GstStateChange

state change from NULL to 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 (in case feature need to be probed).

state change from READY to PAUSED.

• The element pads are activated in order to receive data in PAUSED. Streaming threads are started.
• Some elements might need to return gst.types.StateChangeReturn.Async and complete the state change when they have enough information. It is a requirement for sinks to return gst.types.StateChangeReturn.Async and complete the state change when they receive the first buffer or gst.types.EventType.Eos (preroll). Sinks also block the dataflow when in PAUSED.
• A pipeline resets the running_time to 0.
• Live sources return gst.types.StateChangeReturn.NoPreroll and don't generate data.

state change from PAUSED to PLAYING.

• Most elements ignore this state change.
• The pipeline selects a #GstClock and distributes this to all the children before setting them to PLAYING. This means that it is only allowed to synchronize on the #GstClock in the PLAYING state.
• The pipeline uses the #GstClock and the running_time to calculate the base_time. The 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 gst.types.MessageType.Eos in the PLAYING state. It is not allowed to post gst.types.MessageType.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 gst.types.StateChangeReturn.Success.

state change from PLAYING to PAUSED.

• Most elements ignore this state change.
• The pipeline calculates the running_time based on the last selected #GstClock and the base_time. It stores this information to continue playback when going back to the PLAYING state.
• Sinks unblock any #GstClock wait calls.
• When a sink does not have a pending buffer to play, it returns #GST_STATE_CHANGE_ASYNC from this state change and completes the state change when it receives a new buffer or an gst.types.EventType.Eos.
• Any queued gst.types.MessageType.Eos items are removed since they will be reposted when going back to the PLAYING state. The EOS messages are queued in #GstBin containers.
• Live sources stop generating data and return gst.types.StateChangeReturn.NoPreroll.

state change from PAUSED to READY.

• Sinks unblock any waits in the preroll.
• Elements unblock any waits on devices
• Chain or get_range functions return gst.types.FlowReturn.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

state change from READY to NULL.

• Elements close devices
• Elements reset any internal state.

state change from NULL to NULL. (Since: 1.14)

state change from READY to READY, This might happen when going to PAUSED asynchronously failed, in that case elements should make sure they are in a proper, coherent READY state. (Since: 1.14)

state change from PAUSED to PAUSED. This might happen when elements were in PLAYING state and 'lost state', they should make sure to go back to real 'PAUSED' state (prerolling for example). (Since: 1.14)

state change from PLAYING to PLAYING. (Since: 1.14)