In this chapter follows a list of all defined events that are currently
being used, plus how they should be used/interpreted. Events are stored
in a GstEvent
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 GstStructure
,
but you don't need to worry about that too much for now.
In this chapter, we will discuss the following events:
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 (gst_element_set_eos ()
).
gst_pad_event_default ()
takes care of all this,
so most elements do not need to support this event. Exceptions are
elements that explicitly need to close a resource down on EOS, and
N-to-1 elements. Note that the stream itself is not
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.
The EOS event (GST_EVENT_EOS
) has no properties,
and that makes it one of the simplest events in GStreamer. It is
created using gst_event_new (GST_EVENT_EOS);
.
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.
The flush event is being sent downstream if all buffers and caches
in the pipeline should be emptied. "Queue" elements will
empty their internal list of buffers when they receive this event, for
example. File sink elements (e.g. "filesink") will flush
the kernel-to-disk cache (fdatasync ()
or
fflush ()
) 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.
gst_pad_event_default ()
does just that, so for
most elements, it is enough to forward the event using the default
event handler.
The flush event is created with gst_event_new (GST_EVENT_FLUSH);
.
Like the EOS event, it has no properties.
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.
Depending on the element type, the event can simply be forwarded using
gst_pad_event_default ()
, 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 (GST_FORMAT_BYTES
), 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,
GST_FORMAT_TIME
) further downstream.
The discontinuity event is created using the function
gst_event_new_discontinuous ()
. 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.
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)); [..] }
Elements parsing this event can use macros and functions to access the
various properties. GST_EVENT_DISCONT_NEW_MEDIA (event)
checks the new-media boolean value.
gst_event_discont_get_value (event, format, &value)
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.
Seek events are meant to request a new stream position to elements. This new position can be set in several formats (time, bytes or "units" [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.
Seek events are, like discontinuity events, built up using positions in
specified formats (time, bytes, units). They are created using the
function gst_event_new_seek ()
, 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.
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)); [..] }
Elements parsing this event can use macros and functions to access the
properties. The seek type can be retrieved using
GST_EVENT_SEEK_TYPE (event)
. 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
GST_EVENT_SEEK_FORMAT (event)
or
GST_EVENT_SEEK_METHOD (event)
. The requested
position is available through GST_EVENT_SEEK_OFFSET (event)
,
and is given in the specified format.
The filler event is, as the name says, a "filler" 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 changes.
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 gst_pad_event_default ()
.
The stream filler is created using gst_event_new (GST_EVENT_FILLER);
.
It has no properties.
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.
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.
The interrupt event (gst_event_new (GST_EVENT_INTERRUPT);
)
has no properties.
WRITEME
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 Chapter 18. Most
elements will simply forward the event by calling
gst_pad_event_default ()
.
The tag event is created using the function
gst_event_new_tag ()
. It requires a filled
taglist as argument.
Elements parsing this event can use the function
gst_event_tag_get_list (event)
to acquire the
taglist that was parsed.