Reduce the cost of polygons by culling the labels on partially visibl…

…e polygons, deriving pixel bounds from polygon's latlng bounds, and most importantly cache the TextPainter.

lib/src/layer/polygon_layer/label.dart

@@ -6,42 +6,54 @@ import 'package:flutter_map/src/layer/polygon_layer/polygon_layer.dart';
 import 'package:latlong2/latlong.dart';
 import 'package:polylabel/polylabel.dart';
 
+class PolygonBounds {
+  final Offset min;
+  final Offset max;
+
+  const PolygonBounds(this.min, this.max);
+}
+
 void Function(Canvas canvas)? buildLabelTextPainter({
+  required math.Point<double> mapSize,
   required Offset placementPoint,
-  required List<Offset> points,
-  required String labelText,
+  required PolygonBounds bounds,
+  required TextPainter textPainter,
   required double rotationRad,
   required bool rotate,
-  required TextStyle labelStyle,
   required double padding,
 }) {
-  final textSpan = TextSpan(text: labelText, style: labelStyle);
-  final textPainter = TextPainter(
-    text: textSpan,
-    textAlign: TextAlign.center,
-    textDirection: TextDirection.ltr,
-  )..layout();
+  final dx = placementPoint.dx;
+  final dy = placementPoint.dy;
+  final width = textPainter.width;
+  final height = textPainter.height;
 
-  final dx = placementPoint.dx - textPainter.width / 2;
-  final dy = placementPoint.dy - textPainter.height / 2;
-
-  double maxDx = 0;
-  var minDx = double.infinity;
-  for (final point in points) {
-    maxDx = math.max(maxDx, point.dx);
-    minDx = math.min(minDx, point.dx);
+  // Cull labels where the polygon is still on the map but the label would not be.
+  if (dx + width / 2 < 0 || dx - width / 2 > mapSize.x) {
+    return null;
+  }
+  if (dy + height / 2 < 0 || dy - height / 2 > mapSize.y) {
+    return null;
   }
 
-  if (maxDx - minDx - padding > textPainter.width) {
+  // Note: I'm pretty sure this doesn't work for concave shapes. It would be more
+  // correct to evaluate the width of the polygon at the height of the label.
+  if (bounds.max.dx - bounds.min.dx - padding > width) {
     return (canvas) {
       if (rotate) {
         canvas.save();
-        canvas.translate(placementPoint.dx, placementPoint.dy);
+        canvas.translate(dx, dy);
         canvas.rotate(-rotationRad);
-        canvas.translate(-placementPoint.dx, -placementPoint.dy);
+        canvas.translate(-dx, -dy);
       }
 
-      textPainter.paint(canvas, Offset(dx, dy));
+      textPainter.paint(
+        canvas,
+        Offset(
+          dx - width / 2,
+          dy - height / 2,
+        ),
+      );
+
       if (rotate) {
         canvas.restore();
       }

lib/src/layer/polygon_layer/polygon_layer.dart

@@ -53,6 +53,18 @@ class Polygon {
   LatLngBounds get boundingBox =>
       _boundingBox ??= LatLngBounds.fromPoints(points);
 
+  TextPainter? _textPainter;
+  TextPainter? get textPainter {
+    if (label != null) {
+      return _textPainter ??= TextPainter(
+        text: TextSpan(text: label, style: labelStyle),
+        textAlign: TextAlign.center,
+        textDirection: TextDirection.ltr,
+      )..layout();
+    }
+    return null;
+  }
+
   Polygon({
     required this.points,
     this.holePointsList,
@@ -148,6 +160,13 @@ class PolygonPainter extends CustomPainter {
 
   int? _hash;
 
+  PolygonBounds getBounds(Polygon polygon) {
+    final bbox = polygon.boundingBox;
+    final min = map.getOffsetFromOrigin(bbox.southWest);
+    final max = map.getOffsetFromOrigin(bbox.northEast);
+    return PolygonBounds(min, max);
+  }
+
   List<Offset> getOffsets(List<LatLng> points) {
     return List.generate(
       points.length,
@@ -241,7 +260,7 @@ class PolygonPainter extends CustomPainter {
         }
       }
 
-      if (polygonLabels && !drawLabelsLast && polygon.label != null) {
+      if (!drawLabelsLast && polygonLabels && polygon.textPainter != null) {
         // Labels are expensive because:
         //  * they themselves cannot easily be pulled into our batched path
         //    painting with the given text APIs
@@ -252,10 +271,10 @@ class PolygonPainter extends CustomPainter {
         // The painter will be null if the layouting algorithm determined that
         // there isn't enough space.
         final painter = buildLabelTextPainter(
+          mapSize: map.size,
           placementPoint: map.getOffsetFromOrigin(polygon.labelPosition),
-          points: offsets,
-          labelText: polygon.label!,
-          labelStyle: polygon.labelStyle,
+          bounds: getBounds(polygon),
+          textPainter: polygon.textPainter!,
           rotationRad: map.rotationRad,
           rotate: polygon.rotateLabel,
           padding: 20,
@@ -277,14 +296,13 @@ class PolygonPainter extends CustomPainter {
         if (polygon.points.isEmpty) {
           continue;
         }
-        final offsets = getOffsets(polygon.points);
-
-        if (polygon.label != null) {
+        final textPainter = polygon.textPainter;
+        if (textPainter != null) {
           final painter = buildLabelTextPainter(
+            mapSize: map.size,
             placementPoint: map.getOffsetFromOrigin(polygon.labelPosition),
-            points: offsets,
-            labelText: polygon.label!,
-            labelStyle: polygon.labelStyle,
+            bounds: getBounds(polygon),
+            textPainter: textPainter,
             rotationRad: map.rotationRad,
             rotate: polygon.rotateLabel,
             padding: 20,