(press ESC to stop animation)

The Prototype Bidirectional Widget

by Dov Grobgeld
Email: dov@imagic.weizmann.ac.il

Background

The prototype bidirectional widget is a prototype implementation of a bidirectional widget. It was created as a testbed for development of a BiDi text widget. It's goal is not to be efficient, but rather to test out the various user interaction aspects. It is my hope that once the prototype behaviour has been worked out, this behaviour can be implemented in various widget sets, e.g. in Gtk, Qt, and LessTif.

Implementation

The widget has been implemented on top of a graphics library called cnv that was especially developed for this purpose. cnv is nothing more than a drawing area. Currently there has been implemented a cnv interface for gtk/gdk, but alternative cnv interfaces should easily be developed.

Screen shot

See the animated gif above, which consists of a series of screen shots taken after each character was inserted.

Status

Done

Widget always shows correct bidirectional string.
Uses fribidi.
Mouse click puts cursor at correct logical position in correct direction.
Alt-LShift and Alt-RShift switches keyboard mapping.
Two subsequent Alt-LShift or Alt-RShift makes the line switch its base direction.
Left and right arrows now work in visual mode. Note that the logical cursor positioning corresponding to a visual position depends on from which direction you arrive. E.g. with the following text

visual string: a b c Z Y X
where X, Y, and Z indicate RTL characters.
If you want to continue to write after Z you can put the cursor between Z and Y and then press left arrow. If you want to continue writing after c put the cursor between b and c and press right arrow. In both cases the cursor arrives at the same visual position but in the first case the cursor is logically after Z and in the second case logically after c.
This system allows arriving at all logical positions when working in visual mode. This is in contrast to the broken MS visual mode in which there are logical positions that can not be arrived at.
The selection now works, though it is not currently possible to do anything with it. To create a selection press mouse button 3 and a selection will be created from the cursor position to the mouse position. The selection is logical and may therefor be split visually. In the future I plan to introduce a visual selection as well.

Todo

Selections, cut and paste.
Line breaking algorithm.
Optimizations in redrawing, speed and memory.
Implementing in Gtk!