mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-30 05:31:15 +00:00
82 lines
3.6 KiB
Text
82 lines
3.6 KiB
Text
OUTDATED
|
|
--------
|
|
|
|
Since the plan generation only happens as a result of the state mechanism,
|
|
I'll describe that first.
|
|
|
|
It's supposed to be recursive, such that setting the state on a Bin
|
|
recursively sets all the children. However, this needs to be rethought
|
|
somewhat, in light of some recent ideas on the actual definition of some
|
|
of the states.
|
|
|
|
The mechanism is thus: When you call gst_element_set_state(element,state),
|
|
it calls the change_state() class method. The basic Element-provided
|
|
version just sets or unsets the state. A more complex element like the
|
|
audiosink will switch on the state and do certain things like open or
|
|
close the sound card on transition to/from various states. The success or
|
|
failure of these actions can determine whether or not the state gets
|
|
[un]set as requested.
|
|
|
|
GtkObject signals enter in here, as whenever a state is successfully
|
|
changed, the STATE_CHANGE signal is fired, which gives higher-level code
|
|
the ability to do something based on the change.
|
|
|
|
The Bin's change_state function walks through all its children and sets
|
|
their state. This is where things get interesting, and where things are
|
|
going to need to be changed.
|
|
|
|
The issue is what the states are and mean. Currently the states are as
|
|
follows (from gstelement.h):
|
|
|
|
typedef enum {
|
|
GST_STATE_COMPLETE = (1 << 0),
|
|
GST_STATE_RUNNING = (1 << 1),
|
|
GST_STATE_DISCOVERY = (1 << 2),
|
|
GST_STATE_PREROLL = (1 << 3),
|
|
|
|
GST_STATE_PLAYING = (1 << 4),
|
|
GST_STATE_PAUSED = (1 << 5),
|
|
|
|
GST_STATE_MAX = (1 << 15),
|
|
} GstElementState;
|
|
|
|
COMPLETE means all the necesary information is available to run, i.e. the
|
|
filename for the disksrc, etc. RUNNING means that it's actually doing
|
|
something, but that's fuzzy. PLAYING means there really is data flowing
|
|
through the graph, where PAUSED temporary stops the flow. PLAYING &&
|
|
PAUSED is the same idea as !PLAYING, but there are probably going to be
|
|
many cases where there really is a distinction.
|
|
|
|
DISCOVERY is intended for the autoconnect case, in those instances where
|
|
the only way to determine the input or output type of some pad is for an
|
|
element to actually process some data. The idea in that case is that the
|
|
source element would be responsible for sending the data non-destructively
|
|
(in the case of a network client, it would have to save it all up, unless
|
|
it has seek capabilities over the network), and all downstream elements
|
|
process it in such a way as to not hose their own state. Or rather, when
|
|
they cease to do discovery, they completely wipe their state as if nothing
|
|
ever happened.
|
|
|
|
PREROLL is a local state, used for things like sending the first half of
|
|
an MPEG GOP through the decoder in order to start playback at a frame
|
|
somewhere in the middle of said GOP. Not sure how that will work,
|
|
exactly.
|
|
|
|
|
|
The issue is that these states aren't layered, and it most certainly isn't
|
|
the case that a container isn't able to be of a certain state unless all
|
|
of its children are. I guess I should explain the idea of reconfigurable
|
|
pipelines:
|
|
|
|
Build an MP3 player, give it the ability to use audio effects plugins.
|
|
Since you don't want to have to start the stream over again (especially if
|
|
it's a network stream) every time you change the effect. This means you
|
|
need to be able to freeze the pipeline in place to change it, without
|
|
taking too much time.
|
|
|
|
This matters when you consider that certain state changes should render
|
|
various state bits invalid. In the FROZEN state these won't happen,
|
|
because the assumption is that they're temporary.
|
|
|
|
If you haven't noticed by now, the state system isn't entirely
|
|
self-consistent yet. It needs work, and it needs discussion.
|