title: Draw UIView and all its subview to Context
body: <p>a UIView is created and small snow.png is kept added and removed to UIView to simulate the snowing effect.</p>
  
  <p>Then I would like to capture the screen as an image. </p>
  
  <pre><code>UIGraphicsBeginImageContext(self.uiviewPreview.bounds.size);
  [self.uiviewPreview.layer renderInContext:UIGraphicsGetCurrentContext()];
  UIImage *viewImage = UIGraphicsGetImageFromCurrentImageContext();
  UIGraphicsEndImageContext();
  UIImageWriteToSavedPhotosAlbum(viewImage, nil, nil, nil);
  </code></pre>
  
  <p>Note: self.uiviewPreview is the UIView.</p>
  
  <p>But I can only see the image in UIview in the saved jpg. But all added subviews at that moment are not saved.</p>
  
  <p>I would like to know whether renderInContext is correct or not to use</p>