docs/design/part-gstbin.txt

GstBin
------

GstBin is a container element for other GstElements. This makes it possible
to group elements together so that they can be treated as one single 
GstElement. A GstBin provides a GstBus for the children and collates messages
from them.

Add/removing elements
~~~~~~~~~~~~~~~~~~~~~

The basic functionality of a bin is to add and remove GstElements to/from it.
gst_bin_add() and gst_bin_remove() perform these operations respectively.

The bin maintains a parent-child relationship with its elements (see part-
relations.txt).


Retrieving elements
~~~~~~~~~~~~~~~~~~~

GstBin provides a number of functions to retrieve one or more children from
itself. A few examples of the provided functions:

gst_bin_get_by_name() retrieves an element by name.

gst_bin_iterate_elements() returns an iterator to all the children.


element management
~~~~~~~~~~~~~~~~~~

The most important function of the GstBin is to distribute all GstElement
operations on itself to all of its children. This includes:

 - state changes
 - index get/set
 - clock get/set

The state change distribution is the most complex and is explained in 
part-states.txt.

GstBus
~~~~~~

The GstBin creates a GstBus for its children and distributes it when child
elements are added to the bin. The bin attaches a sync handler to receive
messages from children. The bus for receiving messages from children is 
distinct from the bin's own externally-visible GstBus. 

Messages received from children are forwarded intact onto the bin's 
external message bus, except for EOS and SEGMENT_START/DONE which are 
handled specially.

ASYNC_START/ASYNC_STOP messages received from the children are used to
trigger a recalculation of the current state of the bin, as described in 
part-states.txt.

The application can retrieve the external GstBus and integrate it in the
mainloop or it can just _pop() messages off in its own thread.

When a bin goes to READY it will clear all cached messages.


EOS
~~~

The sink elements will post an EOS message on the bus when they reach EOS. The
EOS message is only posted to the bus when the sink element is in PLAYING.

The bin collects all EOS messages and forwards it to the application as
soon as all the sinks have posted an EOS.

The list of queued EOS messages is cleared when the bin goes to PAUSED
again. This means that all elements should repost the EOS message when going
to PLAYING again.


SEGMENT_START/DONE
~~~~~~~~~~~~~~~~~~

A bin collects SEGMENT_START messages but does not post them to the application.
It counts the number of SEGMENT_START messages and posts a SEGMENT_STOP message
to the application when an equal number of SEGMENT_STOP messages where received.

The cached SEGMENT_START/STOP messages are cleared when going to READY.


DURATION
~~~~~~~~

When a DURATION query is performed on a bin, it will forward the query to all
its sink elements. The bin will calculate the total duration as the MAX of all
returned durations and will then cache the result so that any further query can
use the cached version. The reason for caching the result is because the
duration of a stream typically does not change that often.

A GST_MESSAGE_DURATION_CHANGED posted by an element will clear the cached
duration value so that the bin will query the sinks again. This message is
typically posted by elements that calculate the duration of the stream based
on some average bitrate, which might change while playing the stream. The
DURATION_CHANGED message is posted to the application, which can then fetch
the updated DURATION.


Subclassing
~~~~~~~~~~~

Subclasses of GstBin are free to implement their own add/remove implementations.
It is a good idea to update the GList of children so that the _iterate() functions
can still be used if the custom bin allows access to its children.

Any bin subclass can also implement a custom message handler by overriding the
default message handler.