[TrackShape]: Class for flutter slider with gradient

GradientRectSliderTrackShape.dart

import 'package:flutter/material.dart';

/// Based on https://www.youtube.com/watch?v=Wl4F5V6BoJw

class GradientRectSliderTrackShape extends SliderTrackShape
    with BaseSliderTrackShape {
  const GradientRectSliderTrackShape({
    this.gradient = const LinearGradient(
      colors: [
        Colors.red,
        Colors.yellow,
      ],
    ),
    this.darkenInactive = true,
  });

  final LinearGradient gradient;
  final bool darkenInactive;

  @override
  void paint(
      PaintingContext context,
      Offset offset,
      {
        required RenderBox parentBox,
        required SliderThemeData sliderTheme,
        required Animation<double> enableAnimation,
        required TextDirection textDirection,
        required Offset thumbCenter,
        bool isDiscrete = false,
        bool isEnabled = false,
        double additionalActiveTrackHeight = 2,
      }
    ) {
    assert(sliderTheme.disabledActiveTrackColor != null);
    assert(sliderTheme.disabledInactiveTrackColor != null);
    assert(sliderTheme.activeTrackColor != null);
    assert(sliderTheme.inactiveTrackColor != null);
    assert(sliderTheme.thumbShape != null);
    assert(sliderTheme.trackHeight != null && sliderTheme.trackHeight! > 0);

    final Rect trackRect = getPreferredRect(
      parentBox: parentBox,
      offset: offset,
      sliderTheme: sliderTheme,
      isEnabled: isEnabled,
      isDiscrete: isDiscrete,
    );

    final activeGradientRect = Rect.fromLTRB(
      trackRect.left,
      (textDirection == TextDirection.ltr)
          ? trackRect.top - (additionalActiveTrackHeight / 2)
          : trackRect.top,
      thumbCenter.dx,
      (textDirection == TextDirection.ltr)
          ? trackRect.bottom + (additionalActiveTrackHeight / 2)
          : trackRect.bottom,
    );

    // Assign the track segment paints, which are leading: active and
    // trailing: inactive.
    final ColorTween activeTrackColorTween = ColorTween(
        begin: sliderTheme.disabledActiveTrackColor,
        end: sliderTheme.activeTrackColor);
    final ColorTween inactiveTrackColorTween = darkenInactive
        ? ColorTween(
          begin: sliderTheme.disabledInactiveTrackColor,
          end: sliderTheme.inactiveTrackColor
        )
        : activeTrackColorTween;
    final Paint activePaint = Paint()
      ..shader = gradient.createShader(activeGradientRect)
      ..color = activeTrackColorTween.evaluate(enableAnimation)!;
    final Paint inactivePaint = Paint()
      ..color = inactiveTrackColorTween.evaluate(enableAnimation)!;
    final Paint leftTrackPaint;
    final Paint rightTrackPaint;
    switch (textDirection) {
      case TextDirection.ltr:
        leftTrackPaint = activePaint;
        rightTrackPaint = inactivePaint;
        break;
      case TextDirection.rtl:
        leftTrackPaint = inactivePaint;
        rightTrackPaint = activePaint;
        break;
    }

    final Radius trackRadius = Radius.circular(trackRect.height / 2);
    final Radius activeTrackRadius = Radius.circular(trackRect.height / 2 + 1);

    context.canvas.drawRRect(
      RRect.fromLTRBAndCorners(
        trackRect.left,
        (textDirection == TextDirection.ltr)
            ? trackRect.top - (additionalActiveTrackHeight / 2)
            : trackRect.top,
        thumbCenter.dx,
        (textDirection == TextDirection.ltr)
            ? trackRect.bottom + (additionalActiveTrackHeight / 2)
            : trackRect.bottom,
        topLeft: (textDirection == TextDirection.ltr)
            ? activeTrackRadius
            : trackRadius,
        bottomLeft: (textDirection == TextDirection.ltr)
            ? activeTrackRadius
            : trackRadius,
      ),
      leftTrackPaint,
    );
    context.canvas.drawRRect(
      RRect.fromLTRBAndCorners(
        thumbCenter.dx,
        (textDirection == TextDirection.rtl)
            ? trackRect.top - (additionalActiveTrackHeight / 2)
            : trackRect.top,
        trackRect.right,
        (textDirection == TextDirection.rtl)
            ? trackRect.bottom + (additionalActiveTrackHeight / 2)
            : trackRect.bottom,
        topRight: (textDirection == TextDirection.rtl)
            ? activeTrackRadius
            : trackRadius,
        bottomRight: (textDirection == TextDirection.rtl)
            ? activeTrackRadius
            : trackRadius,
      ),
      rightTrackPaint,
    );
  }
}