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Sun, 2005-Jun-19

A RESTful non-web User Interface Model

I've spent most of today hacking python. The target has been to exploit the possiblity of REST in a non-web programming envioronment. I chose to target a glade replacement built from REST principles.

Glade is a useful bare bones tool for constructing widget trees that can be embedded into software as generated source code or read on the fly by libglade. Python interfaces are available for libglade and gtk, but there are a few things I wanted to resolve and improve.

  1. The event handling model, and
  2. The XML schema

The XML schema is bleugh. Awfully verbose and not much use. It is full of elements like "widget" and "child" and "parameter" that don't mean anything, leaving the real semantic stuff as magic constant values in various attribute fields. In my prototype I've collapsed each parameter into an attribute of its parent, and made all elements either widgets or event handling objects.

The real focus of my attention, though, was the event handling. Currently gtk widgets can emit certain notifications, and it is possible to register with the objects to have your own functions called when the associated event goes off. Because a function call is the only way to interact with these signals, you immediately have to start writing code for potentially trivial applications. I wanted to reduce the complexity of this first line of event handling so much that it could be included into the glade file instead. I wanted to reduce the complexity to a set of GET, and PUT operations.

I've defined my only event handler for the prototype application with four fields:

To make this URL-heavy approach to interface design useful, I put in place a few schemes for internal use in the application. The "literal:" scheme just returns the percent-decoded data of its path component. It can't be PUT to. The "object:" scheme is handled by a resolver that allows objects to register with it by name. Once a parent has been identified, attributes of the parent can be burrowed into by adding path segments.

The prototype example was a simple one. I create a gtk window with a label in it. The label initially reads "Hello, world". On a mouse click, it becomes "Goodbye, world". It's not going to win any awards for oringinality, but it demonstrates a program that can be entirely defined in terms of generic URI-handling and XML loading code and an XML file that contains the program data. An abbreviated version of the XML file I used looks like this:

<gtk.Window id="window1">
        <gtk.Label id="label1" label="literal:Hello,%20world">

You can see that the literal URI schem handling is a little clunky, and needs to have some unicode issues thought out. The Event node is triggered by a button_press_event on label1, and sends the literal "Goodbye, world" to lable1's own "label" attribute. You can see some of the potential power emerging already, though. If you changed either the initial label url to data extracte from a database or from http we have immediately developed not a static snapshot in this file but a dynamic beast. The data in our event handler could refer to an object or XML file that had been built up XForms-style via other event interactions. It is possible that even quite sophisticated applications could be written in this way as combinations of queries and updates to a generic store. Only geniunely complicated interactions would need to be modelled in code, and that code would not be clouded by the needless complexity of widget event handling.

It is my theory that if REST works at all, it will work in this kind of programming environment as well as it could on the web. I'm not trying to preclude the use of other languages or methods for controlling the logic of a single window's behaviour, but I am hoping to clearly define the level of complexity you can apply through a glade-like editor. I think it needs to be higher than it is presently to be really productive, but I don't think it should become a software IDE in itself.

All in all my prototype came to 251 lines of python, including whitespace and very minimal comments. That includes code for the event handler (12 lines), a gtk object loader (54 lines, and generic python object actually), and URI resolution classes including http support through urllib2. The object resolver is the largest file, weighing in at 109 lines. It is made slightly more complex by having to deal with pygtk's use of setters and getters as well as the more generic python approaches to attribute assignment.

My example was trivial, but I do think that this kind of approach could be considerably more powerful than that of existing glade. The real test (either of the method or of gtk's design) will probably come in list and tree handling code. This will be especially true where column data comes from a dynamic source such as an sqlite database. I do anticpate some tree problems, where current even handlers often need to pass strange contexts around in order to correctly place popup windows. It may come together, though.