Compute label position in map/coordinate space rather than pixel space (

#1641)

example/lib/pages/polygon.dart

@@ -131,6 +131,7 @@ class PolygonPage extends StatelessWidget {
                       borderColor: Colors.purple,
                       label: "Rotated!",
                       rotateLabel: true,
+                      labelPlacement: PolygonLabelPlacement.polylabel,
                     ),
                     Polygon(
                       points: holeOuterPoints,

lib/src/layer/label.dart

@@ -3,24 +3,22 @@ import 'dart:math' as math;
 import 'package:collection/collection.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_map/plugin_api.dart';
+import 'package:latlong2/latlong.dart';
 import 'package:polylabel/polylabel.dart';
 
-void Function(Canvas canvas)? buildLabelTextPainter({
-  required String labelText,
+void Function(Canvas canvas)? buildLabelTextPainter(
+  List<LatLng> locs,
+  LatLng labelLoc, {
+  required Offset placementPoint,
   required List<Offset> points,
+  required String labelText,
   required double rotationRad,
   bool rotate = false,
   TextStyle? labelStyle,
-  PolygonLabelPlacement labelPlacement = PolygonLabelPlacement.polylabel,
   double padding = 0,
 }) {
-  final placementPoint = switch (labelPlacement) {
-    PolygonLabelPlacement.centroid => _computeCentroid(points),
-    PolygonLabelPlacement.polylabel => _computePolylabel(points),
-  };
-
-  var dx = placementPoint.dx;
-  var dy = placementPoint.dy;
+  double dx = placementPoint.dx;
+  double dy = placementPoint.dy;
 
   if (dx > 0) {
     final textSpan = TextSpan(text: labelText, style: labelStyle);
@@ -63,17 +61,37 @@ void Function(Canvas canvas)? buildLabelTextPainter({
   return null;
 }
 
-Offset _computeCentroid(List<Offset> points) {
-  return Offset(
-    points.map((e) => e.dx).average,
-    points.map((e) => e.dy).average,
+LatLng computeLabelPosition(
+    PolygonLabelPlacement labelPlacement, List<LatLng> points) {
+  return switch (labelPlacement) {
+    PolygonLabelPlacement.centroid => _computeCentroid(points),
+    PolygonLabelPlacement.polylabel => _computePolylabel(points),
+  };
+}
+
+LatLng _computeCentroid(List<LatLng> points) {
+  return LatLng(
+    points.map((e) => e.latitude).average,
+    points.map((e) => e.longitude).average,
   );
 }
 
-Offset _computePolylabel(List<Offset> points) {
-  final labelPosition = polylabel([
-    List<math.Point>.generate(
-        points.length, (i) => math.Point(points[i].dx, points[i].dy)),
-  ]);
-  return labelPosition.point.toOffset();
+LatLng _computePolylabel(List<LatLng> points) {
+  final labelPosition = polylabel(
+    [
+      List<math.Point>.generate(points.length,
+          (i) => math.Point(points[i].longitude, points[i].latitude)),
+    ],
+    // "precision" is a bit of a misnomer. It's a threshold for when to stop
+    // dividing-and-conquering the polygon in the hopes of finding a better
+    // point with more distance to the polygon's outline. It's given in
+    // point-units, i.e. degrees here. A bigger number means less precision,
+    // i.e. cheaper at the expense off less optimal label placement.
+    precision: 0.000001,
+  );
+  final latlng = LatLng(
+    labelPosition.point.y.toDouble(),
+    labelPosition.point.x.toDouble(),
+  );
+  return latlng;
 }

lib/src/layer/polygon_layer.dart

@@ -43,8 +43,12 @@ class Polygon {
   // of opposing clock-directions cut holes into each other leading to a leaky optimization.
   final bool _filledAndClockwise;
 
-  LatLngBounds? _boundingBox;
+  // Location where to place the label if `label` is set.
+  LatLng? _labelPosition;
+  LatLng get labelPosition =>
+      _labelPosition ??= computeLabelPosition(labelPlacement, points);
 
+  LatLngBounds? _boundingBox;
   LatLngBounds get boundingBox =>
       _boundingBox ??= LatLngBounds.fromPoints(points);
 
@@ -240,12 +244,14 @@ class PolygonPainter extends CustomPainter {
         // The painter will be null if the layouting algorithm determined that
         // there isn't enough space.
         final painter = buildLabelTextPainter(
+          polygon.points,
+          polygon.labelPosition,
+          placementPoint: map.getOffsetFromOrigin(polygon.labelPosition),
           points: offsets,
           labelText: polygon.label!,
           labelStyle: polygon.labelStyle,
           rotationRad: map.rotationRad,
           rotate: polygon.rotateLabel,
-          labelPlacement: polygon.labelPlacement,
           padding: 10,
         );