dispatch_after with the ability to cancel execution

DispatchAfterWithCancel.h

//
//  DispatchAfterWithCancel.h
//
//  Created by Jeff Burt on 4/18/16.
//  Copyright © 2016 Jeff Burt. All rights reserved.
//

#ifndef DispatchAfterWithCancel_h
#define DispatchAfterWithCancel_h

typedef void(^dispatch_after_with_cancel_t)(BOOL shouldCancel);

/**
 *  Provides a dispatch_after block that can be canceled before its execution.
 *  The block will be called on the main thread.
 *
 *  To cancel the block before it executes, simply call `cancel_block` with
 *  the block returned by this function.
 *
 *  This function will always return a non-null `dispatch_after_with_cancel_t`
 *  block, even if the `block` provided is NULL.
 *
 *  @param delay the time to wait before execution
 *  @param block the block to execute
 *
 *  @return a block that will execute after the set delay. Calling `cancel_block`
 *  with this block will cancel the block before it executes.
 */
NS_INLINE dispatch_after_with_cancel_t dispatch_after_with_cancel(NSTimeInterval delay, dispatch_block_t block) {
    __block dispatch_block_t originalBlock = [block copy];
    
    __block dispatch_after_with_cancel_t cancelableBlock = ^(BOOL shouldCancel) {
        if (shouldCancel == NO && originalBlock) {
            originalBlock();
        }
        
        originalBlock = NULL;
    };
    
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, delay * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
        if (cancelableBlock) {
            cancelableBlock(NO);
            cancelableBlock = NULL;
        }
    });
    
    return cancelableBlock;
}

/**
 *  Cancels the block before it executes. Does nothing when a NULL block is
 *  provided.
 *
 *  @param block the block to cancel
 */
NS_INLINE void cancel_block(dispatch_after_with_cancel_t block) {
    if (block) {
        block(YES);
        block = NULL;
    }
}

#endif

ViewController.m

//
//  ViewController.m
//
//  Created by Jeffrey Burt on 4/18/16.
//  Copyright © 2016 Jeff Burt. All rights reserved.
//

#import "ViewController.h"
#import "DispatchAfterWithCancel.h"

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    dispatch_after_with_cancel_t block = dispatch_after_with_cancel(3, ^{
        NSLog(@"Hello world!");
    });
    
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSLog(@"Canceling block early");
        cancel_block(block);
    });
}

@end

Looks like somebody is psyching up for an upcoming Facebook interview and a potential move to the Left Coast. The original Glassdoor article (and question) you're responding to is mine. 😜 Best of luck to you.