Whether or not an event is of specified type is normally tested using event->is_a. Other useful classifications are realised using event->is_up, event->is_down, event->is_drag, event->has_modifier, event<-click_time and event<-click_displacement.

One event requires special treatment. This is the event related to wheel-mice, mice normally having two buttons and a scroll-wheel. Rotations of the scroll-wheel are mapped to an event object with event<-id: wheel. The rotation is available as event<-rotation, which is @nil for all other events. A notch of the wheel is a rotation of 15 degrees. The value is positive if the wheel is rotated from the user and negative when rotated towards the user.

In the X11 version, wheel-events are normally mapped to the virtual mouse-buttons 4 and 5. If the class-variable event.x11_wheel_mouse = @on (default in the X11 version), the low-level event handling will map the button 4 and button 5 events to wheel events.

Horizontal scrolling, e.g., using a trackpad or tilting the wheel, is mapped to an event with event<-id: horizontal_wheel. Its event<-rotation is positive to the right. If a trackpad scrolls diagonally, the event reports the dominant direction.

The event<-detail of both events is a wheel_data object that holds the precise scroll distance in both directions, so a handler may use get(Ev, dx, DX) and get(Ev, dy, DY) for smooth scrolling.

In a window with window<->smooth_scroll @on, the event<-rotation may be a fraction of a notch. See there.

The wheel event is handled by class window, class editor, class list_browser and class scroll_bar. The wheel scrolls vertically, a page if Control is held down. As usual, the wheel with Shift held down scrolls horizontally, as does the wheel on a horizontal scroll_bar. If the receiver cannot scroll horizontally, Shift scrolls one line.

See also

event->is_a