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138 lines
4.3 KiB
Text
138 lines
4.3 KiB
Text
This is a round up from our IRC session on events. It's open for
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discussion of course.
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Definition
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----------
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The event system is designed to be a mechanism for _inter_plugin_
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communication. Their scope is therefore limited in a way that they do
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not serve as a way to communicate between plugins and the app (signals
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and properties are still used for plugin-app communication).
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Events will be generated by either a plugin or the app. It should be
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possible for a plugin to generate an event on one of its pads and it
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should be possible for an app to insert an event on an arbitrary pad in
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the pipeline.
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Event handling
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--------------
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Events can both travel upstream or downstream. Some events, by nature,
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only travel in one direction.
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* downstream events
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- Travel in the same way buffers do. This includes that they are handled
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by the scheduler. The rationale is that the events should be kept
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as close to the buffers are possible.
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- plugins should check the type of the GstData passed in the _chain
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or _loop function and act appropriately. This can be done by either
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doing their own stuff or by calling the default handler.
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- are handled on the sink pad.
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* upstream events
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- are handled with an event handler attached to the srcpad. A default
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handler will be implemented for pads that don't implement their own
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handler.
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- travel as fast as possible. the rationale is that a seek event should
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get to the src element ASAP.
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Possible candidates for events
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------------------------------
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- QoS
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quality of service. Plugins can notify other plugins about the quality
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of the pipeline. A video element can for example say that it receives
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too much frames and that plugins connected to it need to slow down.
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- EOS
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A plugin can notify other plugins that it has run out-of-data.
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- Seek
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Used to notify plugins that they need to seek to a certain byte offset
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or timestamp.
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- discontinuous
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A plugin has detected a discontinuity in the stream. Other plugins
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might need to resync.
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- flush
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Plugins need to get rid of any buffered data ASAP.
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- caps nego??
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- bufferpool get??
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- ...
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application generated events
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----------------------------
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The application can insert events into the pipeline at arbitrary
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places. This will be done by calling gst_pad_event() on a pad.
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A first implementation will only cover inserting events on src pads
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since inserting events on sinkpads needs changes to the scheduler.
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Effects of events on plugins
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----------------------------
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some events are going to change the state of an element. The EOS event
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will for example change the state of an element to the PAUSED state. Not
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sure when or how this will happen.
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use cases
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---------
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1) filesrc ! fakesink
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filesrc will read until it reaches EOF. It will then create a GstEvent
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of type EOS and return it in the _get function. The event will travel
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downstream and will reach the fakesink element. Fakesink will detect
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the event in the _chain function and will call the default handler. The
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default handler will set the element to the paused state. filesrc will
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eventually change its state to PAUSED, probably before sending out the
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event (TBD)
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2) filesrc ! fakesink
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The app wants to perform a seek on filesrc. It'll call the gst_pad_event()
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on filesrcs src pad with the SEEK event type. The event handler will
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react and change filesrcs internal status. filesrc will return a DISCONT
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event before returning the buffer with the new offset.
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3) filesrc ! mpeg2parse video_0! queue ! { mpeg2dec ! xvideosink }
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lost of possibilities here: The app can choose to insert a seek event
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on the filesrc element (byte offset), it can insert a byte/time offset
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seek on the video_0 pad of mpeg2parse or it can insert a time seek event
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on mpeg2decs src pad.
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the event will travel upstream using the handlers and the intermediate
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elements can convert the event from a time to a byte offset (possibly
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using GstTimeCache to speed up things).
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Filesrc will get a byte seek event on its src pad and will proceed as
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in case 2.
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As can be seen from this example the app will generate an event in another
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context than those of the plugins, so this will need proper locking.
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The app can also choose to insert a flush event on one of the src
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pads. The plugins would clear their cached data and forward the event
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to their upstream peer pad(s).
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4)...
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Insert impossible case here..
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