AreaHandler defines the functionality needed for handling events from an Area. Each of the methods on AreaHandler is called from the GUI thread, and every parameter (other than the Area itself) should be assumed to only be valid during the life of the method call (so for instance, do not save AreaDrawParams.AreaWidth, as that might change without generating an event).

Coordinates to Draw and MouseEvent are given in points. Points are generic, floating-point, device-independent coordinates with (0,0) at the top left corner. You never have to worry about the mapping between points and pixels; simply draw everything using points and you get nice effects like looking sharp on high-DPI monitors for free. Proper documentation on the matter is being written. In the meantime, there are several referenes to this kind of drawing, most notably on Apple's website:

For a scrolling Area, points are automatically offset by the scroll position. So if the mouse moves to position (5,5) while the horizontal scrollbar is at position 10 and the horizontal scrollbar is at position 20, the coordinate stored in the AreaMouseEvent structure is (15,25). The same applies to drawing.

AreaHandler is referenced in 1 repository


type AreaHandler interface {
	// Draw is sent when a part of the Area needs to be drawn.
	// dp will contain a drawing context to draw on, the rectangle
	// that needs to be drawn in, and (for a non-scrolling area) the
	// size of the area. The rectangle that needs to be drawn will
	// have been cleared by the system prior to drawing, so you are
	// always working on a clean slate.
	// If you call Save on the drawing context, you must call Release
	// before returning from Draw, and the number of calls to Save
	// and Release must match. Failure to do so results in undefined
	// behavior.
	Draw(a *Area, dp *AreaDrawParams)

	// MouseEvent is called when the mouse moves over the Area
	// or when a mouse button is pressed or released. See
	// AreaMouseEvent for more details.
	// If a mouse button is being held, MouseEvents will continue to
	// be generated, even if the mouse is not within the area. On
	// some systems, the system can interrupt this behavior;
	// see DragBroken.
	MouseEvent(a *Area, me *AreaMouseEvent)

	// MouseCrossed is called when the mouse either enters or
	// leaves the Area. It is called even if the mouse buttons are being
	// held (see MouseEvent above). If the mouse has entered the
	// Area, left is false; if it has left the Area, left is true.
	// If, when the Area is first shown, the mouse is already inside
	// the Area, MouseCrossed will be called with left=false.
	// TODO what about future shows?
	MouseCrossed(a *Area, left bool)

	// DragBroken is called if a mouse drag is interrupted by the
	// system. As noted above, when a mouse button is held,
	// MouseEvent will continue to be called, even if the mouse is
	// outside the Area. On some systems, this behavior can be
	// stopped by the system itself for a variety of reasons. This
	// method is provided to allow your program to cope with the
	// loss of the mouse in this case. You should cope by cancelling
	// whatever drag-related operation you were doing.
	// Note that this is only generated on some systems under
	// specific conditions. Do not implement behavior that only
	// takes effect when DragBroken is called.
	DragBroken(a *Area)

	// KeyEvent is called when a key is pressed while the Area has
	// keyboard focus (if the Area has been tabbed into or if the
	// mouse has been clicked on it). See AreaKeyEvent for specifics.
	// Because some keyboard events are handled by the system
	// (for instance, menu accelerators and global hotkeys), you
	// must return whether you handled the key event; return true
	// if you did or false if you did not. If you wish to ignore the
	// keyboard outright, the correct implementation of KeyEvent is
	// 	func (h *MyHandler) KeyEvent(a *ui.Area, ke *ui.AreaKeyEvent) (handled bool) {
	// 		return false
	// 	}
	KeyEvent(a *Area, ke *AreaKeyEvent) (handled bool)