GCD：在非主线程上返回

Question

我在一些非主线程中使用dispatch_async 。 我们称之为“线程A”。 我没有使用GCD获得线程A。 执行完某些块后，我想返回线程A。我该如何做？

踩A：

dispatch_queue_t fetchQ = dispatch_queue_create("Thread B", NULL);
dispatch_async(fetchQ, ^{
    // Do some stuff...
    // ...

    //Now I want to return in Thread A
});
dispatch_release(fetchQ);

Answer 1

怎么样

dispatch_queue_t fetchA = ...
//..

dispatch_queue_t fetchQ = dispatch_queue_create("Thread B", NULL);
dispatch_async(fetchQ, ^{
    // Do some stuff...
    // ...

    dispatch_async(fetchA, ^{
        // Do stuff on Tread A
    });

});
dispatch_release(fetchQ);

Answer 2

这是种丑陋的做法，但似乎可行。

dispatch_queue_t fetchQ = dispatch_queue_create("Thread B", NULL);

__block BOOL changeWhenComplete = NO;

dispatch_async(fetchQ, ^{

    for (int i =0; i < 2; i++) {

        NSLog(@"doing work");
        sleep(1);
    }
    //Now I want to return in Thread A
    changeWhenComplete = YES;
});
dispatch_release(fetchQ);

while (!changeWhenComplete) {

    sleep(1);
    NSLog(@"waiting for fetchQ to complete");
}

NSLog(@"fetchQ returned");
return;

值得注意的是，这将阻塞线程A。