IMPLEMENTING BACKENDS
=====================

Clutter supports multiple backends for handling windowing systems and
GL/GLES API on different platforms.

The GL and GLES API are abstracted by the COGL library. The windowing
system is handled by the ClutterBackend implementations inside Clutter
itself.

Clutter, at the moment, supports only in-tree backends.

In order to write a new backend for a specific platform you should
create a new sub-directory under clutter/clutter containing:

  <backend>/clutter-backend-<backend>.h
  <backend>/clutter-backend-<backend>.c

  -- The subclass of the ClutterBackend abstract class.

  <backend>/clutter-stage-<backend>.h
  <backend>/clutter-stage-<backend>.c

  -- The implementation of the stage actor.

  <backend>/clutter-event-<backend>.c

  -- The event handling code (optional).

  <backend>/clutter-<backend>.h

  -- A header for the backend-specific API that should be installed
     by Clutter inside the include directory along with the rest of
     the public API headers (optional).


Implementing ClutterBackend
---------------------------

Each backend must implement the

  GType
  _clutter_backend_impl_get_type (void);

function declared inside clutter/clutter-private.h. The implementation
of the function must return the same GType of the backend implementation,
for instance:

  GType
  _clutter_backend_impl_get_type (void)
  {
    return CLUTTER_TYPE_BACKEND_GLX;
  }

The ClutterBackend implementation is a singleton instance, and the
backend must ensure that every time g_object_new() is called the same
pointer is returned (with its reference count increased). The GObject
API reference describes how to use the ::constructor virtual function
to implement a singleton, so you should refer to that.

When implementing the ClutterBackend subclass these virtual functions
can be overridden:

  ClutterBackend::add_options
  -- Use this function to install new, backend-specific GOptionEntry
     definitions to the Clutter GOptionGroup. This function is guaranteed
     to be called just once.

  ClutterBackend::pre_parse
  -- Use this function to check for environment variables or setting
     up default values before the command line arguments are parsed.
     This function is guaranteed to be called just once.

  ClutterBackend::post_parse
  -- Use this function to prepare the backend with the values either
     set inside the ::pre_parse virtual function or by the command
     line options parsing code. This function is guaranteed to be
     called just once.

  ClutterBackend::init_events
  -- Use this function to initialize the event handling. This function
     is guaranteed to be called just once.

  ClutterBackend::get_features
  -- Use this function to retrieve the features detectable at runtime
     from the GL or GLES implementation, plus the eventual backend-specific
     features.

  ClutterBackend::ensure_context
  -- This function is used to ensure that the backend drawing context
     is made current for passed ClutterStage, using the backend-specific
     API.

  ClutterBackend::redraw
  -- This function is used to draw the passed ClutterStage; the backend
     must call clutter_actor_paint() on the ClutterStage that has been
     passed as a parameter and then perform backend-specific tasks, like
     waiting for vertical blanking and swapping the buffers.

  ClutterBackend::create_stage
  -- This function is used to create the stage implementation. It will
     receive as an argument the ClutterStage instance that is "wrapping"
     the actual implementation being created. The backend must create
     its stage implementation and call _clutter_stage_set_window() with
     the wrapper and the stage implementation instance:

       _clutter_stage_set_window (wrapper, CLUTTER_STAGE_WINDOW (impl));

     The backend must also call clutter_actor_realize() on the stage
     implementation, and then check if the stage has been realized, using
     the CLUTTER_ACTOR_IS_REALIZED() macro; if the stage was not
     realized, it must return NULL and set the passed GError.

Implementing the stage
----------------------

ClutterStage acts as a wrapper object relaying all the drawing operations
to the actual implementation. The implementation of the stage can be any
ClutterActor subclass, as long as it does not subclass ClutterStage and
it implements the ClutterStageWindow interface.

The ClutterStageWindow interface contains a set of virtual functions that
should be overridden by backends that support a windowing system, like
::set_title(), ::set_fullscreen(), ::set_cursor_visible(), etc.

The stage implementation actor must implement at least the ::realize and
::unrealize ClutterActor virtual functions. Inside the ::realize function
the stage implementation should:

  - create a new native window handle
  - if the backend doesn't have a drawing context (either GL or GLES),
    create one and assing it to the backend
  - set the CLUTTER_ACTOR_REALIZED flag on *both* the wrapper and the
    stage implementation (this is very important)
  - call clutter_stage_ensure_current() with the wrapper instance

In case of failure, the CLUTTER_ACTOR_REALIZED flag should be unset on
the stage implementation.

Inside the ::unrealize function the stage implementation should:

  - unset the CLUTTER_ACTOR_REALIZED flag on itself
  - destroy the native window handle
  - call clutter_stage_ensure_current() with the wrapper instance

NOTES
=====

If the platform is using X11 you should probably subclass ClutterBackendX11
and ClutterStageX11, which will provide you with a ready to use code
implementation for event handling and window management.

Usual points of failure for backends are:

- not setting the CLUTTER_ACTOR_REALIZED flag on the stage implementation
  and the stage wrapper inside the ::realized virtual function before
  calling clutter_stage_ensure_current();
- calling public API, like clutter_actor_paint(), or checking properties
  on the stage implementation instead of the ClutterStage wrapper.

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