UIMapView does not show content at first time

I have a ViewContr开发者_如何学运维oller has a UIMapView as IBOutlet and the ViewController will be pushed into a navigationViewController. I use loadHtmlString:baseURL: method to set the content of the UIMapView, which works fine. But the content does not show at the first time when I pushed the ViewController to the NavigationViewController

    [richContent loadHTMLString: [NSString stringWithFormat:template, 
      article.articleTitle, 
      stringFromDate, 
      article.bigImage, 
      html] 
  baseURL: [[NSBundle mainBundle] resourceURL]]; 

Dunno where is the problem...

Thanks.