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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 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
indefinate number of interfaces per object instance, and a client
application could then enumerate interfaces and instantiate one.
The API would then look like this:
void gst_element_register_interface (GstElement *element,
const gchar *name,
GstInterfaceFunc func);
const GList *gst_element_list_interfaces (GstElement *element);
GstInterface *gst_element_get_interface (GstElement *element,
const gchar *name);
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'd 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.
For the plugin, it's then as simple as can be. The class_init
function sets the virtual functions in the interface class object,
and the instance_init function registers the object per created
element. The get_interface() handler refs this interface and
returns it. The application unrefs it when it's done. The
appropriate functions will be called by the application when it
thinks it needs to. Perfectly simple!
For applictions, it's even simpler. Request an interface and use
it as documented. When you're done, unref it. It's just like
elements: simple!
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
===========
typedef struct _GstInterface {
GObject object;
} GstInterface;
typedef struct _GstInterfaceClass {
GObjectClass klass;
} GstInterfaceClass;
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...
And alsasink would surely implement the same interface.
/* This is confusing naming... (i.e. FIXME)
* A channel is referred to both as the number of simultaneous
* sounds the input can handle as well as the in-/output itself
*/
typedef struct _GstMixerChannel {
gchar *label;
gint current_num_channels,
max_num_channels;
} GstMixerChannel;
typedef struct _GstMixer {
GstInterface interface;
} GstMixer;
typedef struct _GstMixerClass {
GstInterfaceClass 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);
gboolean (* get_mute) (GstMixer *mixer,
GstMixerChannel *channel);
} GstMixerClass;
Name for in the element list: "mixer". Gstaudio provides wrapper
functions for each of the class' virtual functions. Possibly also
some macros for GST_MIXER_CHANNEL_HAS_FLAG () or _get_channel ().
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.
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... Discussion needed here!
typedef struct _GstOverlayChannel {
gchar *label;
} GstOverlayChannel;
typedef struct _GstOverlayNorm {
gchar *label;
} GstOverlayNorm;
typedef struct _GstOverlay {
GstInterface interface;
} GstOverlay;
typedef struct _GstOverlayClass {
GstInterfaceClass klass;
/* virtual functions */
GList * (* list_channels) (GstOverlay *overlay);
void (* set_channel) (GstOverlay *overlay,
GstOverlayChannel *channel);
const gchar * (* get_channel) (GstOverlay *overlay);
GList * (* list_norms) (GstOverlay *overlay);
void (* set_norm) (GstOverlay *overlay,
GstOverlayNorm *norm);
const gchar * (* get_norm) (GstOverlay *overlay);
void (* set_xwindowid) (GstOverlay *overlay,
XWindowID xid);
} GstOverlayClass;
4c) user input
--------------
And yes, user input could be an interface too. Even better, it
should definately be. And wasn't this one of our key issues for
0.8.0?
No code here. Go implement it, lazy ass!
5) Status of this document
==========================
This is a proposal, nothing more. Nothing is implemented. Target
release is 0.8.0 or any 0.7.x version.
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. ;).
|
