diff --git a/src/ol/observable.js b/src/ol/observable.js
index c3d51b1d24..83fe48cbf9 100644
--- a/src/ol/observable.js
+++ b/src/ol/observable.js
@@ -15,12 +15,27 @@ goog.require('goog.events.EventTarget');
  * @todo stability experimental
  */
 ol.Observable = function() {
+
   goog.base(this);
 
+  /**
+   * @private
+   * @type {number}
+   */
+  this.revision_ = 0;
+
 };
 goog.inherits(ol.Observable, goog.events.EventTarget);
 
 
+/**
+ * @return {number} Revision.
+ */
+ol.Observable.prototype.getRevision = function() {
+  return this.revision_;
+};
+
+
 /**
  * Listen for a certain type of event.
  * @param {string|Array.<string>} type The event type or array of event types.