Monday, December 20, 2010

Cache me if you can

From the producers who brought you 'Tengo and Cache', we present 'Cache me if you can'.

The concept is all about HTML delivery into iOS devices alongside a mechanism for updating the application without going through the Apple app store. Loading HTML from a local file is not a new idea, as a few companies and projects have sprung up around this type of mobile app development have emerged, most notably Phonegap and more recently Apparatio.

The original iOS app I had built, Tengo and Cache, created a way that took this idea in a slightly different direction. The projects above deliver HTML within an application and would rely on Javascript to update any content within the application, using local storage to persist data. What Tengo and Cache offered was a way to download new HTML documents in their entirety, storing the files in writable areas of the file system on a device. This worked by providing a manifest file, based on the HTML5 manifest, on the domain which was the iOS app was trying to download files from.

During a hack day for the Guardian, I extended this work to add a further downstream cache. Although every effort is made to cache resources before loading, some files may be requested which are not included in the manifest. Overriding the NSURLCache class, the application can intercept any calls which attempt to go out to the web. This cache then retrieves the file, stores it, and then serves this instead of continuing down the pipeline. The next time this file is requested, the cache serves the file on disk instead of hitting the web at all.

Now that I had an iOS application that should pre-cache and intercept, I needed some content to install onto a device. I had wanted to use a Wordpress blog, or RSS feed, but felt that this content could prove difficult. Without knowing what content would be delivered, I couldn't build a manifest file that would capture the entire content. Also, I thought it might prove to be bad user experience if something that would work on a web page when online might not work offline. Even something as ubiquitous as search might look to have failed miserably.

I decided to use something I knew I had control over, the Guardian Content API. Using a small NodeJS application, I could retrieve a query and present that as HTML, alongside an appropriate manifest file. Then all that was required was a small property change in the iOS app and a native application was ready to be launched.

1 comment:

BenJam said...

looking forward to 'Cache 22'