How do I tell when a UIWebView is done rendering (not loading)?

I know when its done loading... (webViewDidFinishLoad), but I want to use

[webView.layer renderInContext:UIGraphicsGetCurrentContext()];

to create an image from the UIWebView. Occasionally I get the image prior to the webView finishing its rendering. I can use performSelector to delay the get of the image, but the amount of wait is arbitrary and brittle.