title: OpenLayers 3 How to register Feature Modified event as "featuremodified" in OpenLayer 2
body: <p>I need to implement <strong>Undo / Redo</strong> functionality using <strong>OpenLayers 3</strong> vector editing (just like demonstrated in <a href="http://dev.openlayers.org/examples/modify-feature.html" rel="noreferrer">http://dev.openlayers.org/examples/modify-feature.html</a> for OpenLayers 2).</p>
  
  <p>To keep track of <em>geometry</em> changes of <em>features</em>, I have to manage a memory stack that will hold changed geometric definitions of features upon user interactions. I know OpenLayers 3 provides observable objects. So <code>ol.Feature</code> or <code>ol.Feature.getGeometry()</code> can be observed for changes, but I am looking for explicit events emitted by <code>ol.interaction.Modify</code>, that should notify me when the <em>interaction</em> starts or ends the vector editing operation (just like <code>"beforefeaturemodified"</code> and <code>"featuremodified"</code> events in OpenLayers 2).</p>
  
  <p>A handler listening for observed <em>geometry</em> or <em>feature</em> changes can be used for the purpose but it is too expensive because <em>geometry</em> of the feature under modification is changed with every pixel move.</p>
  
  <p>I have gone through the official <a href="http://openlayers.org/en/v3.0.0/apidoc" rel="noreferrer">documentation</a> of OpenLayers 3 but couldn't find various events that are provided by OpenLayers 2. In most cases the documentation mentions <em>change</em> event only. I wonder if such events are not a consideration for Openlayer 3's architecture. If it is so, any suggestions how can I extend the existing <code>ol.interaction.Modify</code> to incorporate custom events? thanks.</p>