Gray box on UIWebView Screenshot

问题

I save a webview screenshot to a file using this method:

UIGraphicsBeginImageContext(webview.bounds.size);     
[webview.layer renderInContext:UIGraphicsGetCurrentContext()];
UIImage *screenImage = UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();

[imageData writeToFile:imagePath atomically:YES]; 

But sometimes it's have a gray area in it. How i remove that a gray area?

来源：https://stackoverflow.com/questions/6149289/gray-box-on-uiwebview-screenshot