getOuterPath method

@override Path getOuterPath(Rect host, Rect? guest)

Creates a Path that describes the outline of the shape.

The host is the bounding rectangle of the shape.

The guest is the bounding rectangle of the shape for which a notch will be made. It is null when there is no guest.

Implementation

@override
Path getOuterPath(Rect host, Rect? guest) {
  if (guest == null || !host.overlaps(guest)) return Path()..addRect(host);

  // The guest's shape is a circle bounded by the guest rectangle.
  // So the guest's radius is half the guest width.
  final double notchRadius = guest.width / 2.0;

  // We build a path for the notch from 3 segments:
  // Segment A - a Bezier curve from the host's top edge to segment B.
  // Segment B - an arc with radius notchRadius.
  // Segment C - a Bezier curve from segment B back to the host's top edge.
  //
  // A detailed explanation and the derivation of the formulas below is
  // available at: https://goo.gl/Ufzrqn

  const double s1 = 20.0;
  const double s2 = 5.0;

  final double r = notchRadius;
  final double a = -1.0 * r - s2;
  final double b = host.top - guest.center.dy;

  final double n2 = math.sqrt(b * b * r * r * (a * a + b * b - r * r));
  final double p2xA = ((a * r * r) - n2) / (a * a + b * b);
  final double p2xB = ((a * r * r) + n2) / (a * a + b * b);
  final double p2yA = math.sqrt(r * r - p2xA * p2xA);
  final double p2yB = math.sqrt(r * r - p2xB * p2xB);

  final List<Offset?> p = List<Offset?>.filled(6, null);

  // p0, p1, and p2 are the control points for segment A.
  p[0] = Offset(a - s1, b);
  p[1] = Offset(a, b);
  final double cmp = b < 0 ? -1.0 : 1.0;
  p[2] = cmp * p2yA > cmp * p2yB ? Offset(p2xA, p2yA) : Offset(p2xB, p2yB);

  // p3, p4, and p5 are the control points for segment B, which is a mirror
  // of segment A around the y axis.
  p[3] = Offset(-1.0 * p[2]!.dx, p[2]!.dy);
  p[4] = Offset(-1.0 * p[1]!.dx, p[1]!.dy);
  p[5] = Offset(-1.0 * p[0]!.dx, p[0]!.dy);

  // translate all points back to the absolute coordinate system.
  for (int i = 0; i < p.length; i += 1) {
    p[i] = p[i]! + guest.center;
  }

  return Path()
    ..moveTo(host.left, host.top)
    ..lineTo(p[0]!.dx, p[0]!.dy)
    ..quadraticBezierTo(p[1]!.dx, p[1]!.dy, p[2]!.dx, p[2]!.dy)
    ..arcToPoint(
      p[3]!,
      radius: Radius.circular(notchRadius),
      clockwise: false,
    )
    ..quadraticBezierTo(p[4]!.dx, p[4]!.dy, p[5]!.dx, p[5]!.dy)
    ..lineTo(host.right, host.top)
    ..lineTo(host.right, host.bottom)
    ..lineTo(host.left, host.bottom)
    ..close();
}