iOS 栅栏函数

转自

https://www.jianshu.com/p/f6b494c19c92

一、什么是栅栏函数

dispatch_barrier_async函数，在进程管理中起到一个栅栏的作用，它等待所有位于barrier函数之前的操作执行完毕后执行,并且在barrier函数执行之后,barrier函数之后的操作才会得到执行，该函数需要同dispatch_queue_create函数生成的DISPATCH_QUEUE_CONCURRENT队列一起使用。

二、dispatch_barrier_async函数的作用

实现高效率的数据库访问和文件访问

避免数据竞争

三、dispatch_barrier_async实例

dispatch_queue_t queue=dispatch_queue_create("testqueue",DISPATCH_QUEUE_CONCURRENT);dispatch_async(queue,^{NSLog(@"----1-----%@",[NSThread currentThread]);});dispatch_async(queue,^{NSLog(@"----2-----%@",[NSThread currentThread]);});dispatch_barrier_async(queue,^{NSLog(@"----barrier-----%@",[NSThread currentThread]);});dispatch_async(queue,^{NSLog(@"----3-----%@",[NSThread currentThread]);});dispatch_async(queue,^{NSLog(@"----4-----%@",[NSThread currentThread]);});

输出结果

----2-----{number = 3, name = (null)}

----1-----{number = 4, name = (null)}

----barrier-----{number = 4, name = (null)}

----3-----{number = 4, name = (null)}

----4-----{number = 3, name = (null)}

四、dispatch_barrier_async和dispatch_barrier_sync的区别

1. dispatch_barrier_sync

dispatch_queue_t queue=dispatch_queue_create("testqueue",DISPATCH_QUEUE_CONCURRENT);NSLog(@"start");dispatch_async(queue,^{NSLog(@"---test1---");});dispatch_async(queue,^{NSLog(@"---test12---");});dispatch_barrier_sync(queue,^{NSLog(@"----barrier-----");});NSLog(@"aaa");dispatch_async(queue,^{NSLog(@"---test3---");});dispatch_async(queue,^{NSLog(@"---test4---");});NSLog(@"last");

运行结果

start

---test1---

---test12---

----barrier-----

aaa

last

---test3---

---test4---

2. dispatch_barrier_sync

dispatch_queue_t queue=dispatch_queue_create("testqueue",DISPATCH_QUEUE_CONCURRENT);NSLog(@"start");dispatch_async(queue,^{NSLog(@"---test1---");});dispatch_async(queue,^{NSLog(@"---test12---");});dispatch_barrier_async(queue,^{NSLog(@"----barrier-----");});NSLog(@"aaa");dispatch_async(queue,^{NSLog(@"---test3---");});dispatch_async(queue,^{NSLog(@"---test4---");});NSLog(@"last");

运行结果

start

aaa

last

---test1---

---test12---

----barrier-----

---test3---

---test4---

跟sync的情况相比，aaa、last的输出位置完全不同，async的时候aaa的输出在test1结束之前，sync的aaa输出在test1结束之后。

3. 总结

dispatch_barrier_sync(queue,void(^block)())会将queue中barrier前面添加的任务block全部执行后,再执行barrier任务的block,再执行barrier后面添加的任务block.

dispatch_barrier_async(queue,void(^block)())会将queue中barrier前面添加的任务block只添加不执行,继续添加barrier的block,再添加barrier后面的block,同时不影响主线程(或者操作添加任务的线程)中代码的执行!