Color calc

main.dart

import 'dart:math';
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: MyWidget(),
      ),
    );
  }
}

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  Color _testColor;
  Color _testGradientStart;
  Color _testGradientEnd;

  List<Color> get _testGradient => [_testGradientStart, _testGradientEnd];

  @override
  Widget build(BuildContext context) {
    return Container(
      padding: EdgeInsets.all(16),
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: <Widget>[
          Row(
            children: [
              Expanded(
                child: TextField(
                  decoration: InputDecoration(
                    border: OutlineInputBorder(),
                    labelText: 'Color',
                  ),
                  onChanged: (colorStr) => setState(() {
                    _testColor = _parseColor(colorStr);
                  }),
                ),
              ),
              const SizedBox(width: 16),
              Container(width: 48, height: 48, color: _testColor),
            ],
          ),
          const SizedBox(height: 16),
          Text('Color brightness: ${getBrightness(_testColor)}'),
          Text('Color is ${isDark(_testColor) ? 'DARK' : 'LIGHT'}'),
          const SizedBox(height: 16),
          Row(
            children: [
              Expanded(
                child: TextField(
                  decoration: InputDecoration(
                    border: OutlineInputBorder(),
                    labelText: 'Gradient start color',
                  ),
                  onChanged: (colorStr) => setState(() {
                    _testGradientStart = _parseColor(colorStr);
                  }),
                ),
              ),
              const SizedBox(width: 16),
              Container(width: 48, height: 48, color: _testGradient.first),
            ],
          ),
          const SizedBox(height: 16),
          Row(
            children: [
              Expanded(
                child: TextField(
                  decoration: InputDecoration(
                    border: OutlineInputBorder(),
                    labelText: 'Gradient start color',
                  ),
                  onChanged: (colorStr) => setState(() {
                    _testGradientEnd = _parseColor(colorStr);
                  }),
                ),
              ),
              const SizedBox(width: 16),
              Container(width: 48, height: 48, color: _testGradient.last),
            ],
          ),
          const SizedBox(height: 16),
          Row(
            children: [
              Text('Gradient average color: '),
              Container(
                  width: 16, height: 16, color: getAverageColor(_testGradient)),
            ],
          ),
          Text(
              'Gradient brightness: ${getBrightness(getAverageColor(_testGradient))}'),
          Text('Gradient is ${isDark(getAverageColor(_testGradient)) ? 'DARK' : 'LIGHT'}'),
        ],
      ),
    );
  }

  Color _parseColor(String colorStr) {
    String hexColor =
        colorStr?.toUpperCase()?.replaceAll("#", "")?.replaceAll("0X", "");
    if (hexColor?.length != 6 && hexColor?.length != 8) {
      return null;
    }
    if (hexColor != null && hexColor.length == 6) {
      hexColor = "FF" + hexColor;
    }
    try {
      return Color(int.parse(hexColor, radix: 16));
    } catch (_) {
      return null;
    }
  }
}

bool isDark(Color color) {
  if (color == null) {
    return false;
  }

  return getBrightness(color) < 128.0;
}

double getBrightness(Color color) {
  if (color == null) {
    return 0;
  }

  return (color.red * 299 + color.green * 587 + color.blue * 114) / 1000;
}

Color getAverageColor(List<Color> colors) {
  if (colors == null || colors.isEmpty || colors.contains(null)) {
    return null;
  }

  double r = 0;
  double g = 0;
  double b = 0;
  for (final c in colors) {
    r += c.red * c.red;
    g += c.green * c.green;
    b += c.blue * c.blue;
  }
  return Color.fromARGB(
    255,
    sqrt(r / colors.length).round(),
    sqrt(g / colors.length).round(),
    sqrt(b / colors.length).round(),
  );
}