Create a UIImage by rendering UIWebView on a background thread - iPhone

Question

Does someone know of a way, or has a creative idea as to how to obtain an UIImage that is rendered from a UIWebView? The catch is, that is must be on a background thread.

I'll elaborate: I'm trying to obtain the image from multiple UIWebViews every second+-, and display it on screen (iPhone's of course). Because rendering a layer to a CGContext is a CPU consuming job, I wouldn't like to use the main thread, so not to hang the UI.

My attempts so far were:

• I tried to use renderInContext on the webView's layer to the UIGraphicalContext, but the _WebTryThreadLock error crashed the webviews.

• I tried creating a CGBitmapContext, and render the webview's layer to it, but got the same result.

• I tried implementing a copy method (by adding a Category) to CALayer, that deep copied all of the public properties, and sublayers. Afterwards, I tried to renderInContext the layer I copied. I got a UIImage that was partially "correct" - meaning, not all of the layers were rendered, so for example, i would get only the website header (or footer, or body, or search bar, or just a some of the frames). The UIWebview's layer consists of all sort of subclassed CALayers, so this is probably why this approached didn't work.

• I tried setting the kCATransactionDisableActions in a CATransaction, but it didn't appear to change this behavior (neither of them).

I'm pretty close to giving up.

Is there a savior among you people?

Answer 1

UIWebView hates, and I mean really hates, having anything done to it on a background thread. UIKit is not fully thread safe. Drawing to a graphics context is (this was added in iOS 4), but not creating UIViews on a secondary thread.

Are you creating your UIWebViews off the main thread? Do you perhaps have some code to share? I would suspect your issues are being caused by the fact you're trying to perform operations to a UIWebView on a secondary thread. The drawing operation to render the view's contents as an image can happen off the main thread, but creating the view itself can't.