gstreamer/docs/design/part-context.txt

Context
-------

GstContext is a container object, containing a type string and a
generic GstStructure. It is used to store and propagate context
information in a pipeline, like device handles, display server
connections and other information that should be shared between
multiple elements in a pipeline.

For sharing context objects and distributing them between application
and elements in a pipeline, there are downstream queries, upstream
queries, messages and functions to set a context on a complete pipeline.


Context types
~~~~~~~~~~~~~
Context type names should be unique and be put in appropriate namespaces,
to prevent name conflicts, e.g. "gst.egl.EGLDisplay". Only one specific
type is allowed per context type name.


Elements
~~~~~~~~
Elements that need a specific context for their operation would
do the following steps until one succeeds:

 1) Check if the element already has a context of the specific type,
    i.e. it was previously set via gst_element_set_context().

 2) Query downstream with GST_QUERY_CONTEXT for the context and check if
    downstream already has a context of the specific type

 3) Query upstream with GST_QUERY_CONTEXT for the context and check if
    upstream already has a context of the specific type

 4) Post a GST_MESSAGE_NEED_CONTEXT message on the bus with the required
    context types and afterwards check if a usable context was set now
    as in 1). The message could be handled by the parent bins of the
    element and the application.

 4) Create a context by itself and post a GST_MESSAGE_HAVE_CONTEXT message
    on the bus.


Bins will propagate any context that is set on them to their child elements via
gst_element_set_context(). Even to elements added after a given context has
been set.

Bins can handle the GST_MESSAGE_NEED_CONTEXT message, can filter both
messages and can also set different contexts for different pipeline parts.


Applications
~~~~~~~~~~~~
Applications can set a specific context on a pipeline or elements inside
a pipeline with gst_element_set_context().

If an element inside the pipeline needs a specific context, it will post
a GST_MESSAGE_NEED_CONTEXT message on the bus. The application can now
create a context of the requested type or pass an already existing context
to the element (or to the complete pipeline).

Whenever an element creates a context internally it will post a
GST_MESSAGE_HAVE_CONTEXT message on the bus. Bins will cache these
contexts and pass them to any future element that requests them.