Very similar code exists within PhoneGap. In addition to bi-directional message passing, PhoneGap also provides: 1.) JSON serialization/deserialization, 2.) callback tracking, and 3.) white-listing of domains that may pass messages to the host.

https://github.com/phonegap/phonegap-iphone

Here's the relevant part:

https://github.com/marcuswestin/WebViewJavascriptBridge/blob...

Any thoughts on using local XMLHttpRequest instead of the more usual frame reload method?

Great idea! Sounds cleaner but I think it'd be more code injected into the webview than the current approach.