int checkPointToMove(Offset pressedPoint) {
    // 控制點非空才查找
    if (points.isNotEmpty) {
      var pointsToCheck = <Offset>[];
      final maxDistance = 10.0;
      // 查找觸控響應(yīng)范圍內(nèi)的控制點
      for (Offset p in points) {
        if ((p.dx - pressedPoint.dx).abs() < maxDistance &&
            (p.dy - pressedPoint.dy).abs() < maxDistance) {
          pointsToCheck.add(p);
        }
      }
      
      // 未找到
      if (pointsToCheck.length == 0) {
        return -1;
      } else if (pointsToCheck.length == 1) {
        // 只有一個點，直接返回
        return points.indexOf(pointsToCheck[0]);
      } else {
        // 有多個點命中，找到距離最近的點返回
        Offset point = pointsToCheck[0];
        var distance = distanceBetween(pointsToCheck[0], pressedPoint);
        for (int i = 1; i < pointsToCheck.length; i++) {
          var newDistance = distanceBetween(pointsToCheck[i], pressedPoint);
          if (newDistance < distance) {
            point = pointsToCheck[i];
            distance = newDistance;
          }
        }

        return points.indexOf(point);
      }
    }

    return -1;
  }

onPointerMove: ((event) {
  if (indexOfPointToMove != -1) {
    points[indexOfPointToMove] = event.localPosition;
    setState(() {});
  }
}),