trackPointer method
      
void
trackPointer(
    
    
- dynamic event
Implementation
void trackPointer(event) {
  Vector2? position = pointerPositions[event.pointerId];
  if (position == null) {
    position = Vector2();
    pointerPositions[event.pointerId] = position;
  }
  position.setValues(event.pageX, event.pageY);
}