如何实现一个能插队的GCD队列

需求：

批量执行一些任务的时候，可以有其他任务插进来优先执行，待执行完插队的任务后，继续执行之前的任务。

代码：

//
//  ViewController.m
//  GCD插队的任务
//
//  Created by 秦伟 on 2017/4/21.
//  Copyright © 2017年 秦伟. All rights reserved.
//

#import "ViewController.h"

@interface ViewController ()
@property (nonatomic, strong) dispatch_queue_t low;
@property (nonatomic, strong) dispatch_queue_t high;
- (IBAction)buttonAction:(id)sender;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    _low = dispatch_queue_create("com.test.low", DISPATCH_QUEUE_SERIAL);
    _high = dispatch_queue_create("com.test.high", DISPATCH_QUEUE_SERIAL);
    dispatch_set_target_queue(_low, _high);
    
    for (NSInteger i = 0; i < 20; i++) {
        dispatch_async(_low, ^{
            [NSThread sleepForTimeInterval:1.0];
            NSLog(@"low: %ld", i);
        });
    }
    dispatch_async(_low, ^{
        NSLog(@"全部完成！");
    });
}

- (IBAction)buttonAction:(id)sender {
    
    dispatch_suspend(_low);
    dispatch_async(_high,^{
        NSLog(@"high: 插！！！");
        dispatch_resume(_low);
    });
}
@end