  *token = [CancellationToken new];
    dispatch_block_t wrapper = ^{
        if (!token.cancelled) {
            block();
        }
    };
    
    dispatch_after(time, queue, wrapper);
    return token;
}



Use thusly:
CancellationToken *token = cancellable_dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 15*NSEC_PER_SEC), q, ^{
        NSLog(@"Here");
    });
    
    [token cancel];

Marcio · Answer

Just use create a block a cancel using dispath_block_cancel(...)`

dispatch_block_t block = dispatch_block_create(DISPATCH_BLOCK_INHERIT_QOS_CLASS, ^{
        UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"I'm an alert" message:@"done" preferredStyle:UIAlertControllerStyleAlert];
        [self presentViewController:alert animated:YES completion:nil];
    });

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), dispatch_get_main_queue(), block);

dispatch_block_cancel(block); // Canceling the block

Yury Ramanchuk · Answer

You can make assosiation with block (pay attentions to block copy and retain cycle)


-(NSObject *)getAssosiation:(dispatch_block_t)block {
    return objc_getAssociatedObject(block, @selector(getAssosiation:));
}

-(void)setAssosiation:(dispatch_block_t)block {
    objc_setAssociatedObject(block, @selector(getAssosiation:), [NSObject new], OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (void)cancelBlock:(dispatch_block_t)block {
    [self setAssosiation:block];
}

-(dispatch_block_t) dispatch_after_with_cancel:(NSTimeInterval) delay  for:(dispatch_block_t) block {
    
    dispatch_block_t cancellable = ^{
        if (![self getAssosiation:block]) {
            block();

        }
    };
    
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delay * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        cancellable();
    });
    
    return cancellable;
}



- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
    dispatch_block_t block = ^{
        NSLog(@"test");
    };
    
    [self dispatch_after_with_cancel:5 for:block];

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        [self cancelBlock:block];
    });

}

Gui · Answer

Create a function that returns a block which can be used to cancel the operation:

dispatch_block_t cancellable_dispatch_after(dispatch_time_t time, dispatch_queue_t queue, dispatch_block_t block)
{
    __block BOOL cancelled = NO;
    
    dispatch_after(time, queue, ^{
        if (cancelled) return;
        
        block();
    });
    
    return ^{
        cancelled = YES;
    };
}

Usage:

        dispatch_block_t cancelOperation = cancellable_dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"Dispatched");
        });
        
        cancelOperation();

simple · Answer

I don't understand why we would need so complicated solution. This simple solution just works fine for me: 

var pleaseCancel = false

func call() {
DispatchQueue.global().asyncAfter(deadline: DispatchTime.now() + .seconds(4)) {
	print("in the block")
	if !pleaseCancel {
		print("the block's not cancelled")
	}
}
}

call()
pleaseCancel = true

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