FAQ 34.37 What are events?

Asynchronous actions that are fired by ActiveX controls and handled by control containers.

ActiveX Controls (and OCXs) define "events" in addition to properties and methods. Events are asynchronous actions that are fired by the ActiveX control and are handled by the control container. An event is like a method in that it has a name, parameters, and a return value, and is implemented using Automation. Events are different from methods in that methods are defined by the control and implemented by the control, whereas events are defined by the control and implemented by the control container.

Some events are GUI events: if the user clicks on a button that is an ActiveX control, the button could fire an event notifying its control container. Other events are database events: if an ActiveX control is bound to a record in a database and the record is updated by another program, the control could fire an event notifying its control container and the container might query the database and update its display. Many other asynchronous actions can be implemented as events including timer events and network events.

Events are asynchronous. In this context, asynchronous means that the control can fire the event at any time and the firing of the event is not initiated by (or stimulated by) the control container. Asynchronous does not mean nonblocking, as it might in the context of asynchronous message queueing.