Stream Status
-------------

This document describes the design and use cases for the stream status
messages.

STREAM_STATUS messages are posted on the bus when the state of a streaming
thread changes. The purpose of this message is to allow the application to
interact with the streaming thread properties, such as the thread priority or
the threadpool to use.

We accommodate for the following requirements:

 - Application is informed when a streaming thread is about to be created. It
   should be possible for the application to suggest a custom GstTask.
 - Application is informed when the status of a streaming thread is changed.
   This can be interesting for GUI application that want to visualize the status
   of the streaming threads (playing/paused/stopped)
 - Application is informed when a streaming thread is destroyed.

We allow for the following scenarios:

 - Elements require a specific (internal) streaming thread to operate or the
   application can create/specify a thread for the element.
 - Elements allow the application to configure a priority on the threads.


Use cases
~~~~~~~~~

 * boost the priority of the udp receiver streaming thread

    .--------.    .-------.    .------.    .-------.
    | udpsrc |    | depay |    | adec |    | asink |
    |       src->sink    src->sink   src->sink     |
    '--------'    '-------'    '------'    '-------'

  - when going from READY to PAUSED state, udpsrc will require a streaming
    thread for pushing data into the depayloader. It will post a STREAM_STATUS
    message indicating its requirement for a streaming thread.

  - The application will usually react to the STREAM_STATUS messages with a sync
    bus handler.

  - The application can create and configure a custom GstTask to manage the
    streaming thread or it can ignore the message which will make the element
    use its default GstTask.

  - The application can react to the ENTER/LEAVE stream status message to
    configure the thread right before it is started/stopped. This can be used to
    configure the thread priority.

  - Before the GstTask is changed state (start/pause/stop) a STREAM_STATUS
    message is posted that can be used by the application to keep track of
    the running streaming threads.


Messages
~~~~~~~~

 The existing STREAM_STATUS message will be further defined and implemented in
 (selected) elements. The following fields will be contained in the message:

  - "type", GST_TYPE_STREAM_STATUS_TYPE

      - a set of types to control the lifecycle of the thread:
       GST_STREAM_STATUS_TYPE_CREATE: a new streaming thread is going to be
          created. The application has the chance to configure a custom thread.
       GST_STREAM_STATUS_TYPE_ENTER: the streaming thread is about to enter its
          loop function for the first time.
       GST_STREAM_STATUS_TYPE_LEAVE: the streaming thread is about to leave its
          loop.
       GST_STREAM_STATUS_TYPE_DESTROY: a streaming thread is destroyed

      - A set of types to control the state of the threads: 
       GST_STREAM_STATUS_TYPE_START: a streaming thread is started
       GST_STREAM_STATUS_TYPE_PAUSE: a streaming thread is paused
       GST_STREAM_STATUS_TYPE_STOP: a streaming thread is stopped

  - "owner", GST_TYPE_ELEMENT
      The owner element of the thread. The message source will contain the pad
      (or one of the pads) that will produce data by this thread. If this thread
      does not produce data on a pad, the message source will contain the owner
      as well. The idea is that the application should be able to see from the
      element/pad what function this thread has in the context of the
      application and configure the thread appropriatly.

  - "object", G_TYPE, GstTask/GThread
      A GstTask/GThread controlling this streaming thread. This can be NULL when
      the object controlling the streaming thread is not yet created.

  - "flow-return", GstFlowReturn
      A status code for why the thread state changed. when threads are created
      and started, this is usually GST_FLOW_OK but when they are stopping it
      contains the reason code why it stopped.

  - "reason", G_TYPE_STRING
      A string describing the reason why the thread started/stopped/paused.
      Can be NULL if no reason is given.


Events
~~~~~~