Added polyline customisation options

lib/src/layer/polyline_layer.dart

@@ -1,3 +1,4 @@
+import 'dart:math';
 import 'dart:ui';
 
 import 'package:flutter/widgets.dart';
@@ -15,10 +16,16 @@ class Polyline {
   final List<Offset> offsets = [];
   final double strokeWidth;
   final Color color;
+  final double borderStrokeWidth;
+  final Color borderColor;
+  final bool isDotted;
   Polyline({
     this.points,
     this.strokeWidth = 1.0,
     this.color = const Color(0xFF00FF00),
+    this.borderStrokeWidth = 0.0,
+    this.borderColor = const Color(0xFFFFFF00),
+    this.isDotted = false,
   });
 }
 
@@ -88,9 +95,65 @@ class PolylinePainter extends CustomPainter {
     final paint = new Paint()
       ..color = polylineOpt.color
       ..strokeWidth = polylineOpt.strokeWidth;
-    canvas.drawPoints(PointMode.lines, polylineOpt.offsets, paint);
+    final borderPaint = polylineOpt.borderStrokeWidth > 0.0
+        ? (new Paint()
+      ..color = polylineOpt.borderColor
+      ..strokeWidth = polylineOpt.strokeWidth + polylineOpt.borderStrokeWidth)
+        : null;
+    double radius = polylineOpt.strokeWidth / 2;
+    double borderRadius = radius + (polylineOpt.borderStrokeWidth / 2);
+    if (polylineOpt.isDotted) {
+      double spacing = polylineOpt.strokeWidth * 1.5;
+      if (borderPaint != null) {
+        _paintDottedLine(canvas, polylineOpt.offsets, borderRadius, spacing, borderPaint);
+      }
+      _paintDottedLine(canvas, polylineOpt.offsets, radius, spacing, paint);
+    } else {
+      if (borderPaint != null) {
+        _paintLine(canvas, polylineOpt.offsets, borderRadius, borderPaint);
+      }
+      _paintLine(canvas, polylineOpt.offsets, radius, paint);
+    }
+  }
+
+  void _paintDottedLine(Canvas canvas, List<Offset> offsets, double radius, double stepLength, Paint paint) {
+    double startDistance = 0.0;
+    for (int i = 0; i < offsets.length - 1; i++) {
+      Offset o0 = offsets[i];
+      Offset o1 = offsets[i + 1];
+      double totalDistance = _dist(o0, o1);
+      double distance = startDistance;
+      while (distance < totalDistance) {
+        double f1 = distance / totalDistance;
+        double f0 = 1.0 - f1;
+        var offset = Offset(o0.dx * f0 + o1.dx * f1, o0.dy * f0 + o1.dy * f1);
+        canvas.drawCircle(offset, radius, paint);
+        distance += stepLength;
+      }
+      startDistance = distance < totalDistance ? stepLength - (totalDistance - distance) : distance - totalDistance;
+    }
+    canvas.drawCircle(polylineOpt.offsets.last, radius, paint);
+  }
+
+  void _paintLine(Canvas canvas, List<Offset> offsets, double radius, Paint paint) {
+    canvas.drawPoints(PointMode.lines, offsets, paint);
+    for (var offset in offsets) {
+      canvas.drawCircle(offset, radius, paint);
+    }
   }
 
   @override
   bool shouldRepaint(PolylinePainter other) => false;
 }
+
+double _dist(Offset v, Offset w) {
+  return sqrt(_dist2(v, w));
+}
+
+double _dist2(Offset v, Offset w) {
+  return _sqr(v.dx - w.dx) + _sqr(v.dy - w.dy);
+}
+
+double _sqr(double x) {
+  return x * x;
+}