Using grand central dispatch to run a callback after a delay, with hacky partial cancellation. Cancelling allows the callback to be deallocated, but doesn't completely clean up what's in GCD.

gistfile1.m

+(void) runBlock:(VoidBlock)callback
      afterDelay:(NSTimeInterval)delayInSeconds
          unless:(TOCCancelToken*)unlessCancelledToken {
    
    __block VoidBlock callbackForBlock = callback;
    NSObject* lock = [NSObject new];
    
    VoidBlock allowDeallocWrapper = ^{
        VoidBlock callbackPeek;
        @synchronized(lock) {
            callbackPeek = callbackForBlock;
        }
        if (callbackPeek == nil) return; // cancelled
        callbackPeek();
    };

    const int nanosPerSec = 1000*1000*1000;
    // simplification: just assume the double to dispatch_time_t conversion is going to work
    dispatch_time_t callTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds / nanosPerSec));
    
    dispatch_after(callTime,
                   dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
                   allowDeallocWrapper);
    
    // Simplification: assuming the cancellation token is not long lived (will discuss)
    [unlessCancelledToken whenCancelledDo:^{
        @synchronized(lock) {
            callbackForBlock = nil;
        }
    }];
}