mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-04 23:46:43 +00:00
1c17e4700b
Original commit message from CVS: Get rid of NEW_MEDIA event, EOS is supposed to handle that
239 lines
8 KiB
Text
239 lines
8 KiB
Text
This is the current implementation of events, based on an earlier
|
|
document, in this same directory, called "events".
|
|
|
|
Definition
|
|
----------
|
|
|
|
The event system is designed to be a mechanism for _inter_plugin_
|
|
communication. Their scope is therefore limited in a way that they do
|
|
not serve as a way to communicate between plugins and the app (signals
|
|
and properties are still used for plugin-app communication).
|
|
|
|
Events will be generated by either a plugin or the app. It should be
|
|
possible for a plugin to generate an event on one of its pads and it
|
|
should be possible for an app to insert an event on an abitrary pad or
|
|
element in the pipeline.
|
|
|
|
|
|
Event handling
|
|
--------------
|
|
|
|
Events can both travel upstream or downstream. Some events, by nature,
|
|
only travel in one direction.
|
|
|
|
* downstream events
|
|
|
|
- Travel in the same way buffers do. This includes that they are handled
|
|
by the scheduler. The rationale is that the events should be kept
|
|
as close to the buffers are possible.
|
|
|
|
- plugins should check the type of the GstData passed in the _chain
|
|
or _loop function and act appropriatly. This can be done by either
|
|
doing their own stuff or by calling the default handler.
|
|
|
|
- are handled on the sink pad.
|
|
|
|
* upstream events
|
|
|
|
- are handled with an event handler attached to the srcpad. A default
|
|
handler is implemented for pads that don't implement their own
|
|
handler.
|
|
|
|
- travel as fast as possible. the rationale is that a seek event should
|
|
get to the src element ASAP.
|
|
|
|
|
|
|
|
Plugin generated events
|
|
-----------------------
|
|
|
|
We cover the range of events a plugin can generate and how they
|
|
are supposed to handle them.
|
|
|
|
* EOS
|
|
|
|
when the plugin has no more data to push, it pushes an EOS
|
|
event and calls gst_element_set_eos.
|
|
|
|
_get based plugins should call gst_element_set_eos() before
|
|
returning the EOS event.
|
|
|
|
gst_element_set_eos() will put an element into the PAUSED state
|
|
and will emit the eos signal to the app.
|
|
|
|
Elements receiving the EOS event on a pad should not pull anymore
|
|
data from that pad (in case of a loop based element).
|
|
If the plugin cannot operate when it doesn't receive data on that
|
|
pad, the element should go to EOS too. It does this by pushing
|
|
any data it might still have, to the srcpad after which it
|
|
pushes an EOS event on thet pad and calls gst_element_set_eos().
|
|
|
|
The EOS event will typically originate from a source element, it
|
|
will eventually put all elements into the PAUSED state so that the
|
|
pipeline stops.
|
|
|
|
The EOS event is strictly a downstream event.
|
|
|
|
* DISCONTINUOUS
|
|
|
|
The discontinuous event is used to indicate a discontinuity in the
|
|
stream to downstream elements. A disctontinuity happens for the
|
|
following reasons:
|
|
|
|
- a source element is sending the first buffers of a stream.
|
|
- an element has performed a seek operation resulting in a
|
|
discontinuity in the data.
|
|
|
|
elements that receive a discontinuity event should flush any state
|
|
they might have and treat the new data as the start of new data.
|
|
Before sending out the new data the element must send a discont event
|
|
to the downstream element with at least a GST_FORMAT_TIME or
|
|
GST_FORMAT_BYTES as the format/value pair (see below).
|
|
|
|
a DISCONTINUOUS event has a flush flag. If that flag is set, the
|
|
element should also remove any buffered data it might have.
|
|
|
|
In addition to the flush flag, a DISCONTINUOUS event also caries up
|
|
to GST_DISCONTINUOUS_MAX_FORMATS format/value pairs. These values
|
|
might be used by the receiver of the event to resynchronize itself.
|
|
|
|
elements that are using a clock must take special actions upon
|
|
receiving the DISCONTINUOUS event: they must call
|
|
gst_clock_handle_discont() with the GST_FORMAT_TIME value of
|
|
the discont event.
|
|
|
|
The DISCONTINUOUS event is strictly a downstream event.
|
|
|
|
|
|
* SEEK
|
|
|
|
The seek event is used to reposition the upstream elements to
|
|
a new position in the media stream.
|
|
|
|
The seek event caries a bitwise-or of a GstFormat and a GstSeekType.
|
|
|
|
An element receiving the seek event on its srcpad should try to
|
|
reposition itself as closely to the requested location as possible.
|
|
if the ACCURATE flag is set and it cannot reposition itself with
|
|
absolute certainty, it should reposition itself well before the
|
|
requested position. If the ACCURATE flag is not set, the element
|
|
is free to choose a suitable position in the stream before or after
|
|
the requested time.
|
|
|
|
If the flush flag is set it should make sure that it pushes the
|
|
DISCONTINUOUS event the next time it is scheduled, clearing any
|
|
data it might have buffered.
|
|
|
|
The element is free to set the ACCURACY field in the event to notify
|
|
the originator of the event of the result. If no accuracy is set, it
|
|
defaults to FUZZY.
|
|
|
|
The SEEK event is stricly an upstream event.
|
|
|
|
* QOS
|
|
|
|
QoS is sent to indicate Quality of Service to the upstream element(s).
|
|
|
|
The QOS event is stricly an upstream event.
|
|
|
|
* FLUSH
|
|
|
|
A plugin can send a flush event to its src or sink peer to clear the
|
|
buffered contents in the pipeline.
|
|
|
|
|
|
application generated events
|
|
----------------------------
|
|
|
|
The application can insert events into the pipeline at arbirary
|
|
places. This is done by calling gst_pad_send_event() on a pad.
|
|
|
|
An application can also insert events on any element. The element
|
|
can implement its own handler for these events or it can use
|
|
the default handler, which simply forwards the event to the first
|
|
connected sinkpad of the element. Events to an element are send using
|
|
gst_element_send_event().
|
|
|
|
This first implementation only covers inserting events on src pads
|
|
since inserting events on sinkpads needs changes to the scheduler.
|
|
|
|
|
|
* FLUSH
|
|
|
|
A flush event is used to clear any buffered data an element might
|
|
have. It is a downstream and upstream event.
|
|
|
|
Flush events are typically inserted into the pipeline by the app.
|
|
Elements that buffer data should implement an event handler on
|
|
both the sink and src pads it might have and respond to the flush
|
|
event by clearing all data they have buffered.
|
|
|
|
* SEEK
|
|
|
|
The application can insert the seek event into the pipeline on
|
|
a srcpad with gst_pad_send_event () or on an element with
|
|
gst_element_send_event().
|
|
|
|
The semantics of the seek parameters are described above.
|
|
|
|
|
|
Thread safety
|
|
-------------
|
|
|
|
All upstream events can occur outside the element's thread context.
|
|
It is not required to protect the element's data structures with
|
|
mutexes because in principal we don't support sending events to
|
|
a running threaded pipeline.
|
|
|
|
GstQueue in its current form will refuse to pass events upstream if
|
|
it is in the PLAYING state. Future versions might pass the event
|
|
on as soon as the element is scheduled again on the sinkpad.
|
|
|
|
An application cannot insert an event on an element that is PLAYING
|
|
in another thread context. It is therefore strongly recommended to
|
|
PAUSE the threaded pipeline before inserting an event.
|
|
|
|
|
|
use cases
|
|
---------
|
|
|
|
1) filesrc ! fakesink
|
|
|
|
<< explain EOS >>
|
|
|
|
|
|
2) filesrc ! fakesink
|
|
|
|
The app wants to perform a seek on filesrc. It'll call the gst_pad_send_event()
|
|
on filesrcs src pad with the SEEK event type. The event handler will
|
|
react and change filesrcs internal status. filesrc will return a DISCONT
|
|
event before returning the buffer with the new offset.
|
|
|
|
3) filesrc ! mpeg2parse video_0! queue ! { mpeg2dec ! xvideosink }
|
|
|
|
lost of possibilities here: The app can choose to insert a seek event
|
|
on the filesrc element (byte offset), it can insert a byte/time offset
|
|
seek on the video_0 pad of mpeg2parse or it can insert a time seek event
|
|
on mpeg2decs src pad.
|
|
|
|
the event will travel upstream using the handlers and the intermediate
|
|
elements can convert the event from a time to a byte offset (possibly
|
|
using GstTimeCache to speed up things).
|
|
|
|
Filesrc will get a byte seek event on its src pad and will proceed as
|
|
in case 2.
|
|
|
|
As can be seen from this example the app will generate an event in another
|
|
context than those of the plugins, so this will need proper locking.
|
|
|
|
The app can also choose to insert a flush event on one of the src
|
|
pads. The plugins would clear their cached data and forward the event
|
|
to their upstream peer pad(s).
|
|
|
|
4)...
|
|
|
|
Insert impossible case here..
|
|
|
|
|
|
|
|
|