pan method
Implementation
void pan(Offset delta) {
if (delta == Offset.zero) return;
_state = _state.copyWith(offset: _state.offset + delta);
notifyListeners();
}
void pan(Offset delta) {
if (delta == Offset.zero) return;
_state = _state.copyWith(offset: _state.offset + delta);
notifyListeners();
}