Gstreamer State Change

• Some concepts of States
  1. States defined in gstreamer
     • GST_STATE_VOID_PENDING: the state of an element is not set.
     • GST_STATE_NULL: initial state of an element.
     • GST_STATE_READY: all sources are ready and can go to pause.
     • GST_STATE_PAUSE: ready to accept and process data.
     • GST_STATE_PLAYING: the GstClock is running and the data is flowing.
  2. Variables in GstElement
     • current_state: the current state of an element.
     • next_state: the next state of an element.
     • pending_state: the final state the element should go to.
     • target_state: last state the element we set.
     • last_return: the last return value of an element state change.
  3. The states switch
     
• Create a simple pipe to understand what are states

  $ gst-launch-0.10 alsasrc ! audiorate ! filesink location=test.wav
  

• At the beginning, gstreamer core how to initialize each element
  1. Use funtion gst_element_factory_make("alsasrc", "sound") to create alsasrc element
  2. When create instance of alsasrc(sound), gst_element_init() will initial state of the element.

     GST_STATE_TARGET (element) = GST_STATE_NULL;            //target_state

     GST_STATE_NEXT (element) = GST_STATE_VOID_PENDING;      //next_state

     GST_STATE_PENDING (element) = GST_STATE_VOID_PENDING;   //pending_state

     GST_STATE_RETURN (element) = GST_STATE_CHANGE_SUCCESS;  //last_return 

     GST_STATE (element) = GST_STATE_NULL;                   //current_state 

• Use function gst_bin_add_many to add elements to pipeline. Just add each element to GstBin->children, didn't change state of element
• After link success I want to change state of pipeline from GST_STATE_NULL to GST_STATE_PLAYING
  1. Now the state of element is GST_STATE_NULL, first change the state to GST_STATE_READY. All elements are added in GstBin->cheldren, when we change the state of pipeline to GST_STATE_READY, pipeline will call gst_bin_change_state_func to find child elemnt to change state to GST_STATE_READY. each child element will call plugin->change_state which defined by ourselves.
  2. If state change to GST_STATE_READY successfully, update state of element, then try to change state of pipeline from GST_STATE_READY to GST_STATE_PAUSE. Alsa call gst_bin_change_state_func, the same as step 1.
  3. If state change to GST_STATE_PAUSE successfully, update state of element, then try to change state of pipeline from GST_STATE_PAUSE to GST_STATE_READY. Alsa call gst_bin_change_state_func, the same as step 1.
• When accept message GST_MESSAGE_EOS, try to change the state of pipeline from GST_STATE_PLAYING to GST_STATE_NULL