Deferring viewWillAppear until webViewDidFinishLoad

Question

I have an application that uses UIWebViews in several view controllers. The UIWebViews are used to render locally generated html, no slow network access required.

To save memory I only load these on demand as prompted by the viewcontroller viewWillAppear callback. (And unload offscreen instances in response to didReceiveMemoryWarning messages.)

The problem is that the user gets to see the html being rendered, sometimes accompanied by flashes of styling and other assorted unpleasant artifacts. I would much rather the rendering be done offscreen, and reveal the fully rendered view when its ready.

It would be very tidy to be able to have the viewWillAppear not return until the UIWebView is fully rendered. But how?

I tell the UIWebView what to render by sending it a loadHTMLString:baseURL: message. This is asynchronous, and some time (soon) later the webview's delegate gets called back webViewDidFinishLoad.

I experimented with running a runloop inside viewWillAppear, running either the NSDefaultRunLoopMode or UITrackingRunLoopMode. This works in the simulator (it complains to the log

[CATransaction synchronize] called within transaction

but does work) but on a device it deadlocks, with webViewDidFinishLoad never being called.

(Also, it seems like the UIWebView loading property doesn't work. At least, after I call loadHTMLString:baseURL: and before getting the callback it's not true.)

Answer 1

Lots of solutions here I think. A quick one is to load your UIWebView with it's hidden property set to YES. Then set your UIViewController as the UIWebViews delegate and implement:

- (void)webViewDidFinishLoad:(UIWebView *)webView

where you set the property back to NO.

A thing to note is that webViewDidFinishLoad will fire more than once if you have framed/embedded content. So you have to keep track of this. Shouldn't really be a problem if you are loading local content.