onPointerMove method
Implementation
@override
void onPointerMove(int pointerId, Offset offset, Map<int, Offset> pointers) {
if (!_active) return;
super.onPointerMove(pointerId, offset, pointers);
_Vector newVector = _Vector(pointers.values.first, pointers.values.last);
if (newVector.getLength().isNaN) return;
_lastScale = newVector.getLength() / _startVector!.getLength();
mapModel.scaleAround(newVector.getFocalPoint(), _lastScale);
_lastVector = newVector;
}