addZone method
Implementation
int addZone(
{Function(html.MouseEvent e)? onDragOver,
Function(html.MouseEvent e)? onDrop}) {
if (onDragOver != null) {
_dragFunctions[currentZoneId] = onDragOver;
}
if (onDrop != null) {
_dropFunctions[currentZoneId] = onDrop;
}
return currentZoneId++;
}