Allow styles to override feature geometries

With this change, application developers are able to define styles that
render a different geometry than the feature geometry. This can e.g. be
used to render an interior point of a polygon instead of the polygon, or
to render symbols like arrows along lines.

externs/olx.js

@@ -6075,7 +6075,8 @@ olx.style.TextOptions.prototype.stroke;
 
 
 /**
- * @typedef {{fill: (ol.style.Fill|undefined),
+ * @typedef {{geometry: (undefined|string|ol.geom.Geometry|function(ol.Feature): ol.geom.Geometry),
+ *     fill: (ol.style.Fill|undefined),
  *     image: (ol.style.Image|undefined),
  *     stroke: (ol.style.Stroke|undefined),
  *     text: (ol.style.Text|undefined),
@@ -6085,6 +6086,15 @@ olx.style.TextOptions.prototype.stroke;
 olx.style.StyleOptions;
 
 
+/**
+ * Feature property or geometry or function returning a geometry to render
+ * for this style.
+ * @type {undefined|string|ol.geom.Geometry|function(ol.Feature): ol.geom.Geometry}
+ * @api
+ */
+olx.style.StyleOptions.prototype.geometry;
+
+
 /**
  * Fill style.
  * @type {ol.style.Fill|undefined}

src/ol/render/vector.js

@@ -123,8 +123,8 @@ ol.renderer.vector.renderFeature = function(
  */
 ol.renderer.vector.renderFeature_ = function(
     replayGroup, feature, style, squaredTolerance) {
-  var geometry = feature.getGeometry();
-  if (!goog.isDefAndNotNull(geometry)) {
+  var geometry = style.getGeometryFunction()(feature);
+  if (goog.isNull(geometry)) {
     return;
   }
   var simplifiedGeometry = geometry.getSimplifiedGeometry(squaredTolerance);

src/ol/style/style.js

@@ -1,7 +1,9 @@
 goog.provide('ol.style.Style');
+goog.provide('ol.style.defaultGeometryFunction');
 
 goog.require('goog.asserts');
 goog.require('goog.functions');
+goog.require('ol.geom.Geometry');
 goog.require('ol.geom.GeometryType');
 goog.require('ol.style.Circle');
 goog.require('ol.style.Fill');
@@ -24,6 +26,18 @@ ol.style.Style = function(opt_options) {
 
   var options = goog.isDef(opt_options) ? opt_options : {};
 
+  /**
+   * Function that is called with a feature and returns the geometry to render
+   * for this style.
+   * @private
+   * @type {!function(ol.Feature): ol.geom.Geometry}
+   */
+  this.geometryFunction_ = ol.style.defaultGeometryFunction;
+
+  if (goog.isDef(options.geometry)) {
+    this.setGeometry(options.geometry);
+  }
+
   /**
    * @private
    * @type {ol.style.Fill}
@@ -57,6 +71,16 @@ ol.style.Style = function(opt_options) {
 };
 
 
+/**
+ * @return {!function(ol.Feature): ol.geom.Geometry} Function that is called
+ * with a feature and returns the geometry to render instead of the feature's
+ * geometry.
+ */
+ol.style.Style.prototype.getGeometryFunction = function() {
+  return this.geometryFunction_;
+};
+
+
 /**
  * @return {ol.style.Fill} Fill style.
  * @api
@@ -102,6 +126,32 @@ ol.style.Style.prototype.getZIndex = function() {
 };
 
 
+/**
+ * Set a geometry that is rendered instead of the feature's geometry.
+ *
+ * @param {string|ol.geom.Geometry|function(ol.Feature): ol.geom.Geometry} geometry
+ *     Feature property or geometry or function returning a geometry to render
+ *     for this style.
+ * @api
+ */
+ol.style.Style.prototype.setGeometry = function(geometry) {
+  if (goog.isFunction(geometry)) {
+    this.geometryFunction_ = geometry;
+  } else if (goog.isString(geometry)) {
+    this.geometryFunction_ = function(feature) {
+      return feature.get(geometry);
+    };
+  } else if (goog.isDef(geometry)) {
+    goog.asserts.assertInstanceof(geometry, ol.geom.Geometry);
+    this.geometryFunction_ = function() {
+      return geometry;
+    };
+  } else {
+    this.geometryFunction_ = ol.style.defaultGeometryFunction;
+  }
+};
+
+
 /**
  * Set the zIndex.
  *
@@ -264,3 +314,14 @@ ol.style.createDefaultEditingStyles = function() {
 
   return styles;
 };
+
+
+/**
+ * @param {ol.Feature} feature Feature to get the geometry for.
+ * @return {ol.geom.Geometry} Geometry to render.
+ */
+ol.style.defaultGeometryFunction = function(feature) {
+  goog.asserts.assert(!goog.isNull(feature));
+  var geometry = feature.getGeometry();
+  return goog.isDef(geometry) ? geometry : null;
+};