How can I tell when a UIWebView has finished drawing to a context?

Question

In my code I'm trying to show a UIWebView as a page is loading, and then, when it's done, capture an image from the web view to cache and display later (so I don't have to reload and render the web page).

I have something along the lines of:

CGContextRef context = CGBitmapContextCreate(…);
[[webView layer] renderInContext:context];

CGImageRef imageRef = CGBitmapContextCreateImage(context);
UIImage *image = [UIImage imageWithCGImage:imageRef];

The problem I'm running into is that, due to UIWebView's tiling, sometimes only half of the page is rendered to the context by the time I capture the image.

Is there a way to detect or block on UIWebView's background rendering thread so that I can get the image only after all of the rendering has finished?

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UPDATE: It may be that thread race conditions were a red herring (it's unclear from the documentation, at any rate, whether UIWebView's custom layer or a CATiledLayer in general blocks on its background threads).

This may instead have been an invalidation issue (despite several sorts of calls to setNeedsDisplay on both the UIWebView and its layer). Changing the bounds of the UIWebView before rendering it appears to have eliminated the "not drawing the whole thing" problem.

I still ran into a problem where a few tiles were being drawn at the old scale, but calling renderInContext: twice seems to have mitigated that sufficiently.

Answer 1

UIWebView is probably using a CATiledLayer or custom derivative. You may be able to replace the layer with something of your own choosing such as a simple CALayer which does not do threaded drawing. Replace the layer before you start loading content.

If replacing the layer with a standard CALayer does not work, you may have to make your own subclass that emulates the behavior of a CATiledLayer without actually being threaded.

Edit:

From CATiledLayer.h

/* Note: do not attempt to directly modify the `contents' property of
 * an CATiledLayer object - doing so will effectively turn it into a
 * regular CALayer. */

So you may just be able to set the contents to nil before calling renderInContext: