Context is a container object used to store contexts like a device context, a display server connection and similar concepts that should be shared between multiple elements.
Applications can set a context on a complete pipeline by using 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 CONTEXT for the context 3. Query upstream with CONTEXT for the context 4. Post a 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 HAVE_CONTEXT message on the bus.
Bins will catch 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.
Context can be persistent. A persistent Context is kept in elements when they reach NULL, non-persistent ones will be removed. Also, a non-persistent context won't override a previous persistent context set to an element.