Give precedence to feature style

src/ol/style/style.js

@@ -1,5 +1,7 @@
 goog.provide('ol.style.Style');
 
+goog.require('goog.asserts');
+goog.require('goog.functions');
 goog.require('ol.style.Fill');
 goog.require('ol.style.Image');
 
@@ -93,3 +95,47 @@ ol.style.Style.prototype.getText = function() {
 ol.style.Style.prototype.getZIndex = function() {
   return this.zIndex_;
 };
+
+
+/**
+ * A function that takes an {@link ol.Feature} and a `{number}` representing
+ * the view's resolution. The function should return an array of
+ * {@link ol.style.Style}. This way e.g. a vector layer can be styled.
+ *
+ * @typedef {function(ol.Feature, number): Array.<ol.style.Style>}
+ * @api
+ */
+ol.style.StyleFunction;
+
+
+/**
+ * Convert the provided object into a style function.  Functions passed through
+ * unchanged.  Arrays of ol.style.Style or single style objects wrapped in a
+ * new style function.
+ * @param {ol.style.StyleFunction|Array.<ol.style.Style>|ol.style.Style} obj
+ *     A style function, a single style, or an array of styles.
+ * @return {ol.style.StyleFunction} A style function.
+ */
+ol.style.createStyleFunction = function(obj) {
+  /**
+   * @type {ol.style.StyleFunction}
+   */
+  var styleFunction;
+
+  if (goog.isFunction(obj)) {
+    styleFunction = /** @type {ol.style.StyleFunction} */ (obj);
+  } else {
+    /**
+     * @type {Array.<ol.style.Style>}
+     */
+    var styles;
+    if (goog.isArray(obj)) {
+      styles = obj;
+    } else {
+      goog.asserts.assertInstanceof(obj, ol.style.Style);
+      styles = [obj];
+    }
+    styleFunction = goog.functions.constant(styles);
+  }
+  return styleFunction;
+};