Re: Emacs and Gnome Canvas

[Jan Djärv]

Eli Zaretskii skrev 2010-07-15 08.55:
>   . Canvas seems to need GTK+.  What does this mean for platforms where
>     GTK+ is unavailable or not maintained?  What about supporting the
>     TTY?  Do these issues mean we will need to keep the existing
>     display engine in parallel with the Canvas-based one?

Gtk+ is required, you would have to have a parallel display engine for all 
non-Gtk+ cases.
>   . The Canvas redisplay runs from the GTK+ idle handlers.  In Emacs,
>     the idle loop, in addition to triggering redisplay, also checks for
>     input from the keyboard and from subprocesses.  Does this mean that
>     part of the input handling will need to be run by GTK+?

Yes.  Now we run the Gtk+ event loop (or actually not the loop, just a step) 
only when we know there is events pending.  Again, parallel implementation for 
non-Gtk+.  But normal redisplay for Gtk+ also happens in idle handlers.  We 
are in the Gtk+ event loop often enough for this to happen often enough. 
Sometimes though (scroll bars mainly) we force redisplay in Gtk+.  I assume 
you can do that for the Canvas also.

>   . Canvas redisplay is caused by requests from the application to
>     update some "canvas item" when the underlying application's objects
>     are modified; these requests are then served when GTK+ idle
>     handlers are run.  Emacs display engine works differently: changes
>     that require redisplay are not considered until redisplay is
>     entered; the "requests" to update the display are implicitly
>     recorded in the buffers and in the various related data structures
>     (text properties and overlays, display strings, etc.), but not
>     explicitly translated to display terms until redisplay time, and as
>     an inherent part of redisplay itself.  These two very different
>     models will need to be reconciled in some reasonably efficient way.

The changes that require redisplay would have to notify the Canvas that things 
changed.  I think this style is more efficient.

But I'm not sure it is the right way to go for Emacs unless Gtk+ can run on 
all platforms now.  Even if it can, do we want Gtk+ look and feel on W32, OSX, 
Nextstep etc.?  TTY is still a separate issue though.

        Jan D.