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There's no technique to cancel blocks which were defined within a dispatch_after call. There's no handle or reference to them.

Yes. There's no technique to cancel blocks which were defined within a dispatch_after call.

But you can use a boolean that wrap the block to determine whether execute in the block or not.

Inside of the dispatch_after function block parameter, add a flag to determine if the code is to be run or not. If the flag is set to cancel when the block is executed, nothing will happen and effectively cancels the 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");
}