Maintain's event-driven processing is based on triggers. Triggers are links between events and actions. An event is something that an end user does, such as pressing a function key; an action is something that the application executes, such as a function. (Functions are called cases by the Winform Painter.)

For example, you might define a trigger that links the PF7 key to the UpdateSalary function. At run time, each time an end user presses PF7, Maintain invokes UpdateSalary. In this case, you have defined pressing the PF7 key as the trigger event, and you have defined the UpdateSalary function as the trigger action. (Trigger actions are also known as event handlers.)

There are several kinds of triggers:

• Form-level triggers, which are triggered when an end user presses the specified function key when the cursor is anywhere on the form (on any spot not occupied by a control). You can assign 25 form-level triggers to each form (one trigger each for function keys PF1 through PF24, and one trigger for the Enter key).
• Control-level triggers, which are triggered when an end user presses the specified function key when the cursor is on the specified control, such as a particular list box. You can assign 25 control-level triggers to each control (one trigger each for function keys PF1 through PF24, and one trigger for the Enter key). The only exceptions are frames (which do not take any kind of trigger), and buttons (which take button shortcuts).
• Button shortcuts. Buttons do not have control-level triggers; instead, you can assign each button one shortcut key. You can assign any function key as a button's shortcut key. Like a control-level trigger, it is assigned to a single control (that is, to the button). Like a form-level trigger, it is active everywhere on the form (on any spot not occupied by a control, other than the button itself).
• System actions, which are similar to form-level triggers, but instead of invoking functions, they invoke special system-defined actions that do things like close the current form or exit the current procedure. You can assign 25 system actions to each form (one system action each for function keys PF1 through PF24, and one system action for the Enter key).

It is possible to assign the same key to several triggers and a system action. What would happen if an end user pressed that key? What action(s) would the key execute? That would be determined by the general order of precedence for resolving a key's assignments: control-level trigger, then form-level trigger or button shortcut, then system action. When an end user presses a function key, if:

1. The cursor is on a control, and one of the control's triggers has been assigned to that key, that trigger's Maintain function is performed. (Maintain functions—usually simply called functions—are referred to in the Winform Painter as cases.)

   If a system action has also been assigned to that key, the system action is performed immediately following the trigger's function. (If the trigger function closes the Winform, the system action is not performed.)

2. The cursor is not on a control—or it is on a control for which no trigger has been assigned to that key—but a form-level trigger or a button shortcut has been assigned to that key, the form-level trigger or button trigger's function is performed.

   If a system action has also been assigned to that key, the system action is performed immediately following the trigger's function. (If the trigger function closes the Winform, the system action is not performed.)

3. No form-level triggers have been assigned to that key, and no control-level triggers for that key are in effect, but a system action has been assigned to that key, the system action is performed.