iOS dispatch_barrier_async 函数使用

//
//  ViewController.m
//  test_dispatch_assync_barry_01
//
//  Created by jeffasd on 17/4/26.
//  Copyright © 2017年 jeffasd. All rights reserved.
//

#import "ViewController.h"

@interface ViewController ()
{
    int _count;
}
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    _count = 0;
    
}

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{
    [self testDispatchBarrierAsync];
}

- (void)testDispatchBarrierAsync{
    if (_count >= 100) {
        return;
    }
    
    NSLog(@"----------------------------");
    
    dispatch_queue_t concurrentQueue = dispatch_queue_create("my.concurrent.queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(concurrentQueue, ^(){
        NSLog(@"dispatch-1");
    });
    dispatch_async(concurrentQueue, ^(){
        NSLog(@"dispatch-2");
    });
    dispatch_barrier_async(concurrentQueue, ^(){
        NSLog(@"dispatch-barrier");
    });
    dispatch_async(concurrentQueue, ^(){
        NSLog(@"dispatch-3");
    });
    dispatch_async(concurrentQueue, ^(){
        NSLog(@"dispatch-4");
    });
    _count++;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        [self testDispatchBarrierAsync];
    });
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}


@end

2017-04-26 13:55:02.117 test_dispatch_assync_barry_01[9348:1159895] ----------------------------

2017-04-26 13:55:02.118 test_dispatch_assync_barry_01[9348:1159979] dispatch-2

2017-04-26 13:55:02.118 test_dispatch_assync_barry_01[9348:1159981] dispatch-1

2017-04-26 13:55:02.118 test_dispatch_assync_barry_01[9348:1159981] dispatch-barrier

2017-04-26 13:55:02.118 test_dispatch_assync_barry_01[9348:1159981] dispatch-3

2017-04-26 13:55:02.118 test_dispatch_assync_barry_01[9348:1159979] dispatch-4

2017-04-26 13:55:11.200 test_dispatch_assync_barry_01[9348:1159895] ----------------------------

2017-04-26 13:55:11.200 test_dispatch_assync_barry_01[9348:1159981] dispatch-1

2017-04-26 13:55:11.200 test_dispatch_assync_barry_01[9348:1159979] dispatch-2

2017-04-26 13:55:11.201 test_dispatch_assync_barry_01[9348:1159979] dispatch-barrier

2017-04-26 13:55:11.201 test_dispatch_assync_barry_01[9348:1159981] dispatch-4

2017-04-26 13:55:11.201 test_dispatch_assync_barry_01[9348:1159979] dispatch-3

dispatch_barrier_async 作用是在并行队列中，等待前面两个操作并行操作完成，这里是并行输出

dispatch-1，dispatch-2

然后执行

dispatch_barrier_async中的操作，(现在就只会执行这一个操作)执行完成后，即输出

"dispatch-barrier，
最后该并行队列恢复原有执行状态，继续并行执行

dispatch-3,dispatch-4

可以看出来 1和2的执行是没有先后顺序的，3和4也没有先后顺序 但是  dispatch-barrier 肯定是在 12 结束后43是在dispatch-barrier结束后执行

参考文章:

http://www.cnblogs.com/zhidao-chen/p/3598161.html