aves/lib/widgets/common/fx/dashed_path_painter.dart

import 'dart:ui' as ui;
import 'dart:math' as math;

import 'package:flutter/material.dart';

// from https://stackoverflow.com/a/71099304/786656
class DashedPathPainter extends CustomPainter {
  final Path originalPath;
  final Color pathColor;
  final double strokeWidth;
  final double dashGapLength;
  final double dashLength;
  late DashedPathProperties _dashedPathProperties;

  DashedPathPainter({
    required this.originalPath,
    required this.pathColor,
    this.strokeWidth = 3.0,
    this.dashGapLength = 5.0,
    this.dashLength = 10.0,
  });

  @override
  void paint(Canvas canvas, Size size) {
    _dashedPathProperties = DashedPathProperties(
      path: Path(),
      dashLength: dashLength,
      dashGapLength: dashGapLength,
    );
    final dashedPath = _getDashedPath(originalPath, dashLength, dashGapLength);
    canvas.drawPath(
      dashedPath,
      Paint()
        ..style = PaintingStyle.stroke
        ..color = pathColor
        ..strokeWidth = strokeWidth,
    );
  }

  @override
  bool shouldRepaint(DashedPathPainter oldDelegate) => oldDelegate.originalPath != originalPath || oldDelegate.pathColor != pathColor || oldDelegate.strokeWidth != strokeWidth || oldDelegate.dashGapLength != dashGapLength || oldDelegate.dashLength != dashLength;

  Path _getDashedPath(
    Path originalPath,
    double dashLength,
    double dashGapLength,
  ) {
    final metricsIterator = originalPath.computeMetrics().iterator;
    while (metricsIterator.moveNext()) {
      final metric = metricsIterator.current;
      _dashedPathProperties.extractedPathLength = 0.0;
      while (_dashedPathProperties.extractedPathLength < metric.length) {
        if (_dashedPathProperties.addDashNext) {
          _dashedPathProperties.addDash(metric, dashLength);
        } else {
          _dashedPathProperties.addDashGap(metric, dashGapLength);
        }
      }
    }
    return _dashedPathProperties.path;
  }
}

class DashedPathProperties {
  double extractedPathLength;
  Path path;

  final double _dashLength;
  double _remainingDashLength;
  double _remainingDashGapLength;
  bool _previousWasDash;

  DashedPathProperties({
    required this.path,
    required double dashLength,
    required double dashGapLength,
  })  : assert(dashLength > 0.0, 'dashLength must be > 0.0'),
        assert(dashGapLength > 0.0, 'dashGapLength must be > 0.0'),
        _dashLength = dashLength,
        _remainingDashLength = dashLength,
        _remainingDashGapLength = dashGapLength,
        _previousWasDash = false,
        extractedPathLength = 0.0;

  bool get addDashNext {
    if (!_previousWasDash || _remainingDashLength != _dashLength) {
      return true;
    }
    return false;
  }

  void addDash(ui.PathMetric metric, double dashLength) {
    // Calculate lengths (actual + available)
    final end = _calculateLength(metric, _remainingDashLength);
    final availableEnd = _calculateLength(metric, dashLength);
    // Add path
    final pathSegment = metric.extractPath(extractedPathLength, end);
    path.addPath(pathSegment, Offset.zero);
    // Update
    final delta = _remainingDashLength - (end - extractedPathLength);
    _remainingDashLength = _updateRemainingLength(
      delta: delta,
      end: end,
      availableEnd: availableEnd,
      initialLength: dashLength,
    );
    extractedPathLength = end;
    _previousWasDash = true;
  }

  void addDashGap(ui.PathMetric metric, double dashGapLength) {
    // Calculate lengths (actual + available)
    final end = _calculateLength(metric, _remainingDashGapLength);
    final availableEnd = _calculateLength(metric, dashGapLength);
    // Move path's end point
    ui.Tangent tangent = metric.getTangentForOffset(end)!;
    path.moveTo(tangent.position.dx, tangent.position.dy);
    // Update
    final delta = end - extractedPathLength;
    _remainingDashGapLength = _updateRemainingLength(
      delta: delta,
      end: end,
      availableEnd: availableEnd,
      initialLength: dashGapLength,
    );
    extractedPathLength = end;
    _previousWasDash = false;
  }

  double _calculateLength(ui.PathMetric metric, double addedLength) {
    return math.min(extractedPathLength + addedLength, metric.length);
  }

  double _updateRemainingLength({
    required double delta,
    required double end,
    required double availableEnd,
    required double initialLength,
  }) {
    return (delta > 0 && availableEnd == end) ? delta : initialLength;
  }
}