GCD学习之dispatch_barrier_async

    iOS常见的多线程开发方式有NSThread、NSOPeration和GCD，抽象程度依次提高，GCD是最抽象的，使用起来最简单，但相对来说功能有限，比如不能cancel任务，这也算是一点遗憾吧。

    今天主要记录下学习dispatch_barrier_async中遇到的问题（其实是自己没仔细看Apple文档），dispatch_barrier_async是在执行完前面的任务后它才执行，而且它后面的任务等它执行完成之后才会执行（先后顺序是按照添加到queue的次序），但该API适用的场景是dispatch queue必须是用DISPATCH_QUEUE_CONCURRENT属性创建的queue，而不能是用系统定义好的dispatch_get_global_queue，下面是测试代码：

1、当dispatch queue是dispatch_get_global_queue时，dispatch_barrier_async失效：

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    NSLog(@"main thread:%p",[NSThread currentThread]);
    [self testBarrierBlockWithGlobalQueue];
}
- (void)testBarrierBlockWithGlobalQueue
{
    NSLog(@"current iOS Version:%.1f",[[[UIDevice currentDevice] systemVersion] floatValue]);
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:2];
        NSLog(@"Excute block 1:%p",[NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:3];
        NSLog(@"Excute block 2:%p",[NSThread currentThread]);
    });
    dispatch_barrier_async(queue, ^{
        NSLog(@"Excute block 3:%p",[NSThread currentThread]);
        [NSThread sleepForTimeInterval:4];
    });
    dispatch_async(queue, ^{
        NSLog(@"Excute block 4:%p",[NSThread currentThread]);
    });
}
打印结果：
2015-06-09 23:16:33.799 testGCD[46920:3369866] main thread:0x7f87f8c0af70
2015-06-09 23:16:33.800 testGCD[46920:3369866] current iOS Version:8.3
2015-06-09 23:16:33.800 testGCD[46920:3370037] Excute block 4:0x7f87f8e14a60
2015-06-09 23:16:33.800 testGCD[46920:3370029] Excute block 3:0x7f87f8f222d0
2015-06-09 23:16:35.802 testGCD[46920:3370027] Excute block 1:0x7f87f8f27a90
2015-06-09 23:16:36.804 testGCD[46920:3370026] Excute block 2:0x7f87f8c1e610

从打印结果可以看到不是预想的block3在block1、block2后面，最后再执行block4，说明在global_queue下dispatch_barrier_async失效。

2、当dispatch queue是用DISPATCH_QUEUE_CONCURRENT属性创建的queue时，dispatch_barrier_async有效：

- (void)testBarrierBlockWithCreateQueue
{
    NSLog(@"current iOS Version:%.1f",[[[UIDevice currentDevice] systemVersion] floatValue]);
    dispatch_queue_t queue = dispatch_queue_create("com.testBarrierGCD", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:2];
        NSLog(@"Excute block 1:%p",[NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:3];
        NSLog(@"Excute block 2:%p",[NSThread currentThread]);
    });
    dispatch_barrier_async(queue, ^{
        NSLog(@"Excute block 3:%p",[NSThread currentThread]);
        [NSThread sleepForTimeInterval:4];
    });
    dispatch_async(queue, ^{
        NSLog(@"Excute block 4:%p",[NSThread currentThread]);
    });
}
打印结果：
2015-06-09 23:21:55.671 testGCD[46963:3373180] main thread:0x7fa51a428130
2015-06-09 23:21:55.672 testGCD[46963:3373180] current iOS Version:8.3
2015-06-09 23:21:57.676 testGCD[46963:3373293] Excute block 1:0x7fa51a65a890
2015-06-09 23:21:58.673 testGCD[46963:3373294] Excute block 2:0x7fa51a71cf50
2015-06-09 23:21:58.674 testGCD[46963:3373294] Excute block 3:0x7fa51a71cf50
2015-06-09 23:22:02.676 testGCD[46963:3373294] Excute block 4:0x7fa51a71cf50

从打印结果可以看出block执行顺序是按照我们预期的顺序在执行，至此说明了dispatch_barrier_async适用的场景是dispatch queue必须是用DISPATCH_QUEUE_CONCURRENT属性创建的queue（其实Apple文档中有说明的，以后用新的API时一定得仔细阅读文档，避免多走弯路）～祝大家玩的愉快～

测试代码已上传至GitHub：https://github.com/iOSGeek0829/testGCDWithBarrierBlock