GStreamer State Transition
overview
Both elements and pads can be in different states. The states of the pads are linked to the state of the element so the design of the states is mainly focused around the element states.
An element can be in 4 states. NULL, READY, PAUSED and PLAYING. When an element is initially instantiated, it is in the NULL state.
State definitions
- NULL: This is the initial state of an element.
- READY: The element should be prepared to go to PAUSED.
- PAUSED: The element should be ready to accept and process data. Sink elements, however, only accept one buffer and then block.
- PLAYING: The same as PAUSED except for live sources and sinks. Sinks accept and render data. Live sources produce data.
We call the sequence NULL→PLAYING an upwards state change and PLAYING→NULL a downwards state change.
State transitions
the following state changes are possible:
NULL -> READY:
The element must check if the resources it needs are available. Device sinks and sources typically try to probe the device to constrain their caps.
The element opens the device, this is needed if the previous step requires the device to be opened.
READY -> PAUSED:
The element pads are activated in order to receive data in PAUSED. Streaming threads are started.
Some elements might need to return ASYNC and complete the state change when they have enough information. It is a requirement for sinks to return ASYNC and complete the state change when they receive the first buffer or EOS event (preroll). Sinks also block the dataflow when in PAUSED.
A pipeline resets the running_time to 0.
Live sources return NO_PREROLL and don't generate data.
PAUSED -> PLAYING:
Most elements ignore this state change.
The pipeline selects a clock and distributes this to all the children before setting them to PLAYING. This means that it is only allowed to synchronize on the clock in the PLAYING state.
The pipeline uses the clock and the running_time to calculate the base_time. This base_time is distributed to all children when performing the state change.
Sink elements stop blocking on the preroll buffer or event and start rendering the data.
Sinks can post the EOS message in the PLAYING state. It is not allowed to post EOS when not in the PLAYING state.
While streaming in PAUSED or PLAYING elements can create and remove sometimes pads.
Live sources start generating data and return SUCCESS.
PLAYING -> PAUSED:
Most elements ignore this state change.
The pipeline calculates the running_time based on the last selected clock and the base_time. It stores this information to continue playback when going back to the PLAYING state.
Sinks unblock any clock wait calls.
When a sink does not have a pending buffer to play, it returns ASYNC from this state change and completes the state change when it receives a new buffer or an EOS event.
Any queued EOS messages are removed since they will be reposted when going back to the PLAYING state. The EOS messages are queued in GstBins.
Live sources stop generating data and return NO_PREROLL.
PAUSED -> READY:
Sinks unblock any waits in the preroll.
Elements unblock any waits on devices
Chain or get_range() functions return FLUSHING.
The element pads are deactivated so that streaming becomes impossible and all streaming threads are stopped.
The sink forgets all negotiated formats
Elements remove all sometimes pads
READY -> NULL:
Elements close devices
Elements reset any internal state.
reference
https://gstreamer.freedesktop.org/documentation/additional/design/states.html?gi-language=c