Increase scope of polygon draw batching for polygons with labels (fle…

…aflet#1607)

* Polygon labels are expensive. Add a layer-level switch to disable labels to allow reusing the same polygons while toggling the labels, e.g. depending on zoom level.

Also a bunch of small optimizations to minimize canvas operations (save/restore) and reduce strain on garbage collection (fewer ephemeral allocations)

* Improve scope of polygon draw batching by flushing polygons with labels only if the label would actually be drawn, as determined by the layouting algorithm.

lib/src/layer/label.dart

@@ -5,55 +5,47 @@ import 'package:flutter/material.dart';
 import 'package:flutter_map/plugin_api.dart';
 import 'package:polylabel/polylabel.dart';
 
-@immutable
-class Label {
-  final List<Offset> points;
-  final String? labelText;
-  final TextStyle? labelStyle;
-  final double rotationRad;
-  final bool rotate;
-  final PolygonLabelPlacement labelPlacement;
+void Function(Canvas canvas)? buildLabelTextPainter({
+  required String labelText,
+  required List<Offset> points,
+  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),
+  };
 
-  const Label({
-    required this.points,
-    this.labelText,
-    this.labelStyle,
-    required this.rotationRad,
-    this.rotate = false,
-    this.labelPlacement = PolygonLabelPlacement.polylabel,
-  });
-
-  void paintText(Canvas canvas) {
-    final placementPoint = switch (labelPlacement) {
-      PolygonLabelPlacement.centroid => _computeCentroid(points),
-      PolygonLabelPlacement.polylabel => _computePolylabel(points),
-    };
-
-    var dx = placementPoint.dx;
-    var dy = placementPoint.dy;
+  var dx = placementPoint.dx;
+  var dy = placementPoint.dy;
 
+  if (dx > 0) {
     final textSpan = TextSpan(text: labelText, style: labelStyle);
     final textPainter = TextPainter(
       text: textSpan,
       textAlign: TextAlign.center,
       textDirection: TextDirection.ltr,
       maxLines: 1,
     );
-    if (dx > 0) {
-      textPainter.layout();
-      dx -= textPainter.width / 2;
-      dy -= textPainter.height / 2;
 
-      var maxDx = 0.0;
-      var minDx = double.infinity;
-      for (final point in points) {
-        maxDx = math.max(maxDx, point.dx);
-        minDx = math.min(minDx, point.dx);
-      }
+    textPainter.layout();
+    dx -= textPainter.width / 2;
+    dy -= textPainter.height / 2;
+
+    var maxDx = 0.0;
+    var minDx = double.infinity;
+    for (final point in points) {
+      maxDx = math.max(maxDx, point.dx);
+      minDx = math.min(minDx, point.dx);
+    }
 
-      if (maxDx - minDx > textPainter.width) {
-        canvas.save();
+    if (maxDx - minDx - padding > textPainter.width) {
+      return (canvas) {
         if (rotate) {
+          canvas.save();
           canvas.translate(placementPoint.dx, placementPoint.dy);
           canvas.rotate(-rotationRad);
           canvas.translate(-placementPoint.dx, -placementPoint.dy);
@@ -62,22 +54,26 @@ class Label {
           canvas,
           Offset(dx, dy),
         );
-        canvas.restore();
-      }
+        if (rotate) {
+          canvas.restore();
+        }
+      };
     }
   }
+  return null;
+}
 
-  Offset _computeCentroid(List<Offset> points) {
-    return Offset(
-      points.map((e) => e.dx).toList().average,
-      points.map((e) => e.dy).toList().average,
-    );
-  }
+Offset _computeCentroid(List<Offset> points) {
+  return Offset(
+    points.map((e) => e.dx).average,
+    points.map((e) => e.dy).average,
+  );
+}
 
-  Offset _computePolylabel(List<Offset> points) {
-    final labelPosition = polylabel([
-      points.map((p) => math.Point(p.dx, p.dy)).toList(),
-    ]);
-    return labelPosition.point.toOffset();
-  }
+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();
 }

lib/src/layer/polygon_layer.dart

@@ -66,18 +66,22 @@ class Polygon {
   }) : _filledAndClockwise = isFilled && isClockwise(points);
 
   /// Used to batch draw calls to the canvas.
-  int get renderHashCode => Object.hash(
-        holePointsList,
-        color,
-        borderStrokeWidth,
-        borderColor,
-        isDotted,
-        isFilled,
-        strokeCap,
-        strokeJoin,
-        labelStyle,
-        _filledAndClockwise,
-      );
+  int get renderHashCode {
+    _hash ??= Object.hash(
+      holePointsList,
+      color,
+      borderStrokeWidth,
+      borderColor,
+      isDotted,
+      isFilled,
+      strokeCap,
+      strokeJoin,
+      _filledAndClockwise,
+    );
+    return _hash!;
+  }
+
+  int? _hash;
 }
 
 @immutable
@@ -87,10 +91,14 @@ class PolygonLayer extends StatelessWidget {
   /// screen space culling of polygons based on bounding box
   final bool polygonCulling;
 
+  // Turn on/off per-polygon label drawing on the layer-level.
+  final bool polygonLabels;
+
   const PolygonLayer({
     super.key,
     this.polygons = const [],
     this.polygonCulling = false,
+    this.polygonLabels = true,
   });
 
   @override
@@ -105,7 +113,7 @@ class PolygonLayer extends StatelessWidget {
         : polygons;
 
     return CustomPaint(
-      painter: PolygonPainter(pgons, map),
+      painter: PolygonPainter(pgons, map, polygonLabels),
       size: size,
       isComplex: true,
     );
@@ -116,8 +124,10 @@ class PolygonPainter extends CustomPainter {
   final List<Polygon> polygons;
   final MapCamera map;
   final LatLngBounds bounds;
+  final bool polygonLabels;
 
-  PolygonPainter(this.polygons, this.map) : bounds = map.visibleBounds;
+  PolygonPainter(this.polygons, this.map, this.polygonLabels)
+      : bounds = map.visibleBounds;
 
   int get hash {
     _hash ??= Object.hashAll(polygons);
@@ -219,19 +229,32 @@ class PolygonPainter extends CustomPainter {
         }
       }
 
-      if (polygon.label != null) {
-        // Labels are expensive. The `paintText` below is a canvas draw
-        // operation and thus requires us to reset the draw batching here.
-        drawPaths();
-
-        Label(
+      if (polygonLabels && polygon.label != null) {
+        // Labels are expensive because:
+        //  * they themselves cannot easily be pulled into our batched path
+        //    painting with the given text APIs
+        //  * therefore, they require us to flush the batch of polygon draws to
+        //    ensure polygons and labels are stacked correctly, i.e.:
+        //    p1, p1_label, p2, p2_label, ... .
+
+        // The painter will be null if the layouting algorithm determined that
+        // there isn't enough space.
+        final painter = buildLabelTextPainter(
           points: offsets,
-          labelText: polygon.label,
+          labelText: polygon.label!,
           labelStyle: polygon.labelStyle,
           rotationRad: map.rotationRad,
           rotate: polygon.rotateLabel,
           labelPlacement: polygon.labelPlacement,
-        ).paintText(canvas);
+          padding: 10,
+        );
+
+        if (painter != null) {
+          // Flush the batch before painting to preserve stacking.
+          drawPaths();
+
+          painter(canvas);
+        }
       }
     }