Add support for the extremely awesomely super-comprehensive rule based styling

that Andreas has been working so hard on. I think this is the single most
awesome commit I've ever had the pleasure of committing. The results of this
commit are described on http://trac.openlayers.org/wiki/Styles: essentially,
this makes it possible to style features in all kinds of fun ways based on
rules, and will also form the underlying basis for #533. Things this patch
adds:

 * OL.Rule classes. These classes allow you to do tests against the propertie
   of a feature, and set a style based on these properties -- so you can
   compare the 'speedlimit' property of a line, and test if it is > 60, and if
   it is greater than 60, render it in a different color. You can also test
   combinations of rules using the OL.Rule.Logical class, and test featureids
   with the FeatureID class. 
 * OL.Style class: The OL.Style class lets you wrap up Rules into styles that 
   can be used with drawFeature to draw the feature in the selected style.
 * OL.Layer.Vector.drawFeature will check if the given style is an OL.Style
   object, and if so, it will draw the feature accordingly.

examples/georss-flickr.html shows usage of these classes.

Many, many thanks go to Andreas for all his hard work on this: this code really
is very pretty, and includes unit tests for all the classes (and we know that I
am a big fan of tests.)

Three cheers for Andreas: Hip hip, hooray! hip hip, hooray! hip hip, hooray!


git-svn-id: http://svn.openlayers.org/trunk/openlayers@5429 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Rule.js

@@ -0,0 +1,90 @@
+/* Copyright (c) 2006 MetaCarta, Inc., published under a modified BSD license.
+ * See http://svn.openlayers.org/trunk/openlayers/repository-license.txt 
+ * for the full text of the license. */
+
+
+/**
+ * @requires OpenLayers/Util.js
+ * @requires OpenLayers/Style.js
+ *
+ * Class: OpenLayers.Rule
+ * 
+ * This class represents a OGC Rule, as being used for rule-based SLD styling
+ */
+OpenLayers.Rule = OpenLayers.Class({
+    
+    /**
+     * APIProperty: name
+     * {String} name of this rule
+     */
+    name: 'default',
+
+    /**
+     * Property: symbolizer
+     * {Object} Hash of styles for this rule. Contains hashes of feature
+     * styles. Keys are one or more of ["Point", "Line", "Polygon"]
+     */
+    symbolizer: null,
+    
+    /**
+     * APIProperty: minScale
+     * {Number} or {String} minimum scale at which to draw the feature.
+     * In the case of a String, this can be a combination of text and
+     * propertyNames in the form "literal ${propertyName}"
+     */
+    minScale: null,
+
+    /**
+     * APIProperty: maxScale
+     * {Number} or {String} maximum scale at which to draw the feature.
+     * In the case of a String, this can be a combination of text and
+     * propertyNames in the form "literal ${propertyName}"
+     */
+    maxScale: null,
+
+    /** 
+     * Constructor: OpenLayers.Rule
+     * Creates a Rule.
+     *
+     * Parameters:
+     * options - {Object} An optional object with properties to set on the
+     *           rule
+     * 
+     * Returns:
+     * {<OpenLayers.Rule>}
+     */
+    initialize: function(options) {
+        this.symbolizer = {};
+
+        OpenLayers.Util.extend(this, options);
+    },
+
+    /** 
+     * APIMethod: destroy
+     * nullify references to prevent circular references and memory leaks
+     */
+    destroy: function() {
+        for (var i in this.symbolizer) {
+            this.symbolizer[i] = null;
+        }
+        this.symbolizer = null;
+    },
+    
+    /**
+     * APIMethod: evaluate
+     * evaluates this rule for a specific feature
+     * 
+     * Parameters:
+     * feature - {<OpenLayers.Feature>} feature to apply the rule to.
+     * 
+     * Returns:
+     * {boolean} true if the rule applies, false if it does not.
+     * This rule is the default rule and always returns true.
+     */
+    evaluate: function(feature) {
+        // Default rule always applies. Subclasses will want to override this.
+        return true;
+    },
+    
+    CLASS_NAME: "OpenLayers.Rule"
+});