An input device trigger is a physical device associated with a trigger mapping. The operator uses a trigger to indicate significant moments in time. At these times we say that the trigger is fired. A given operator action (e.g., pressing a mouse button) fires at most one trigger. Several input devices can refer to the same trigger.
A trigger represents a validation process, an active and independent process which sends a message to one or more receivers when it is fired. An input device is the receiver of its trigger if there is a waiting REQUEST for it or if it is in EVENT mode. These two conditions may be true simultaneously for different input devices. The validation process exists if there is at least one receiver for a trigger. Otherwise the process does not exist.
If a REQUEST for an input device is waiting when the device trigger is fired, the measure made by this device is used to satisfy the REQUEST. If one or more devices containing a given trigger are in EVENT mode when the trigger is fired, the identification of these devices and their measure values are passed to the input file mechanism as distinct event records.
When the use of a trigger succeeds in satisfying a REQUEST, or in adding event records to the input file, GPHIGS sends the operator an acknowledgement the format of which depends on the implementation of the input device. This acknowledgement cannot be controlled by any function.