Visual Basic Concepts

How an Event-Driven Application Works

An event is an action recognized by a form or control. Event-driven applications execute Basic code in response to an event. Each form and control in Visual Basic has a predefined set of events. If one of these events occurs and there is code in the associated event procedure, Visual Basic invokes that code.

Although objects in Visual Basic automatically recognize a predefined set of events, it is up to you to decide if and how they will respond to a particular event. A section of code — an event procedure — corresponds to each event. When you want a control to respond to an event, you write code in the event procedure for that event.

The types of events recognized by an object vary, but many types are common to most controls. For example, most objects recognize a Click event — if a user clicks a form, code in the form's Click event procedure is executed; if a user clicks a command button, code in the button's Click event procedure is executed. The actual code in each case will most likely be quite different.

Here's a typical sequence of events in an event-driven application:

The application starts and a form is loaded and displayed.
The form (or a control on the form) receives an event. The event might be caused by the user (for example, a keystroke), by the system (for example, a timer event), or indirectly by your code (for example, a Load event when your code loads a form).
If there is code in the corresponding event procedure, it executes.
The application waits for the next event.

Note Many events occur in conjunction with other events. For example, when the DblClick event occurs, the MouseDown, MouseUp, and Click events also occur.