An event is an action that occurs. This action can take place on the part of the user of an application (such as when the user clicks a command button), on the part of application code (such as when a change is made to a record in a recordset), or on the part of the operating system (such as a timer event). When an event occurs, we say that the event is raised, or fired.
Each event has a source. This is the object to which the action is applied, such as the button that was clicked. The source is responsible for alerting the operating system that an event has occurred. It does so by sending an event notification message, generally to its parent or container window. For this reason, Microsoft refers to the event source as the sender.
An event often has an event argument, which is simply data that pertains to the event. For instance, the press of a keyboard key generates an event that includes event arguments describing the keycode of the key pressed and information on the state of modifier keys (the Shift, Alt, and Ctrl keys). The event arguments are part of the message sent by the event source.
An event handler is a procedure (or method) that is executed as a result of event notification. The process of declaring an event handler for an event is called binding the procedure to the event.
Most controls have a large number of built-in events associated with them. For instance, the textbox control has events associated with changing ...