1.dispatch_block_cancel

iOS8之后可以调用dispatch_block_cancel来取消（需要注意必须用dispatch_block_create创建dispatch_block_t）

let task2 = dispatch_block_create(DISPATCH_BLOCK_BARRIER, { print("output after 5 seconds") })
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(5 * Double(NSEC_PER_SEC))), dispatch_get_main_queue(), task2)
dispatch_block_cancel(task2)

2.Swift模拟取消

实际上delay时间到了之后还是会执行延时block，只是这时真正要执行的任务block已经为空

import Foundation

typealias Task = (cancel: Bool) -> Void

func gcdDelay(time: NSTimeInterval, task: ()->()) -> Task? {
    func dispatch_later(block: ()->()) {
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(time * Double(NSEC_PER_SEC))), dispatch_get_main_queue(), block)
    }
    
    var closure: dispatch_block_t? = task
    var result: Task?
    
    let delayedClosure: Task = {
        cancel in
        if let internalClosure = closure {
            if cancel == false {
                dispatch_async(dispatch_get_main_queue(), internalClosure)
            }
        }
        closure = nil
        result = nil
    }
    
    result = delayedClosure
    
    dispatch_later { 
        if let delayedClosure = result {
            delayedClosure(cancel: false)
        }
    }
    
    return result
}

func gcdCancel(task: Task?) {
    task?(cancel: true)
}

func gcdTest(){
    let task = gcdDelay(5) {
        print("output after 5 seconds")
    }
    gcdCancel(task)
}

3.Objective-C模拟取消

原理与2类似
GCDDelay.h

#import <Foundation/Foundation.h>

typedef void(^GCDTask)(BOOL cancel);
typedef void(^gcdBlock)();

@interface GCDDelay : NSObject

+(GCDTask)gcdDelay:(NSTimeInterval)time task:(gcdBlock)block;
+(void)gcdCancel:(GCDTask)task;

+(void)gcdTest;

@end

GCDDelay.m

@implementation GCDDelay

+(void)gcdLater:(NSTimeInterval)time block:(gcdBlock)block
{
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(time * NSEC_PER_SEC)), dispatch_get_main_queue(), block);
}

+(GCDTask)gcdDelay:(NSTimeInterval)time task:(gcdBlock)block
{
    __block dispatch_block_t closure = block;
    __block GCDTask result;
    GCDTask delayedClosure = ^(BOOL cancel){
        if (closure) {
            if (!cancel) {
                dispatch_async(dispatch_get_main_queue(), closure);
            }
        }
        closure = nil;
        result = nil;
    };
    result = delayedClosure;
    [self gcdLater:time block:^{
        if (result)
            result(NO);
    }];
    
    return result;
}

+(void)gcdCancel:(GCDTask)task
{
    task(YES);
}

+(void)gcdTest
{
    GCDTask task = [self gcdDelay:5 task:^{
        NSLog(@"oc output after 5 seconds");
    }];
    [self gcdCancel:task];
}

@end

参考：
1.swifter.tips之GCD和延时调用