Block execution by a flag of blocks queued with dispatch_after.

cancel_dispatcher_by_flag.m

//Flag
_isCancelled = @(NO);
 
//Create a queue
dispatch_queue_t queue = dispatch_queue_create("queue", NULL);
 
//Insert 3.000 blocks executed with a delay of 1 sec
for (int i=0; i<3000; i++) {
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1* NSEC_PER_SEC)), queue, ^(void){
        if(![_isCancelled boolValue])
            NSLog(@"Block #%d executed", i);
        else
            NSLog(@"Block #%d deleted", i);
    });
}
 
//After 2 secs set the flag to doesn't execute the other blocks
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2* NSEC_PER_SEC)), dispatch_get_main_queue(), ^(void){
    NSLog(@"DELETE BLOCKS FROM NOW");
    _isCancelled = @(YES);
});
 
/*
2014-03-22 23:43:33.891 test[2415:1303] Block #0 executed
2014-03-22 23:43:33.892 test[2415:1303] Block #1 executed
2014-03-22 23:43:33.893 test[2415:1303] Block #2 executed
2014-03-22 23:43:33.893 test[2415:1303] Block #3 executed
2014-03-22 23:43:33.893 test[2415:1303] Block #4 executed
.............
2014-03-22 23:43:34.889 test[2415:1303] Block #2380 executed
2014-03-22 23:43:34.889 test[2415:1303] Block #2381 executed
2014-03-22 23:43:34.890 test[2415:1303] Block #2382 executed
2014-03-22 23:43:34.890 test[2415:1303] Block #2383 executed
2014-03-22 23:43:34.890 test[2415:70b] DELETE BLOCKS FROM NOW
2014-03-22 23:43:34.890 test[2415:1303] Block #2384 executed
2014-03-22 23:43:34.890 test[2415:1303] Block #2385 deleted
2014-03-22 23:43:34.890 test[2415:1303] Block #2386 deleted
2014-03-22 23:43:34.891 test[2415:1303] Block #2387 deleted
2014-03-22 23:43:34.891 test[2415:1303] Block #2388 deleted
2014-03-22 23:43:34.891 test[2415:1303] Block #2389 deleted
.............
2014-03-22 23:43:35.153 test[2415:1303] Block #2998 deleted
2014-03-22 23:43:35.153 test[2415:1303] Block #2999 deleted
*/