Using UIWebView as a JavaScript Interpreter

Posted by David Hamrick on

I’ve written before about wanting to create metadata driven applications but did not go in to specifics about how to achieve that. For many of the applications I have worked on that means creating plist files that ship with the application. Those plist files might contain details about what URLs to access in staging/test/production or styling details for UI elements.

This approach works well but I have always wished for more expressiveness, especially as these files grew in size as more and more things needed to be configured. I wanted to be able to use a scripting language to create this metadata. I even went as far as creating my own small scripting language based on Scheme. Creating my own scripting language had it’s advantages, but mostly disadvantages, mostly that I was spending time maintaining it instead of working on the logic that I wanted from the script.

As a recent node.js convert, I decided to see if I could embed JavaScript in my application. I see that many other have had success embedding a full interpreter. Dominic Szablewski embedded Safari’s Javascript engine (JavaScriptCore) in his iOS app and Couchbase embedded Firefox’s JavaScript engine (SpiderMonkey) for scripting CouchDB. While I considered doing this, I thought it may be overkill for my simpler needs. All I wanted to do was input a script and output some JSON that had the metadata that I wanted. But there was another option, use the built in UIWebView.


I created a small adaptor class called JavascriptContext. This may seem like an unnecessary class but if I ever need to replace the UIWebView with a full-featured interpreter all of my interactions with the UIWebView will all be happening in 1 class and I can replace them all at once.

To be honest I was somewhat surprised it was this easy. I thought that using the JavaScript interpreter would require having some HTML loaded, or that it would be extremely slow. The way that I used it was to construct an object and then use JSON.stringify to return the value to my app, then use JSONKit to convert that in to an NSDictionary.

Using a UIWebView instead of embedding a proper interpreter reminded me of something that Chris Hanson said.

Always use the highest-level abstraction available to you, and drop down to lower-level abstractions when measurement shows that they are needed.

If measurements show me that my approach is using too much memory, or too slow, I will then consider another approach. Until that happens I will use the more straight-forward approach.


David Hamrick is a partner at Hamrick Software, the makers of VueScan, the worlds most popular 3rd party scanner software.