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.

Answer 2

I've been working on this, too.

In a thread I create a view hierarchy, loop until the web-view has finished loading content.

The webview I create is inside of a UIViewController's viewdidload-- I've tried doing

if ([NSThread isMainThread] == NO) {[self performselectorOnMainThread: @selector(viewDidLoad)return;)}

And I've done the same for dealloc'ing the webView.

But that didn't work.. I've only found that we avoid UIWebView exceptions UNTIL we hit the autorelease pool...

I'm using instruments to figure out why.

Here's my attack strategy... I'm going to perform the render operation on the main thread with an off-screen view, having a separate thread running some sort of queue to manage them. I'm worried about UI lag, so it'll have to be fairly efficient.