Graphical applications are often referred to as event-driven applications. Unlike the hierarchical, menu-driven applications typical in a character terminal environment, graphical applications put the user more in control of the sequence of events. Using the mouse, menus, and active controls (like buttons on the screen that can respond to user actions), the user can navigate through the application with much more flexibility than in most older applications.

But this flexibility does not happen automatically. You, the developer, must build it into the application by programming procedures or blocks of code that respond to user actions. These are called triggers. The construction of the user interface of an application around blocks of trigger code is the single most fundamental difference in the architecture of event-driven applications.

You already saw some examples of the language constructs the AVM uses to establish triggers and respond to events when you looked through and extended the AppBuilder-generated code for the CustOrders window in Use Basic ABL Constructs In this section, you look at those constructs in a little more detail.