Dart: updating instance's premitive type members

main.dart

void main() {
  final pos = Positions();
  
  print('\n-------------Issue demo---------------\n');
  updatePositionIssue(pos.top);
  print('positionAttr: ${pos.top}');
  updatePositionIssue(pos.left);
  print('positionAttr: ${pos.left}');
  updatePositionIssue(pos.right);
  print('positionAttr: ${pos.right}');
  updatePositionIssue(pos.bottom);
  print('positionAttr: ${pos.bottom}');
  
  print('\n-------------Possible solution---------------\n');
  updatePositionSolution(() => pos.top, (value) => pos.top = value);
  print('positionAttr: ${pos.top}');
  updatePositionSolution(() => pos.left, (value) => pos.left = value);
  print('positionAttr: ${pos.left}');
  updatePositionSolution(() => pos.right, (value) => pos.right = value);
  print('positionAttr: ${pos.right}');
  updatePositionSolution(() => pos.bottom, (value) => pos.bottom = value);
  print('positionAttr: ${pos.bottom}');
}

updatePositionIssue(double positionAttr) {
  // updates the local copy only
  positionAttr = 10;
  print('positionAttr: ${positionAttr}');
}


updatePositionSolution(double Function() getPosition, void Function(double newPosition) setPosition) {
  //updates actual member value in instance
  setPosition(10);
  print('positionAttr: ${getPosition()}');
}

class Positions {
  double top;
  double left;
  double right;
  double bottom;
  

  Positions() {
    top = 0;
    left = 0;
    right = 0;
    bottom = 0;
  }
}