gstreamer/docs/design/part-events.txt

Events
------

Events are objects passed around in parallel to the buffer dataflow to
notify elements of various events.

Events are received on pads using the event function. Some events should
be interleaved with the data stream so they require taking the STREAM_LOCK,
others don't.

Different types of events exist to implement various functionalities.

  GST_EVENT_FLUSH_START:   data is to be discarded
  GST_EVENT_FLUSH_STOP:    data is allowed again
  GST_EVENT_EOS: 	   no more data is to be expected on a pad.
  GST_EVENT_NEWSEGMENT:    A new group of buffers with common start time  
  GST_EVENT_TAG:           Stream metadata.
  GST_EVENT_BUFFERSIZE:    Buffer size requirements
  GST_EVENT_QOS:           A notification of the quality of service of the stream
  GST_EVENT_SEEK:          A seek should be performed to a new position in the stream
  GST_EVENT_NAVIGATION:    A navigation event.
  GST_EVENT_DRAIN:         Play all data downstream before returning.


FLUSH_START/STOP
----------------

A flush event is sent both downstream and upstream to clear any pending data 
from the pipeline. This might be needed to make the graph more responsive 
when the normal dataflow gets interrupted by for example a seek event.

Flushing happens in two stages.

 1) a source filter sends the FLUSH_START event to the downstream peer element. The
    downstream element starts rejecting buffers from the upstream elements. It
    sends the flush event further downstream and discards any buffers it is 
    holding as well as return from the chain function as soon as possible.
    This makes sure that all upstream elements get unblocked.
    This event is not synchronized with the STREAM_LOCK and can be done in the 
    application thread.

 2) a source filter sends the FLUSH_STOP event to indicate
    that the downstream element can accept buffers again. The downstream 
    element sends the flush event to its peer elements. After this step dataflow
    continues. The FLUSH_STOP call is synchronized with the STREAM_LOCK so any
    data used by the chain function can safely freed here if needed. Any 
    pending EOS events should be discarded too.

After the flush completes the second stage, data is flowing again in the pipeline
and all buffers are more recent than those before the flush.

For elements that use the pullrange function, they send both flush events to
the upstream pads in the same way to make sure that the pullrange function
unlocks and any pending buffers are cleared in the upstream elements.


EOS
---

The EOS event can only be sent on a sinkpad. It is typically emited by the
source element when it has finished sending data. This event is mainly sent
in the streaming thread but can also be sent from the application thread.

An EOS event sent on a srcpad returns GST_FLOW_UNEXPECTED.

The downstream element should forward the EOS event to its downstream peer
elements. This way the event will eventually reach the renderers which should
then post an EOS message on the bus when in PLAYING.

An element might want to flush its internally queued data before forwarding
the EOS event downstream. This flushing can be done in the same thread as
the one handling the EOS event.

For elements with multiple sink pads it might be possible to wait for EOS on
all the pads before forwarding the event.

The EOS event should always be interleaved with the data flow, therefore the
STREAM_LOCK should be taken.

Sometimes the EOS event is generated by another element than the source, for 
example a demuxer element can generate an EOS event before the source element.
This is not a problem, the demuxer does not send an EOS event to the upstream
element but returns GST_FLOW_UNEXPECTED, causing the source element to stop
sending data.

An element that sends EOS on a pad should stop sending data on that pad. Source
elements typically pause() their task for that purpose.

By default, a GstBin collects all EOS events from all its sinks before 
posting the EOS message to its parent.

The EOS is only posted on the bus by the sink elements in the PLAYING state. If
the EOS event is received in the PAUSED state, it is queued until the element
goes to PLAYING.

A seek event on an element flushes all pending EOS messages.


NEWSEGMENT
-------------

A newsegment event is sent downstream by an element to indicate that the following
group of buffers start and end at the specified positions. The newsegment event
also contains the playback speed of the stream.

Since the stream time is always set to 0 at start and after a seek, a 0
point for all next buffer's timestamps has to be propagated through the
pipeline using the NEWSEGMENT event.

Before sending buffers, an element must send a NEWSEGMENT event. An element is
free to refuse buffers if they were not preceeded by a NEWSEGMENT event.

Elements that sync to the clock should store the NEWSEGMENT start and end values
and substract the start value from the buffer timestamp before comparing
it against the stream time (see part-clocks.txt).

An element is allowed to send out buffers with the NEWSEGMENT start time already
substracted from the timestamp. If it does so, it needs to send a corrected
NEWSEGMENT downstream, ie, one with start time 0.

A NEWSEGMENT event should be generated as soon as possible in the pipeline and
is usually generated by a demuxer or source. The event is generated before 
pushing the first buffer and after a seek, right before pushing the new buffer.

The NEWSEGMENT event can be sent from both the application and the streaming
thread and should be serialized with the buffers.

Buffers should be clipped within the range indicated by the newsegment event
start and stop values. Sinks must to drop buffers with timestamps out
of the indicated newsegment range.

If a newsegment arrives at an element not preceeded by a flush event, the 
streamtime of the pipeline will not be reset to 0 so any element that syncs
to the clock must use the stop times of the previous newsegment events to
make the buffer timestamps increasing (part-segments.txt).

TAG
---    
  
The tag event is sent downstream when an element has discovered metadata
tags in a media file. Encoders can use this event to adjust their tagging
system. A tag is serialized with buffers.


BUFFERSIZE
----------

An element can suggest a buffersize for downstream elements. This is
typically done by elements that produce data on multiple source pads
such as demuxers.


QOS
---

A QOS, or quality of service message, is generated in an element to report
to the upstream elements about the current quality of the stream. This
is typically done by the sinks that measure the amount of framedrops they
have. (see part-qos.txt)


SEEK
----

A seek event is issued by the application to start playback of a new
position in the stream. It is called form the application thread and
travels upstream.

The seek event contains the new start and end position of playback
after the seek is performed. Optionally the end position can be left
at -1 to continue playback to the end of the stream. The seek event
also contains the new playback rate of the stream, 1.0 is normal playback,
2.0 double speed and negative values mean backwards playback.

A seek usually flushes the graph to minimize latency after the seek. This
behaviour is triggered by using the SEEK_FLUSH flag. 

The seek event is passed along from element to element until it reaches
an element that can perform the seek. No intermediate element is allowed
to assume that a seek to this location will happen. It is allowed to
modify the start and stop times if it needs to do so. this is typically
the case if a seek is requested for a non-time position.

The actual seek is performed in the application thread so that success
or failure can be reported as a return value of the seek event. It is
therefore important that before executing the seek, the element acquires
the STREAM_LOCK so that the streaming thread and the seek gets serialized.

The general flow of executing the seek with FLUSH is as follows:

 1) unblock the streaming threads, they could be blocked in a chain 
    function. This is done by sending a FLUSH_START on all srcpads.
    The flush will make sure that all downstream elements unlock and
    that control will return to this element chain/loop function.
    We cannot lock the STREAM_LOCK before doing this since it might
    cause a deadlock.

 2) acquire the STREAM_LOCK. This will work since the chain/loop function
    was unlocked in step 1).

 3) perform the seek. since the STREAM_LOCK is held, the streaming thread
    will wait for the seek to complete. Most likely, the stream thread
    will pause because the peer elements are flushing.

 4) send a FLUSH_STOP event to all peer elements to allow streaming again.

 5) send a NEWSEGMENT event to signal the new buffer timestamp base time.
    This can also be done from the streaming thread.

 6) start stopped tasks and unlock the STREAM_LOCK, dataflow will continue
    now from the new position.

More information about the different seek types can be found in 
part-seeking.txt.


NAVIGATION
----------

A navigation event is generated by a sink element to signal the elements
of a navigation event such as a mouse movement or button click.
Navigation events travel upstream.


DRAIN
-----

Drain event indicates that upstream is about to perform a real-time event, such
as pausing to present an interactive menu or such, and needs to wait for all
data it has sent to be played-out in the sink.

Drain should only be used by live elements, as it may otherwise occur during
prerolling.

Usually after draining the pipeline, an element either needs to modify timestamps,
or FLUSH to prevent subsequent data being discarded at the sinks for arriving
late (only applies during playback scenarios).