Class Context

Applications can set a context on a complete pipeline by using gst_element_set_context(), which will then be propagated to all child elements. Elements can handle these in #GstElementClass::set_context and merge them with the context information they already have.

When an element needs a context it will do the following actions in this order until one step succeeds:

1. Check if the element already has a context
2. Query downstream with %GST_QUERY_CONTEXT for the context
3. Query upstream with %GST_QUERY_CONTEXT for the context
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
5. Create a context by itself and post a %GST_MESSAGE_HAVE_CONTEXT message on the bus.

Bins will catch %GST_MESSAGE_NEED_CONTEXT messages and will set any previously known context on the element that asks for it if possible. Otherwise the application should provide one if it can.

#GstContext can be persistent. A persistent #GstContext is kept in elements when they reach %GST_STATE_NULL, non-persistent ones will be removed. Also, a non-persistent context won't override a previous persistent context set to an element.