Cache the buffered extent value

Assuming that the maxExtent and resolution property are immutable

src/ol/render/canvas/canvasreplay.js

@@ -62,10 +62,17 @@ ol.render.canvas.Replay = function(tolerance, maxExtent, resolution) {
 
   /**
    * @protected
+   * @const
    * @type {ol.Extent}
    */
   this.maxExtent = maxExtent;
 
+  /**
+   * @private
+   * @type {ol.Extent}
+   */
+  this.bufferedMaxExtent_ = null;
+
   /**
    * @protected
    * @type {number}
@@ -74,6 +81,7 @@ ol.render.canvas.Replay = function(tolerance, maxExtent, resolution) {
 
   /**
    * @protected
+   * @const
    * @type {number}
    */
   this.resolution = resolution;
@@ -964,12 +972,14 @@ ol.render.canvas.LineStringReplay.prototype.drawFlatCoordinates_ =
  * @inheritDoc
  */
 ol.render.canvas.LineStringReplay.prototype.getBufferedMaxExtent = function() {
-  var extent = this.maxExtent;
+  if (goog.isNull(this.bufferedMaxExtent_)) {
-  if (this.maxLineWidth) {
+    this.bufferedMaxExtent_ = ol.extent.clone(this.maxExtent);
-    extent = ol.extent.buffer(
+    if (this.maxLineWidth > 0) {
-        extent, this.resolution * (this.maxLineWidth + 1) / 2);
+      var width = this.resolution * (this.maxLineWidth + 1) / 2;
+      ol.extent.buffer(this.bufferedMaxExtent_, width, this.bufferedMaxExtent_);
+    }
   }
-  return extent;
+  return this.bufferedMaxExtent_;
 };
 
 
@@ -1116,7 +1126,12 @@ ol.render.canvas.LineStringReplay.prototype.setFillStrokeStyle =
   var strokeStyleMiterLimit = strokeStyle.getMiterLimit();
   this.state_.miterLimit = goog.isDef(strokeStyleMiterLimit) ?
       strokeStyleMiterLimit : ol.render.canvas.defaultMiterLimit;
-  this.maxLineWidth = Math.max(this.maxLineWidth, this.state_.lineWidth);
+
+  if (this.state_.lineWidth > this.maxLineWidth) {
+    this.maxLineWidth = this.state_.lineWidth;
+    // invalidate the buffered max extent cache
+    this.bufferedMaxExtent_ = null;
+  }
 };
 
 
@@ -1369,12 +1384,14 @@ ol.render.canvas.PolygonReplay.prototype.finish = function() {
  * @inheritDoc
  */
 ol.render.canvas.PolygonReplay.prototype.getBufferedMaxExtent = function() {
-  var extent = this.maxExtent;
+  if (goog.isNull(this.bufferedMaxExtent_)) {
-  if (this.maxLineWidth) {
+    this.bufferedMaxExtent_ = ol.extent.clone(this.maxExtent);
-    extent = ol.extent.buffer(
+    if (this.maxLineWidth > 0) {
-        extent, this.resolution * (this.maxLineWidth + 1) / 2);
+      var width = this.resolution * (this.maxLineWidth + 1) / 2;
+      ol.extent.buffer(this.bufferedMaxExtent_, width, this.bufferedMaxExtent_);
+    }
   }
-  return extent;
+  return this.bufferedMaxExtent_;
 };
 
 
@@ -1412,7 +1429,12 @@ ol.render.canvas.PolygonReplay.prototype.setFillStrokeStyle =
     var strokeStyleMiterLimit = strokeStyle.getMiterLimit();
     state.miterLimit = goog.isDef(strokeStyleMiterLimit) ?
         strokeStyleMiterLimit : ol.render.canvas.defaultMiterLimit;
-    this.maxLineWidth = Math.max(this.maxLineWidth, state.lineWidth);
+
+    if (state.lineWidth > this.maxLineWidth) {
+      this.maxLineWidth = state.lineWidth;
+      // invalidate the buffered max extent cache
+      this.bufferedMaxExtent_ = null;
+    }
   } else {
     state.strokeStyle = undefined;
     state.lineCap = undefined;