Allow registration of custom functions for expressions

examples/style-rules.js

@@ -49,7 +49,7 @@ var style = new ol.style.Style({rules: [
       }),
       new ol.style.Text({
         color: '#bada55',
-        text: new ol.Expression('label'),
+        text: ol.expression.parse('label'),
         fontFamily: 'Calibri,sans-serif',
         fontSize: 14
       })

examples/vector-layer.js

@@ -1,8 +1,7 @@
-goog.require('ol.Expression');
 goog.require('ol.Map');
 goog.require('ol.RendererHint');
 goog.require('ol.View2D');
-goog.require('ol.filter.Filter');
+goog.require('ol.expression');
 goog.require('ol.layer.TileLayer');
 goog.require('ol.layer.Vector');
 goog.require('ol.parser.GeoJSON');
@@ -19,6 +18,11 @@ var raster = new ol.layer.TileLayer({
   source: new ol.source.MapQuestOpenAerial()
 });
 
+// TODO: discuss scale dependent rules
+ol.expression.register('resolution', function() {
+  return map.getView().getView2D().getResolution();
+});
+
 var vector = new ol.layer.Vector({
   source: new ol.source.Vector({
     projection: ol.proj.get('EPSG:4326')
@@ -32,13 +36,11 @@ var vector = new ol.layer.Vector({
       ]
     }),
     new ol.style.Rule({
-      filter: new ol.filter.Filter(function() {
-        return map.getView().getResolution() < 5000;
-      }),
+      filter: 'resolution() < 5000',
       symbolizers: [
         new ol.style.Text({
           color: '#bada55',
-          text: new ol.Expression('name'),
+          text: ol.expression.parse('name'),
           fontFamily: 'Calibri,sans-serif',
           fontSize: 12
         })

src/ol/expression/expression.exports

@@ -1 +1,2 @@
 @exportSymbol ol.expression.parse
+@exportSymbol ol.expression.register

src/ol/expression/expression.js

@@ -32,7 +32,7 @@ ol.expression.evaluateFeature = function(expr, opt_feature) {
 
 
 /**
- * Parse an expression
+ * Parse an expression.
  * @param {string} source The expression source (e.g. `'foo + 2'`).
  * @return {ol.expression.Expression} An expression instance that can be
  *     evaluated within some scope to provide a value.
@@ -43,6 +43,18 @@ ol.expression.parse = function(source) {
 };
 
 
+/**
+ * Register a library function to be used in expressions.
+ * @param {string} name The function name (e.g. 'myFunc').
+ * @param {function(this:ol.Feature)} func The function to be called in an
+ *     expression.  This function will be called with a feature as the `this`
+ *     argument when the expression is evaluated in the context of a features.
+ */
+ol.expression.register = function(name, func) {
+  ol.expression.lib[name] = func;
+};
+
+
 /**
  * Determines whether an expression is a call expression that calls one of the
  * `ol.expression.lib` functions.