34c7611407
Clarify the Backend::create_context() vfunc role, and the Stage::realize() vfunc with regards to creating the GL/GLES context.
169 lines
6.1 KiB
Plaintext
169 lines
6.1 KiB
Plaintext
IMPLEMENTING BACKENDS
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=====================
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Clutter supports multiple backends for handling windowing systems and
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GL/GLES API on different platforms.
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The GL and GLES API are abstracted by the COGL library. The windowing
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system is handled by the ClutterBackend implementations inside Clutter
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itself.
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Clutter, at the moment, supports only in-tree backends.
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In order to write a new backend for a specific platform you should
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create a new sub-directory under clutter/clutter containing:
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<backend>/clutter-backend-<backend>.h
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<backend>/clutter-backend-<backend>.c
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-- The subclass of the ClutterBackend abstract class.
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<backend>/clutter-stage-<backend>.h
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<backend>/clutter-stage-<backend>.c
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-- The implementation of the stage actor.
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<backend>/clutter-event-<backend>.c
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-- The event handling code (optional).
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<backend>/clutter-<backend>.h
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-- A header for the backend-specific API that should be installed
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by Clutter inside the include directory along with the rest of
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the public API headers (optional).
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Implementing ClutterBackend
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---------------------------
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Each backend must implement the
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GType
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_clutter_backend_impl_get_type (void);
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function declared inside clutter/clutter-private.h. The implementation
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of the function must return the same GType of the backend implementation,
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for instance:
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GType
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_clutter_backend_impl_get_type (void)
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{
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return CLUTTER_TYPE_BACKEND_GLX;
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}
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The ClutterBackend implementation is a singleton instance, and the
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backend must ensure that every time g_object_new() is called the same
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pointer is returned (with its reference count increased). The GObject
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API reference describes how to use the ::constructor virtual function
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to implement a singleton, so you should refer to that.
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The ClutterBackend implementation should hold a single drawing context
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for its entire lifetime; stage implementations should be "made current"
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when needed.
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When implementing the ClutterBackend subclass these virtual functions
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can be overridden:
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ClutterBackend::add_options
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-- Use this function to install new, backend-specific GOptionEntry
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definitions to the Clutter GOptionGroup. This function is guaranteed
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to be called just once.
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ClutterBackend::pre_parse
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-- Use this function to check for environment variables or setting
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up default values before the command line arguments are parsed.
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This function is guaranteed to be called just once.
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ClutterBackend::post_parse
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-- Use this function to prepare the backend with the values either
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set inside the ::pre_parse virtual function or by the command
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line options parsing code. This function is guaranteed to be
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called just once.
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ClutterBackend::init_events
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-- Use this function to initialize the event handling. This function
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is guaranteed to be called just once.
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ClutterBackend::get_features
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-- Use this function to retrieve the features detectable at runtime
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from the GL or GLES implementation, plus the eventual backend-specific
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features.
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ClutterBackend::create_context
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-- This function is used to create the drawing context to be used
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by Clutter. Clutter will call this function during the initialization
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phase. A GL (or GLES) context must always be available after the
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initialization, so that Cogl and Clutter can query it for capabilities.
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ClutterBackend::ensure_context
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-- This function is used to ensure that the backend drawing context
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is made current for passed ClutterStage, using the backend-specific
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API.
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ClutterBackend::redraw
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-- This function is used to draw the passed ClutterStage; the backend
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must call clutter_actor_paint() on the ClutterStage that has been
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passed as a parameter and then perform backend-specific tasks, like
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waiting for vertical blanking and swapping the buffers.
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ClutterBackend::create_stage
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-- This function is used to create the stage implementation. It will
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receive as an argument the ClutterStage instance that is "wrapping"
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the actual implementation being created. The backend must create
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its stage implementation, initialise it and then return it; in case
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of error, the backend must return NULL and set the passed GError.
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Implementing the stage
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----------------------
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ClutterStage acts as a wrapper object relaying all the drawing operations
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to the actual implementation. The implementation of the stage can be any
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GObject subclass, as long as it implements the ClutterStageWindow interface.
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The ClutterStageWindow interface contains a set of virtual functions that
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should be overridden by backends that support a windowing system, like
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::set_title(), ::set_fullscreen(), ::set_cursor_visible(), etc.
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The stage implementation actor must implement:
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• ClutterStageWindow::get_wrapper()
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• ClutterStageWindow::realize() and ::unrealize()
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• ClutterStageWindow::show() and ::hide()
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• ClutterStageWindow::resize()
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• ClutterStageWindow::get_geometry()
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The ::get_wrapper() implementation should return the pointer to the
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ClutterStage actor using the ClutterStageWindow implementation.
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In the ::realize virtual function the stage implementation should:
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- create a new native window handle
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- ensure that there is a GL (or GLES) context
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- make sure that the native window handle is compatible with
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the GL (or GLES) context
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The return value should be TRUE if the stage implementation was successfully
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realized, and FALSE otherwise.
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Inside the ::unrealize function the stage implementation should destroy
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the native window handle created in ::realize().
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The ::resize() virtual function implementation should cause an update
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of the COGL viewport.
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The stage implementation actor can optionally implement:
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• ClutterStageWindow::get_pending_swaps()
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The get_pending_swaps() implementation should return the number of swap
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buffer requests pending completion. This is only relevent for backends
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that also support CLUTTER_FEATURE_SWAP_EVENTS.
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NOTES
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=====
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If the platform is using X11 you should probably subclass ClutterBackendX11
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and ClutterStageX11, which will provide you with a ready to use code
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implementation for event handling and window management.
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