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>Chapter 20. Events: Seeking, Navigation and More</TD
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><DIV
CLASS="sect1"
><H1
CLASS="sect1"
><A
NAME="section-events-definitions"
>20.3. All Events Together</A
></H1
><P
> In this chapter follows a list of all defined events that are currently
being used, plus how they should be used/interpretted. Events are stored
in a <CODE
CLASS="classname"
>GstEvent</CODE
> structure, which is simply a big
C union with the types for each event in it. For the next development
cycle, we intend to switch events over to <CODE
CLASS="classname"
>GstStructure</CODE
>,
but you don't need to worry about that too much for now.
</P
><P
> In this chapter, we will discuss the following events:
</P
><P
></P
><UL
><LI
><P
><A
HREF="section-events-definitions.html#section-events-eos"
>End of Stream (EOS)</A
></P
></LI
><LI
><P
><A
HREF="section-events-definitions.html#section-events-flush"
>Flush</A
></P
></LI
><LI
><P
><A
HREF="section-events-definitions.html#section-events-discont"
>Stream Discontinuity</A
></P
></LI
><LI
><P
><A
HREF="section-events-definitions.html#section-events-seek"
>Seek Request</A
></P
></LI
><LI
><P
><A
HREF="section-events-definitions.html#section-events-filler"
>Stream Filler</A
></P
></LI
><LI
><P
><A
HREF="section-events-definitions.html#section-events-interrupt"
>Interruption</A
></P
></LI
><LI
><P
><A
HREF="section-events-definitions.html#section-events-nav"
>Navigation</A
></P
></LI
><LI
><P
><A
HREF="section-events-definitions.html#section-events-tag"
>Tag (metadata)</A
></P
></LI
></UL
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="section-events-eos"
>20.3.1. End of Stream (EOS)</A
></H2
><P
> End-of-stream events are sent if the stream that an element sends out
is finished. An element receiving this event (from upstream, so it
receives it on its sinkpad) will generally forward the event further
downstream and set itself to EOS (<CODE
CLASS="function"
>gst_element_set_eos ()</CODE
>).
<CODE
CLASS="function"
>gst_pad_event_default ()</CODE
> takes care of all this,
so most elements do not need to support this event. Exceptions are
elements that explicitely need to close a resource down on EOS, and
N-to-1 elements. Note that the stream itself is <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>not</I
></SPAN
>
a resource that should be closed down on EOS! Applications might seek
back to a point before EOS and set the pipeline to PLAYING again.
N-to-1 elements have been discussed previously in
<A
HREF="chapter-loopbased-loopfn.html#section-loopfn-multiinput"
>Multi-Input Elements</A
>.
</P
><P
> The EOS event (<CODE
CLASS="classname"
>GST_EVENT_EOS</CODE
>) has no properties,
and that makes it one of the simplest events in <SPAN
CLASS="application"
>GStreamer</SPAN
>. It is
created using <CODE
CLASS="function"
>gst_event_new (GST_EVENT_EOS);</CODE
>.
</P
><P
> Some elements support the EOS event upstream, too. This signals the
element to go into EOS as soon as possible and signal the EOS event
forward downstream. This is useful for elements that have no concept
of end-of-stream themselves. Examples are TV card sources, audio card
sources, etc. This is not (yet) part of the official specifications of
this event, though.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="section-events-flush"
>20.3.2. Flush</A
></H2
><P
> The flush event is being sent downstream if all buffers and caches
in the pipeline should be emptied. <SPAN
CLASS="QUOTE"
>"Queue"</SPAN
> elements will
empty their internal list of buffers when they receive this event, for
example. File sink elements (e.g. <SPAN
CLASS="QUOTE"
>"filesink"</SPAN
>) will flush
the kernel-to-disk cache (<CODE
CLASS="function"
>fdatasync ()</CODE
> or
<CODE
CLASS="function"
>fflush ()</CODE
>) when they receive this event. Normally,
elements receiving this event will simply just forward it, since most
filter or filter-like elements don't have an internal cache of data.
<CODE
CLASS="function"
>gst_pad_event_default ()</CODE
> does just that, so for
most elements, it is enough to forward the event using the default
event handler.
</P
><P
> The flush event is created with <CODE
CLASS="function"
>gst_event_new (GST_EVENT_FLUSH);</CODE
>.
Like the EOS event, it has no properties.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="section-events-discont"
>20.3.3. Stream Discontinuity</A
></H2
><P
> A discontinuity event is sent downstream to indicate a discontinuity in
the data stream. This can happen because the application used the seek
event to seek to a different position in the stream, but it can also be
because a real-time network source temporarily lost the connection.
After the connection is restored, the data stream will continue, but
not at the same point where it got lost. Therefore, a discontinuity
event is being sent downstream, too.
</P
><P
> Depending on the element type, the event can simply be forwarded using
<CODE
CLASS="function"
>gst_pad_event_default ()</CODE
>, or it should be parsed
and a modified event should be sent on. The last is true for demuxers,
which generally have a byte-to-time conversion concept. Their input
is usually byte-based, so the incoming event will have an offset in
byte units (<CODE
CLASS="classname"
>GST_FORMAT_BYTES</CODE
>), too. Elements
downstream, however, expect discontinuity events in time units, so that
it can be used to update the pipeline clock. Therefore, demuxers and
similar elements should not forward the event, but parse it, free it
and send a new discontinuity event (in time units,
<CODE
CLASS="classname"
>GST_FORMAT_TIME</CODE
>) further downstream.
</P
><P
> The discontinuity event is created using the function
<CODE
CLASS="function"
>gst_event_new_discontinuous ()</CODE
>. It should set a
boolean value which indicates if the discontinuity event is sent
because of a new media type (this can happen if - during iteration -
a new location was set on a network source or on a file source).
then, it should give a list of formats and offsets in that format. The
list should be terminated by 0 as format.
</P
><PRE
CLASS="programlisting"
> static void
my_filter_some_function (GstMyFilter *filter)
{
GstEvent *event;
[..]
event = gst_event_new_discontinuous (FALSE,
GST_FORMAT_BYTES, 0,
GST_FORMAT_TIME, 0,
0);
gst_pad_push (filter->srcpad, GST_DATA (event));
[..]
}
</PRE
><P
> Elements parsing this event can use macros and functions to access the
various properties. <CODE
CLASS="function"
>GST_EVENT_DISCONT_NEW_MEDIA (event)</CODE
>
checks the new-media boolean value.
<CODE
CLASS="function"
>gst_event_discont_get_value (event, format, &value)</CODE
>
gets the offset of the new stream position in the specified format. If
that format was not specified when creating the event, the function
returns FALSE.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="section-events-seek"
>20.3.4. Seek Request</A
></H2
><P
> Seek events are meant to request a new stream position to elements.
This new position can be set in several formats (time, bytes or
<SPAN
CLASS="QUOTE"
>"units"</SPAN
> [a term indicating frames for video, samples for
audio, etc.]). Seeking can be done with respect to the end-of-file,
start-of-file or current position, and can happen in both upstream and
downstream direction. Elements receiving seek events should, depending
on the element type, either forward it (filters, decoders), change the
format in which the event is given and forward it (demuxers), handle
the event by changing the file pointer in their internal stream
resource (file sources) or something else.
</P
><P
> Seek events are, like discontinuity events, built up using positions in
specified formats (time, bytes, units). They are created using the
function <CODE
CLASS="function"
>gst_event_new_seek ()</CODE
>, where the first
argument is the seek type (indicating with respect to which position
[current, end, start] the seek should be applied, and the format in
which the new position is given (time, bytes, units), and an offset
which is the requested position in the specified format.
</P
><PRE
CLASS="programlisting"
> static void
my_filter_some_function (GstMyFilter *filter)
{
GstEvent *event;
[..]
/* seek to the start of a resource */
event = gst_event_new_seek (GST_SEEK_SET | GST_FORMAT_BYTES, 0);
gst_pad_push (filter->srcpad, GST_DATA (event));
[..]
}
</PRE
><P
> Elements parsing this event can use macros and functions to access the
properties. The seek type can be retrieved using
<CODE
CLASS="function"
>GST_EVENT_SEEK_TYPE (event)</CODE
>. This seek type
contains both the indicator of with respect to what position the seek
should be applied, and the format in which the seek event is given.
To get either one of these properties separately, use
<CODE
CLASS="function"
>GST_EVENT_SEEK_FORMAT (event)</CODE
> or
<CODE
CLASS="function"
>GST_EVENT_SEEK_METHOD (event)</CODE
>. The requested
position is available through <CODE
CLASS="function"
>GST_EVENT_SEEK_OFFSET (event)</CODE
>,
and is given in the specified format.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="section-events-filler"
>20.3.5. Stream Filler</A
></H2
><P
> The filler event is, as the name says, a <SPAN
CLASS="QUOTE"
>"filler"</SPAN
> of the
stream which has no special meaning associated with itself. It is used
to provide data to downstream elements and should be interpreted as a
way of assuring that the normal data flow will continue further
downstream. The event is especially intended for real-time MIDI source
elements, which only generate data when something <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>changes</I
></SPAN
>.
MIDI decoders will therefore stall if nothing changes for several
seconds, and therefore playback will stop. The filler event is sent
downstream to assure the MIDI decoder that nothing changed, so that the
normal decoding process will continue and playback will, too. Unless
you intend to work with MIDI or other control-language-based data
types, you don't need this event. You can mostly simply forward it
with <CODE
CLASS="function"
>gst_pad_event_default ()</CODE
>.
</P
><P
> The stream filler is created using <CODE
CLASS="function"
>gst_event_new (GST_EVENT_FILLER);</CODE
>.
It has no properties.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="section-events-interrupt"
>20.3.6. Interruption</A
></H2
><P
> The interrupt event is generated by queue elements and sent downstream
if a timeout occurs on the stream. The scheduler will use this event to
get back in its own main loop and schedule other elements. This
prevents deadlocks or a stream stall if no data is generated over a
part of the pipeline for a considerable amount of time. The scheduler
will process this event internally, so any normal elements do not need
to generate or handle this event at all.
</P
><P
> The difference between the filler event and the interrupt event is that
the filler event is a real part of a pipeline, so it will reach fellow
elements, which can use it to "do nothing else than what I used to do".
The interrupt event never reaches fellow elements.
</P
><P
> The interrupt event (<CODE
CLASS="function"
>gst_event_new (GST_EVENT_INTERRUPT);</CODE
>)
has no properties.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="section-events-nav"
>20.3.7. Navigation</A
></H2
><P
> WRITEME
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="section-events-tag"
>20.3.8. Tag (metadata)</A
></H2
><P
> Tagging events are being sent downstream to indicate the tags as parsed
from the stream data. This is currently used to preserve tags during
stream transcoding from one format to the other. Tags are discussed
extensively in <A
HREF="chapter-advanced-tagging.html"
>Chapter 19</A
>. Most
elements will simply forward the event by calling
<CODE
CLASS="function"
>gst_pad_event_default ()</CODE
>.
</P
><P
> The tag event is created using the function
<CODE
CLASS="function"
>gst_event_new_tag ()</CODE
>. It requires a filled
taglist as argument.
</P
><P
> Elements parsing this event can use the function
<CODE
CLASS="function"
>gst_event_tag_get_list (event)</CODE
> to acquire the
taglist that was parsed.
</P
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