gstreamer/docs/random/interfaces

INTERFACES & ELEMENTS
---------------------

1) Introduction
===============
Interfaces are descriptions on how to handle an object, without actually
implementing the object. This allows for multiple objects to be instantiated
based on this interface. Each of them can then be handled equally by an
application.
Glib, apparently (unchecked), has a way of creating interfaces, probably
by means of a class struct without actually defining the object. The object,
then, does not define a class and these two add up. Benjamin knows more
about interfaces, I didn't study interfaces & glib too deeply, yet. I know
them just from Java.
Interfaces are cool! It allows for some sort of random element creation
without needing to link to the implementation. This is similar to how
GStreamer currently handles media plugins. GStreamer itself could be seen
as an interface too, in that respect.

2) So why do we need interfaces?
================================
Because GStreamer doesn't handle it all. GStreamer in itself is a media
framework for streams of data from one element to the next. There's lots
of things that's media-related, but not handled in this description.
Several examples will probably clarify this: think of the Xvideo output
plugin. We can create an overlay here (Xv-based), and we currently control
this X-connection using glib properties. However, what property name is
associated with what control? And does it work the same as v4lsrc's
overlay image control?
The same goes for a mixer, for image control, audio control, and probably
a lot more. The general idea is simple: *this needs to be documented*.
But properties aren't all - they simply cannot do this all. Some things
cannot be described in a simple one-argument property thing. Of course,
we could give a pointer to a struct as argument, but that's merely a hack
and requires both plugin and app to know the ABI of the struct. This
kills the whole idea of making the plugin independent of the app.
In short: we want interfaces for this.

3) How to integrate an interface in GStreamer
=============================================
Let us start with some starting point: an interface is associated
with an element. It is a feature exported by that specific element,
not by a pipeline or anything more complex. Pipelines are already
handled just fine by GStreamer (or you wouldn't be reading all
this).
Obviously, a pipeline can be a fallback for an interface. Imagine
that we're looking for an audio sink that exposes a mixer, but our
fakesink audio output doesn't ("I wonder why"). We could then create
a pipeline with the volume element in it to "fake" a mixer. Ideally,
the volume element would implement a mixer interface itself.

How are we going to do that in programmatic way? We currently use
properties. Their huge advantage is that we do not need to care
about adding new functions or whatever. Their disadvantage is that
they're limited to one argument. Anything more complex requires
app/plugin knowledge about the shared data, and that defeats the
point of them: to have no dependency on each other. This could be
solved partially by using action signals, but that makes the whole
picture quite complex (since you use multiple methods for doing one
simple thing). Also, they are quite slow compared to functions
because of the table lookups. In short: it'd work, but I'm not in
facour of it...
OK, so an element exposes interfaces. This allows us to think of
the idea of embedding interfaces (dynamically, of course) in the
GstElement object. Think of an object being able to register an
indefinite number of interfaces per object instance, and a client
application could then enumerate interfaces and instantiate one.
Glib gives us GInterface for this purpose. The disadvantage of
this is that it's on a per-class basis, not a per-instance basis.
This is a problem in case of elements where it depends on several
properties whether it supports an interface or not. This can be
solved by simply making one generic virtual function "supported ()"
in a generic derived object of GInterface (GstInterface?).

GstInterface is then a generic thing that is inherited by specific
interfaces (see examples). Obviously, the client will need to know
about the ABI/API of this struct, but that'll happen either way.
Surely, there needs to binary linkage, but I don't consider that a
bad thing. It does improve performance compared to action signals!

So an element contains interfaces. But where are these interfaces
described? And who creates them? I suggest that we do that just as
we handle gstvideo and gstaudio right now (these libs do *nothing*
useful currently, so this would make them a lot more interesting).
These interfaces inherit from GstInterface. The functions that
are needed, can be provided through a class object. The element is
then responsible for storing variables and so on. gstvideo/audio
provides wrapper functions for the class functions. That's also how
glib suggest us to use GInterfaces.

Plugin and application then handle and retrieve interfaces as
documented in the glib documentation, which is available at:
http://www.gnome.org/~mathieu/gobject/main.html

So the most important part left is to document the interfaces
and make sure all elements exporting them work equally. For this,
I'll give two examples.

4) Examples
===========

/* This small extra virtual function is here to provide an
 * interface functionality on a per-instance basis rather
 * than a per-class basis, which is the case for glib.
 */
typedef struct _GstInterfaceClass {
  GTypeInterface parent;

  /* virtual functions */
  gboolean (* supported) (GstInterface *iface);
} GstInterfaceClass;

There would probably be a convenience function that checks
a specific interface's implementation (glib allows for this)
and checks for ->supported () to be set and to return TRUE:

gboolean
gst_element_implements_interface (GstElement *element,
				  GType       iface_type)
{
  if (G_TYPE_CHECK_INSTANCE_TYPE (G_OBJECT (element),
				  type)) {
    GstInterface *iface;
    GstInterfaceClass *ifclass;

    iface = G_TYPE_CHECK_INSTANCE_CAST (G_OBJECT (element),
					type, GstInterface)
    ifclass = GST_INTERFACE_GET_CLASS (iface);

    if (ifclass->supported != NULL &&
        ifclass->supported (iface) == TRUE) {
      return TRUE;
    }
  }

  return FALSE;
}

Let's now add some functions so we can abuse this in case/check
functions.

GstInterface *
gst_interface_cast (gpointer from,
		    GType    type)
{
  GstInterface *iface;

  /* check cast, give warning+fail if it's invalid */
  if (!(iface = G_TYPE_CHECK_INSTANCE_CAST (G_OBJECT (element),
					    type, GstInterface))) {
    return NULL;
  }

  /* if we're an element, take care that this interface
   * is actually implemented */
  if (GST_IS_ELEMENT (from)) {
    gboolean interface_is_implemented =
	gst_element_implements_interface (GST_ELEMENT (from),
					  type);
    g_return_val_if_fail (interface_is_implemented == TRUE, NULL);
  }

  return iface;
}

gboolean
gst_interface_check (gpointer from,
		     GType    type)
{
  GstInterface *iface;

  /* check cast, return FALSE if it fails, don't give a warning... */
  if (!G_TYPE_CHECK_INSTANCE_CAST (from, type,
				   GstInterface)) {
    return FALSE;
  }

  iface = G_TYPE_CHECK_INSTANCE_CAST (G_OBJECT (element),
				      type, GstInterface);

  /* now, if we're an element (or derivative), is this thing
   * actually implemented for real? */
  if (GST_IS_ELEMENT (from)) {
    if (!gst_element_implements_interface (GST_ELEMENT (from),
					   type)) {
      return FALSE;
    }
  }

  return TRUE;
}

#define GST_INTERFACE_CHECK_INSTANCE_CAST(obj, type, cast_t) \
  ((cast_t *) gst_interface_cast ((obj), (type))
#define GST_INTERFACE_CHECK_INSTANCE_TYPE(obj, type) \
  (gst_interface_check ((obj), (type))

We could use this in the GST_IS_... () macros. For example, the
macros GST_IS_MIXER () and GST_MIXER () would then look like this:

/* Note that this is a non-standard macro, and with a reason! */
#define GST_MIXER(obj) \
  (GST_INTERFACE_CHECK_INSTANCE_CAST ((obj), \
				      GST_TYPE_MIXER,
				      GstMixer))
#define GST_IS_MIXER(obj) \
  (GST_INTERFACE_CHECK_INSTANCE_TYPE ((obj), \
				      GST_TYPE_MIXER))

So the application would just tread it with the known macro, and
everything would look extremely simple to the end user.

4a) mixer
---------
A mixer is a way of controlling volume and input/output channels.
This doesn't mean that you control which channel is the subwoofer,
all that is supposed to be done automatically. It is really meant
as a way of representing system-level volumes and such. It could
also be used to turn on/off certain outputs or inputs.
As you've noticed, I'm not only talking about output, but also
input. Indeed, I want both osssrc *and* osssink to export the
same mixer interface! Or at least a very similar one. Volume
control works the same for both. You could say that osssrc should
enumerate the input channels (such as microphone, line-in). Of
course, osssink should not. Or maybe it should, not sure... Maybe,
we'd need a parent osselement which implements all mixer channels.
And alsa* would surely implement the same interface.

/* This is confusing naming... (i.e. FIXME)
 * A channel is referred to both as the number of simultaneous
 * sound streams the input can handle as well as the in-/output
 * itself
 */

#define GST_MIXER_CHANNEL_INPUT  (1<<0)
#define GST_MIXER_CHANNEL_OUTPUT (1<<1)
#define GST_MIXER_CHANNEL_MUTE   (1<<2)
#define GST_MIXER_CHANNEL_RECORD (1<<3)

typedef struct _GstMixerChannel {
  gchar *label;
  gint   current_num_channels,
         max_num_channels,
         flags;
} GstMixerChannel;

typedef struct _GstMixerClass {
  GTypeInterface klass;

  /* virtual functions */
  GList *  (* list_channels) (GstMixer        *mixer);

  void     (* set_volume)    (GstMixer        *mixer,
			      GstMixerChannel *channel,
			      gint            *volumes);
  void     (* get_volume)    (GstMixer        *mixer,
			      GstMixerChannel *channel,
			      gint            *volumes);

  void     (* set_mute)      (GstMixer        *mixer,
			      GstMixerChannel *channel,
			      gboolean         mute);
  void     (* set_record)    (GstMixer        *mixer,
			      GstMixerChannel *channel,
			      gboolean         record);
} GstMixerClass;

libgstmixer.la/so provides wrapper functions for each of the
class' virtual functions. Possibly also some macros for
GST_MIXER_CHANNEL_HAS_FLAG () or _get_channel ().

The rest is done automatically, as described in the already-
mentioned glib documentation for GInterface. This includes
things like the base_init () function of the GstMixerClass,
which fills all the virtual functions for the mixer, and the
actual function implementations. The mixer, basically, operates
as an element on its own. It gets the file descriptor from
the interface->element (every oss... is a osscommon, etc.).

4b) overlay
-----------
Overlay is used in both in- and output, too. Think of v4lsrc,
v4l2src, v4lmjpegsrc, xvideosink - all overlays. But where do
we position the overlay window? Control of this can be done at
various levels: locational control (over the server, asynchronous)
or XID control (but that makes you depend on X and limits the
ability to broaden it over to non-X elements such as fbsink).

However, simplicity *is* an issue here. Do we really care about
overlay? In the end, users will have to link against either FB
or X anyway, so we might want to create separate interfaces for
both. On the other hand, we want to be general too... This is a
decision that we need to make as early as possible in this process.
For now, I propose making X- and FB-based interfaces.

Let's assume that we take X as a basis. Then, overlay becomes as
simple as one function. Possible extendible by providing inputs
(like in the mixer) and norms, although that only applies to
input-to-analog, not to-digital... Others simply return NULL.

typedef struct _GstOverlayClass {
  GTypeInterface klass;

  /* virtual functions */
  void    (* set_xwindowid) (GstOverlay        *overlay,
			     XID                xid);
} GstOverlayClass;

That's all! It would look similar for FB & co.

4c) user input
--------------
And yes, user input could be an interface too. Even better, it
should definitely be. And wasn't this one of our key issues for
0.8.0?

No code here. Go implement it, lazy ass!

General ways of thinking: input can come from a plugin, or from
the application (we don't have modules for joystick input et all).
However, plugins handling input (such as dvdsrc) need to be able
to handle each. So we get both input-to-application as well as
input-from-application APIs.

5) Status of this document
==========================
The interfaces are implemented, more (for metadata, framebuffer-
overlay, video balancing (brightness), user input etc. are all
pending.

6) Copyright and blabla
=======================
(c) Ronald Bultje, 2003 <rbultje@ronald.bitfreak.net> under the
terms of the GNU Free Documentation License. See http://www.gnu.org/
for details.

And no, I'm not for hire. ;).