[compiz] A mouse cursor theming plugin in Compiz

[Luca Saiu]

Hello.

I wrote a Compiz plugin to set the mouse cursor theme.

I'm attaching the code here, but of course I'm open to resubmit it on
launchpad or modify it as needed.  My point here is mostly to discuss
and show what I did, which might be controversial.

Mouse themes would be conceptually simple: there is only a theme to
select among the ones installed under /usr/share/icons (not all of which
are necessarily *mouse* themes; only directories with a "cursors/"
subdirectory count), and a cursor size.  The libXcursor library accepts
a theme name (which corresponds to a directory in /usr/share/icons or in
$XCURSOR_PATH), and a cursor size in pixels; the size which is closest
to the requested one is used.

Of course only "well-behaved" program will obey our settings; each X
application is free to use its own policy for setting cursors and
disregard what we tell it.  All modern large applications however do
respond to cursor theme or size changes, normally using gsettings -- but
each desktop environment has its own different schema; moreover GTK has
its own way of specifying cursor themes in ~/.icons/default/index.theme
, and old-school X applications (that I've ignored) use X resources
instead.

The main target for us at Hypra consists in low-vision users who are not
necessarily technically minded, so we absolutely need a configuration
GUI.  CCSM is great, but its way of specifying options is not a perfect
fit for the mouse theming problem.  As far as I can see from the code
the configuration of Compiz and its plugins is specified in XML (.in)
files which contain the type, range and default values of each option.
The XML files are used to generate C++ code holding configuration data
structures, and also installed to be consulted at run time by CCSM.

Differently from every other case I've seen, the mouse theme plugin
needs to present a choice of one among a set of options which are only
known at run time -- the currently installed mouse themes.  The ideal
way to do that would be with an option of type "int" and a _name
subelement, which CCSM renders as nice combo boxes; but that won't work
in our case because the set of installed themes would need to be
specified in the XML file, which is much too early; the set of installed
themes can change even after compiz is installed.

My solution, which is admittedly kludgy, consists in specifying an
option of type "list" where the first element is the one which is
actually selected; the plugin C++ initialization code searches for the
installed themes and fills the list at run time --- which is
unfortunately not possible using a combo box without huge changes in the
XSLT code generator and CCSM itself, as far as I can see.

The dynamic initialization of the list, if naïvely performed at every
plugin initialization as specified above, would override the user
selection of her current theme.  In order to prevent that I added a
second option storing the set of *installed* themes (alphabetically
sorted for normalization).  The initialization code checks whether the
actually installed themes have changed with respect to the saved value
of the option; if not, the list of *selected* themes is not touched.
This is clunky and it would be best to make the list of installed themes
invisible to the user (I think that could be done with a new "invisible"
type of hint to be specified in the XML file), but I find the current
interface already not too terrible.

I've uploaded a demo screencast at
http://ageinghacker.net/scratch-hypra/mousetheme.webm .

The user can also set a shell command line to actually set the theme
for running applications, and of course the cursor size.

The C++ code in Compiz is responsible for updating the cursor in the
root window, for every screen.  In order to make that cursor dynamically
changeable I had to modify screen.h and screen.cpp, but that is not
intrusive: Compiz only uses a "normal" and a "busy" cursor, so I simply
added two methods setNormalCursor and setBusyCursor accepting a
parameter of type Cursor.  The window manager decorator is separate, and
behaves as an ordinary process.

What do you think of all this?  I'm open to any suggestion.

Thanks,

-- 
Luca Saiu
Hypra team (Accessible website in construction)
-------------- next part --------------
A non-text attachment was scrubbed...
Name: mousetheme-version-1.diff
Type: text/x-diff
Size: 58925 bytes
Desc: not available
URL: <http://lists.freedesktop.org/archives/compiz/attachments/20150331/7ed2f65a/attachment-0001.diff>