objective-c - 为什么下面这个简单的 grand central dispatch 程序不能正常工作？

Question

所以我期待下面的程序打印两行。但是它不打印任何东西。关于需要解决的问题有什么想法吗？

#import <Foundation/Foundation.h>
#import <dispatch/dispatch.h>

int main(int argc, char **argv)
{
 dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0), ^{
 printf("Done outer async\n");
    dispatch_async(dispatch_get_main_queue(),^{
         printf("Done inner sync");
    });
 });

 return 0;
}

谢谢

Answer 1

如果您的程序没有事件循环，您必须调用 dispatch_main():

int main(int argc, char **argv)
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0), ^{
        printf("Done outer async\n");
        dispatch_async(dispatch_get_main_queue(),^{
            printf("Done inner sync");
        });
    });

    dispatch_main();
    return 0;
}

来自文档:

This function "parks" the main thread and waits for blocks to be submitted to the main queue. Applications that call UIApplicationMain (iOS), NSApplicationMain (Mac OS X), or CFRunLoopRun on the main thread must not call dispatch_main.