GCD之dispatch_barrier详解

多线程开发过程中,我们如果想在前面的任务完成后再去执行后面的某些操作,我们可以采用dispatch_barrier_async来轻松实现

• dispatch_barrier是跟dipatch_async/dispatch_sync 相似的api,用来将任务提交到指定队列中,它的使用注意点:queue必须是DISPATCH_QUEUE_CONCURRENT类型的.queue如果是global或者不是DISPATCH_QUEUE_CONCURRENT类型的其他queue,它的作用就和普通的dipatch_async/dispatch_sync一样.

dispatch_queue_t concurrentQueue = dispatch_queue_create("com.GCDDemo.concurrentQueue",DISPATCH_QUEUE_CONCURRENT);
for (int i = 0; i < 3; i ++) {
dispatch_async(concurrentQueue, ^{
[NSThread sleepForTimeInterval:1.0];
NSLog(@"download task %d successed!",i);
});
}
dispatch_barrier_async(concurrentQueue, ^{
[NSThread sleepForTimeInterval:2.0];
NSLog(@"barrier blcok success!");
});
dispatch_async(concurrentQueue, ^{
[NSThread sleepForTimeInterval:1.0];
NSLog(@"upload task successed!");
});
//输出台:
download task 2 successed!
download task 1 successed!
download task 0 successed!
barrier blcok success!
upload task successed!

• 假如queue是global queue

dispatch_queue_t globalQueue = dispatch_get_global_queue(0, DISPATCH_QUEUE_PRIORITY_DEFAULT);
for (int i = 0; i < 3; i ++) {
dispatch_async(globalQueue, ^{
[NSThread sleepForTimeInterval:1.0];
NSLog(@"download task %d successed!",i);
});
}
dispatch_barrier_async(globalQueue, ^{
[NSThread sleepForTimeInterval:2.0];
NSLog(@"barrier blcok success!");
});
dispatch_async(globalQueue, ^{
[NSThread sleepForTimeInterval:1.0];
NSLog(@"upload task successed!");
});
//输出台:
download task 1 successed!
download task 0 successed!
download task 2 successed!
upload task successed!
barrier blcok success!