mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-14 05:12:09 +00:00
75 lines
3.3 KiB
Text
75 lines
3.3 KiB
Text
OUTDATED
|
|
--------
|
|
|
|
|
|
GST State Bits and Transition Rules (graph to follow)
|
|
-----------------------------------
|
|
|
|
These are the 'new' state bits and what they mean.
|
|
|
|
What the state bits are:
|
|
GST_STATE_COMPLETE: if the element has enough data, but is not in any kind
|
|
of running or explicitely stopped state. ready to be used.
|
|
GST_STATE_RUNNING: this is the normal state of the pipeline, where data
|
|
goes all the way through the pipeline normally.
|
|
GST_STATE_DISCOVERY: anything the element does in this state must be reset
|
|
after discovery. any data read from sync source must be cached.
|
|
GST_STATE_PREROLL: not a lot different from PLAYING, except sinks don't
|
|
render what they're getting. useful for elements that require
|
|
data to get in sync, such as an MPEG video decoder that needs
|
|
IBBPBB before starting at the next P.
|
|
|
|
|
|
|
|
Basic transition rules:
|
|
|
|
Completeness is based on the element having enough information to actually
|
|
do something. GST_STATE_COMPLETE is required for any other state to be
|
|
valid, though the only invariant is that you can't be RUNNING unless
|
|
you're COMPLETE. In fact, AFAICT, that's the *only* invariant.
|
|
|
|
The element is entirely in control of this bit at all times. There is no
|
|
way to externally change this bit except by changing the state of the
|
|
element in such a way as to effect a change.
|
|
|
|
|= GST_STATE_COMPLETE
|
|
setting whatever the last bit of info the element was looking for
|
|
(gst_object_set)
|
|
|
|
&= ~GST_STATE_COMPLETE
|
|
changing anything that invalidates the complete state of the element
|
|
|
|
|
|
Whether the element is running or not, on the other hand, is almost
|
|
entirely out of the hands of the individual element. This is generally
|
|
turned on by way of gst_element_run() as called by the parent (ultimately
|
|
by the Pipeline), which happens to optionally call a function private to
|
|
the element to prepare it. As per docs/random/gboolean, very likely this
|
|
function should return a TRUE/FALSE.
|
|
|
|
Generally, I think if there is no such function, the generic element code
|
|
should go ahead and set the state, and trigger the state_changed signal,
|
|
returning TRUE. If there is a function, call it. If it returns TRUE,
|
|
fire off the signal (since the signal is actually an Element signal
|
|
anyway, why eat another function call?). Return the result regardless.
|
|
|
|
|= GST_STATE_RUNNING
|
|
starting up the pipeline with gst_pipeline_start
|
|
|
|
~= ~GST_STATE_RUNNING
|
|
stopping the pipeline with gst_pipeline_stop, or some error state
|
|
|
|
gst_pipeline_start() simply calls the gst_element_start() function on each
|
|
of the elements in it. This sets the RUNNING bit of each element, and for
|
|
GstBin's it loops through that list. gst_pipeline_start() is just a
|
|
special case version of gst_bin_start(). All start() functions are
|
|
GstElementClass functions, meaning you can start any element the same way.
|
|
|
|
The pipeline can be stopped the same way, but more likely the pipeline
|
|
will be stopped due to some stoppage condition, such as EOF on the source
|
|
file, or the parser being told to stop the stream. In the EOF case, it
|
|
would turn its RUNNING bit off, then call the stop() class function on its
|
|
parent. This would trigger an up-hill, breath-first traversal of the
|
|
whole graph. Alternately, if each element lists its uber-parent (the
|
|
Pipeline) it can simply inform the pipeline directly, causing a
|
|
depth-first traversal just like the start() case.
|