Interfaces to objects

[Jasper St. Pierre]

On Tue, Jul 8, 2014 at 7:51 AM, Stephen Lee <stephenl7797 at gmail.com> wrote:

> Hey, I'm working on my own window manager and I'm having a hard time
> tracking through the Weston code to see where a function is registered as
> part of an interface. Right now I am looking at surfaces. I guess my
> question is more related to the relation of surface and shell surface. In
> the main compositor file, their is a struct called surface_interface which
> is passed into 'wl_resource_set_implementation' along with a resource
> 'wl_surface_interface'. Okay, that is clear enough, but now in shell.c
> their are a bunch of functions in the 'shell_surface_implementation' struct
> which again has a resource created and implemented for
> 'wl_shell_surface_interface'. Also in the module_init their is a global
> created to 'bind' the compositor to another struct for 'desktop' stuff. How
> do all of these things relate? Is surface similar to a base class and
> shell_surface similar to an inherited class?
>

Hey

So, there's four different objects you've found: wl_compositor, wl_surface,
wl_shell and wl_shell_surface.

These are four different protocol objects and you can all find their
interfaces separately in the protocol XML file [0]. That's what the
"interface" structs are implementing inside Weston: the request handlers
for these interfaces.

wl_shell_surface is a "role" for wl_surface, which is sort of like an
inherited class, but not quite. Instead of saying "wl_shell_surface extends
from wl_surface", you make a wl_surface, and then make a wl_shell_surface
from that wl_surface, which tags it as a wl_shell_surface. You can mostly
treat it as inheritance for the purposes of understanding.

A wl_surface is a bag of bits where you can attach an output buffer, and
also get pointer input on and so forth. Attaching a wl_shell_surface role
allows you to put the window on screen, drag it around, make it resizable,
maximize it, etc. These are things that other UIs like cars, TVs, and some
tablet interfaces might not want, so they're hidden in an optional portion
of the spec.

Then there's the two factory objects. The way protocol extensions work,
there's a series of globals, which is how these objects get created in the
first place. The wl_compositor global creates wl_surfaces, and the wl_shell
global creates wl_shell_surfaces.


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-- 
  Jasper
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