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在我绘制图表的应用程序中，可以选择将图表作为图像发送并将图表保存到照片库。但是图表可能包含多达 4000 个或更多的数据，因此获取图像需要时间。因此，我在一个单独的线程上进行这项工作，并使用进度 hud 来指示活动，如下所示：

- (void)sendImageAsMail
{
    if ([MFMailComposeViewController canSendMail])
    {
        // Configure HUD
        self.myHUD.mode = MBProgressHUDModeIndeterminate;
        self.myHUD.labelText = @"Acquiring image, please wait";
        self.myHUD = [MBProgressHUD showHUDAddedTo:self.view animated:YES];
        __block UIImage *graphImage;

        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
        ^{
            CPTGraph *currentGraph = self.hostView.hostedGraph;
            graphImage = [currentGraph imageOfLayer];
            self.imageData = UIImagePNGRepresentation(graphImage);
            [[NSNotificationCenter defaultCenter] postNotificationName:@"mailReady" object:nil];
        });
    }
    else
    {
        [self showAlertWithTitle:@"Cannot Send Mail" andText:@"Your mail configuration is incorrect"];
    }
}

- (void)mailReady:(NSNotification *)notification
{
    MFMailComposeViewController *mailVC = [[MFMailComposeViewController alloc] init];
    mailVC.mailComposeDelegate = self;
    [mailVC setSubject:[NSString stringWithFormat:@"some subject here (%@)", [[[iDSConstants sharedConstants]codeTypes] objectAtIndex:_codeIndex]]];
    [mailVC setMessageBody:@"some body" isHTML:NO];
    [mailVC addAttachmentData:self.imageData mimeType:@"image/png" fileName:@"Design Spectrum"];

    [MBProgressHUD hideAllHUDsForView:self.view animated:YES];

    [self presentViewController:mailVC animated:YES completion:nil];
}

现在有趣的是，在显示 mailviewcontroller 之前，hud 没有被解除（或隐藏，如果可能的话）；如果我按下取消并关闭电子邮件，我可以看到平视显示器在旋转，并且它会继续旋转几秒钟。我尝试了其他方法，例如在代码的不同位置关闭 hud：

（上面代码的一部分）

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
^{
    CPTGraph *currentGraph = self.hostView.hostedGraph;
    graphImage = [currentGraph imageOfLayer];
    self.imageData = UIImagePNGRepresentation(graphImage);

    **// get main thread for dismissing the hud
    dispatch_async(dispatch_get_main_queue(), ^{
       [MBProgressHUD hideHUDForView:self.view animated:YES];
    });**

    [[NSNotificationCenter defaultCenter] postNotificationName:@"mailReady" object:nil];
});

但我仍然可以看到之前应该隐藏的平视显示器。对此问题的任何见解将不胜感激。

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