UIWebView to UIImage

I tried to capture image from UIWebView using this method but the image contains only visible area of the screen. How do I capture full content of UIWebView including invisible areas, i.e. the entire web page into one single image?

-(UIImage*)captureScreen:(UIView*) viewToCapture{
  UIGraphicsBeginImageContext(viewToCapture.bounds.size);
  [viewToCapture.layer renderInContext:UIGraphicsGetCurrentContext()];
  UIImage *viewImage = UIGraphicsGetImageFromCurrentImageContext();
  UIGraphicsEndImageContext();
  return viewImage;
}