Tuesday, January 15, 2013

Hurdles #1 - Apache Pivot - BXML Text Validators

Learning new technologies, frameworks, and languages can be hard. This is especially true when working alone from online documentation when the subject matter is either new, not widely adopted, or the documentation is a work in progress. Minor hurdles can derail the best intentioned learner because the solution is so blindingly obvious to anyone with knowledge of the subject that a solution is never stated.

I have started this post series titled "Hurdles" to track the minor, obvious, but frustrating issues I encounter when learning new things. Perhaps someone, sometime will find one of these posts useful, but if not it will at least be a log of lessons learned.

This article is about Apache Pivot, an open-source Java UI library specifically targeted towards creating rich interface applications for the web or standalone, along with a number of supporting libraries that simplify things like creating REST-ful services. After reading up on Pivot it intrigued me enough to give it a try to evaluate it and perhaps find a use for it on personal projects or at work.

My first impressions, the BXML definition and binding structures have strong flavours of WPF. There are significant differences of course, but the feel of familiarity and the relative ease of putting together a simple application based on the tutorials that had a rich interface layer and used simple web services gave me a good first impression.

The first real hurdle came when I was designing my first input form and wanted to attach a validator to a TextInput control using BXML. The APIs make attaching a validator a trivial exercise in Java code, the TextInput object contains a method called "setValidator" and taking a "org.apache.pivot.wtk.validation.Validator" type. The available validators themselves are simple but varied and selecting an IntValidator for this task was easy to do.

However, adding a new IntValidator instance into a TextInput tag in BXML was not as simple as it seemed. I tried a variety of [validator="obj"] attributes, using dereference, parameter, and variable syntax. All I got for my trouble was a heap of error messages and invalid cast exceptions.

I eventually found my solution in the Apache Pivot - Users nabble forum (for reference, here is the link to the Apache Pivot - Developers forum too) in a topic titled "Hi,   ". The conclusion to my problem was to create a child tag for the property that I wanted to set within the TextInput definition, and then create a child tag of that tag with the validator instance definition. This is the standard process for setting Collection property instances, but also applies to single Object property instances as well. Below is the simplest BXML source to illustrate the example.

        <IntValidator xmlns="org.apache.pivot.wtk.validation"/>