Overview of Event Notification
A filter notifies the Filter Graph Manager about an event by posting an event notification. The event could be something expected, such as the end of a stream, or it could represent an error, such as a failure to render a stream. The Filter Graph Manager handles some filter events by itself, and it leaves others for the application to handle. If the Filter Graph Manager does not handle a filter event, it places the event notification into a queue. The filter graph can also queue its own event notifications for the application.
An application retrieves events from the queue and responds to them based on the type of event. Event notification in DirectShow is therefore similar to the Microsoft Windows message queuing scheme. An application can also cancel the Filter Graph Manager's default behavior for a given event type. The Filter Graph Manager then puts those events directly into the queue for the application to handle.
This mechanism enables
- The Filter Graph Manager to communicate with the application.
- Filters to communicate with both the application and the Filter Graph Manager.
- The application to determine its degree of involvement in handling events.