polyline with gradient (#452)

* add polyline with gradient

* resolve gradientColors empty case

* code formatting

* code foramtting

Author: SebWojd

example/lib/pages/polyline.dart

@@ -14,6 +14,13 @@ class PolylinePage extends StatelessWidget {
       LatLng(53.3498, -6.2603),
       LatLng(48.8566, 2.3522),
     ];
+
+    var pointsGradient = <LatLng>[
+      LatLng(55.5, -0.09),
+      LatLng(54.3498, -6.2603),
+      LatLng(52.8566, 2.3522),
+    ];
+
     return Scaffold(
       appBar: AppBar(title: Text('Polylines')),
       drawer: buildDrawer(context, PolylinePage.route),
@@ -43,7 +50,20 @@ class PolylinePage extends StatelessWidget {
                           strokeWidth: 4.0,
                           color: Colors.purple),
                     ],
-                  )
+                  ),
+                  PolylineLayerOptions(
+                    polylines: [
+                      Polyline(
+                        points: pointsGradient,
+                        strokeWidth: 4.0,
+                        gradientColors: [
+                          Color(0xffE40203),
+                          Color(0xffFEED00),
+                          Color(0xff007E2D),
+                        ],
+                      ),
+                    ],
+                  ),
                 ],
               ),
             ),

lib/src/layer/polyline_layer.dart

@@ -8,6 +8,7 @@ import 'package:latlong/latlong.dart';
 
 class PolylineLayerOptions extends LayerOptions {
   final List<Polyline> polylines;
+
   PolylineLayerOptions({this.polylines = const [], rebuild})
       : super(rebuild: rebuild);
 }
@@ -19,13 +20,18 @@ class Polyline {
   final Color color;
   final double borderStrokeWidth;
   final Color borderColor;
+  final List<Color> gradientColors;
+  final List<double> colorsStop;
   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.gradientColors,
+    this.colorsStop,
     this.isDotted = false,
   });
 }
@@ -85,6 +91,7 @@ class PolylineLayer extends StatelessWidget {
 
 class PolylinePainter extends CustomPainter {
   final Polyline polylineOpt;
+
   PolylinePainter(this.polylineOpt);
 
   @override
@@ -95,12 +102,19 @@ class PolylinePainter extends CustomPainter {
     final rect = Offset.zero & size;
     canvas.clipRect(rect);
     final paint = Paint()
-      ..color = polylineOpt.color
       ..strokeWidth = polylineOpt.strokeWidth
       ..strokeCap = StrokeCap.round
       ..strokeJoin = StrokeJoin.round
       ..blendMode = BlendMode.src;
 
+    if (polylineOpt.gradientColors == null) {
+      paint.color = polylineOpt.color;
+    } else {
+      polylineOpt.gradientColors.isNotEmpty
+          ? paint.shader = _paintGradient()
+          : paint.color = polylineOpt.color;
+    }
+
     final filterPaint = Paint()
       ..color = polylineOpt.borderColor.withAlpha(255)
       ..strokeWidth = polylineOpt.strokeWidth
@@ -177,6 +191,23 @@ class PolylinePainter extends CustomPainter {
     }
   }
 
+  ui.Gradient _paintGradient() => ui.Gradient.linear(polylineOpt.offsets.first,
+      polylineOpt.offsets.last, polylineOpt.gradientColors, _getColorsStop());
+
+  List<double> _getColorsStop() => (polylineOpt.colorsStop != null &&
+          polylineOpt.colorsStop.length == polylineOpt.gradientColors.length)
+      ? polylineOpt.colorsStop
+      : _calculateColorsStop();
+
+  List<double> _calculateColorsStop() {
+    final colorsStopInterval = 1.0 / polylineOpt.gradientColors.length;
+    return polylineOpt.gradientColors
+        .map((gradientColor) =>
+            polylineOpt.gradientColors.indexOf(gradientColor) *
+            colorsStopInterval)
+        .toList();
+  }
+
   @override
   bool shouldRepaint(PolylinePainter other) => false;
 }