Custom Star Painter

star.dart

import 'dart:math' as math;

import 'package:flutter/gestures.dart';
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

const double _kRadiansPerDegree = math.pi / 180;

class Star extends StatelessWidget {
  final Color color;
  final double ratio;
  final double startAngle;
  final int spikes;
  const Star({
    Key key,
    this.color = Colors.yellow,
    this.ratio = 0.5,
    this.spikes = 21,
    this.startAngle = -90.0,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final Size size = MediaQuery.of(context).size;

    return CustomPaint(
      size: size,
      painter: StarPainter(
        color: color,
        ratio: ratio,
        startAngle: startAngle,
        spikes: spikes,
      ),
    );
  }
}

class StarPainter extends CustomPainter {
  StarPainter({
    this.color,
    this.startAngle,
    this.ratio = 0.5,
    this.spikes = 10,
  }) : super();

  final Color color;
  final double startAngle;
  final double ratio;
  final int spikes;

  Path _buildStarPath(
    double radius,
    double innerRadius,
  ) {
    final Offset center = Offset(radius, radius);

    final Path path = Path();

    var distance = innerRadius;

    path.moveTo(center.dx, center.dy);

    double step = 360 / (spikes * 2);
    var stopAngle = 360 + startAngle;
    double x;
    double y;
    double angle;
    for (angle = startAngle; angle <= stopAngle; angle = angle + step) {
      distance = distance == radius ? innerRadius : radius;

      x = distance * math.cos(angle * _kRadiansPerDegree);
      y = distance * math.sin(angle * _kRadiansPerDegree);

      path.lineTo(center.dx + x, center.dy + y);
    }

    if (angle != stopAngle) {
      distance = distance == radius ? innerRadius : radius;
      x = distance * math.cos(startAngle * _kRadiansPerDegree);
      y = distance * math.sin(startAngle * _kRadiansPerDegree);
      path.lineTo(center.dx + x, center.dy + y);
    }

    path.close();

    return path;
  }

  @override
  void paint(Canvas canvas, Size size) {
    final double radius = size.width / 2;
    final double innerRadius = radius * (1 - ratio);

    final path = _buildStarPath(radius, innerRadius);

    final Paint slicePaint = Paint()
      ..style = PaintingStyle.fill
      ..isAntiAlias = true
      ..color = color;

    canvas.drawPath(path, slicePaint);
  }

  @override
  bool shouldRepaint(StarPainter oldDelegate) => false;
}

Usage:

Star(
  spikes: 21,
  ratio: 0.5,
  color: Colors.yellow,
);

Star(
  spikes: 100,
  ratio: 0.5,
  color: Colors.yellow,
);

Star(
  spikes: 300,
  ratio: 0.5,
  color: Colors.yellow,
);

Star(
   spikes: 5,
   ratio: 0.6,
   color: Colors.white,
);