ios – 使用GCD和webView进行死锁

我发现了一个似乎导致WebKit陷入僵局的问题.如果我从我的主线程运行此代码,我正确地看到一个警报.我可以点击警报上的“确定”按钮,它会解散并且一切正常：

[theWebView stringByEvaluatingJavaScriptFromString:@"alert('hi');"];

如果我进行了一些修改,那么仍然会出现警告消息,但是无法点击“确定”按钮 – 您无法关闭警报,如果您闯入应用程序,它将挂在stringByEvaluatingJavaScriptFromString调用中：

dispatch_async(dispatch_get_global_queue(disPATCH_QUEUE_PRIORITY_DEFAULT,0),^{
    dispatch_async(dispatch_get_main_queue(),^{
        [theWebView stringByEvaluatingJavaScriptFromString:@"alert('hi');"];
    });
});

这两者中唯一不同的是,在第二个中,它在调度队列的上下文中在主线程中运行JS.

另一方面,如果我执行以下操作,则不会发生挂起：

- (void) showHi:(id) it
{
    [(UIWebView*)it stringByEvaluatingJavaScriptFromString:@"alert('hi');"];
}

....

dispatch_async(dispatch_get_global_queue(disPATCH_QUEUE_PRIORITY_DEFAULT,^{
    [self performSelectorOnMainThread:@selector(showHi:) withObject:theWebView waitUntilDone:NO];
});

有人可以对导致挂起的问题有所了解吗？

编辑：

相关问题：

Perform UI Changes on main thread using dispatch_async or performSelectorOnMainThread?
Whats the difference between performSelectorOnMainThread and dispatch_async on main queue?
Grand Central Dispatch (GCD) vs. performSelector – need a better explanation

非常相似的问题：

UIWebView stringByEvaluatingJavaScriptFromString hangs on iOS5.0/5.1 when called using GCD

dispatch_async(dispatch_get_global_queue(disPATCH_QUEUE_PRIORITY_DEFAULT,^{
        UIAlertView *message = [[UIAlertView alloc] initWithTitle:@"Test"
                                                          message:@"Test"
                                                         delegate:nil
                                                cancelButtonTitle:@"OK"
                                                otherButtonTitles:nil];
        [message show];

        [theWebView stringByEvaluatingJavaScriptFromString:@"alert('hi');"];
    });
});