My Ideas on Compositors

[Carl-Philip Haensch]

Hi,

after a discussion on #wayland, I want to write down some ideas about  
compositor plugins for wayland at all.

At first, the wayland reference implementation should be as basic as  
possible to make wayland also a choice for OpenBox users. On the other  
hand, you want to have nice effects on your screen, wobbling windows  
etc.

So the solution would be a plugin architecture that extends the  
compositor for effects, app switchers and so on.
My idea is a shared library that is loaded by wayland and that has a  
bunch of callbacks wayland can call.
There will be only one .so file loaded into wayland but that's no  
reason to not implement plugin multiplexers and other interesting stuff.

So what we need to design is a plugin API that lets you do everything  
you want. The compositor will have a list of procedure variables  
containing the linked callbacks of the master plugin. Each function  
call will be done if the pointer to the callback is not null. If the  
plugin replaces something (for example drawing a surface), the  
"native" implementation is not executed if there is a plugin function  
for it.
Now a list of functions that will have to be in the plugin API. Please  
read through it and add things I forgot:
  - onDrawBackground (draws the back image)
  - onAfterDrawBackground(for background animations, panels)
  - onSurfaceUpdate (to manipulate the position matrix of the window,  
called after you have matrix and matrix_inv)
  - onSurfaceDraw (replaces the draw code of the display.. for  
wobbling windows)
  - onAfterDraw (to draw notifications)
  - onSurfaceCreate
  - onSurfaceDelete
  - onKeyPress (to fish out global hot keys like Alt+Tab; maybe needs  
events like up/down/press)
  - Functions to manipulate the visibility of the surfaces
  - Functions to request meta data from surfaces (fullscreen,  
maximized, minimized, toolbox)
  - Functions to set size and position

So the following things are possible with this plugin API:
  - Having a clock and CPU load applets in background
  - Multiple desktops
  - desktop overview
  - desktop cube
  - whobbling windows
  - notifications
  - panels and window placement around the panel
  - app switcher
  - properly workigng fullscreen

What did I forget? What do you think about the idea or the concept of  
implementing it?