More Autodesk Forge

Back in August, I blogged about attending the Autodesk Forge DevCon is San Francisco. This month I’m again extremely fortunate and am attending Autodesk University in Las Vegas with work.

Since my previous blog, I’ve been busy on a proof of concept that marries our NBS Create specification product and the Autodesk Forge Viewer. There will be more to follow in the coming months, but for now I just wanted to capture a few features I implemented incase they are useful to anyone else.

1. Creating an extension that captures object selection

The application I’m prototyping needs to extract data from the model when an object is clicked. The Forge Viewer api documentation covers how to create and register an extension to get selection events etc. Adding functionality as an extension, is the recommended approach for adding custom functionality to the viewer.

The data my application needs from the viewer can only be obtained when the viewer has fully loaded the model’s geometry and object tree. So we have to be sure we subscribe to the appropriate events.

Create and register the extension

function NBSExtension(viewer, options) {, viewer, options);

NBSExtension.prototype = Object.create(Autodesk.Viewing.Extension.prototype);
NBSExtension.prototype.constructor = NBSExtension;

Autodesk.Viewing.theExtensionManager.registerExtension('NBSExtension', NBSExtension);

Subscribe and handle the events

My extension needs to handle the SELECTION_CHANGED_EVENT, GEOMETRY_LOADED_EVENT and OBJECT_TREE_CREATED_EVENT. The events are bound on the extensions “load” method.

NBSExtension.prototype.load = function () {
  console.log('NBSExtension is loaded!');

  this.onSelectionBinded = this.onSelectionEvent.bind(this);
  this.viewer.addEventListener(Autodesk.Viewing.SELECTION_CHANGED_EVENT, this.onSelectionBinded);

  this.onGeometryLoadedBinded = this.onGeometryLoadedEvent.bind(this);
  this.viewer.addEventListener(Autodesk.Viewing.GEOMETRY_LOADED_EVENT, this.onGeometryLoadedBinded);

  this.onObjectTreeCreatedBinded = this.onObjectTreeCreatedEvent.bind(this);
  this.viewer.addEventListener(Autodesk.Viewing.OBJECT_TREE_CREATED_EVENT, this.onObjectTreeCreatedBinded);

  return true;

A well behaved extension should also clean up after it’s unloaded.

NBSExtension.prototype.unload = function () {
  console.log('NBSExtension is now unloaded!');

  this.viewer.removeEventListener(Autodesk.Viewing.SELECTION_CHANGED_EVENT, this.onSelectionBinded);
  this.onSelectionBinded = null;

  this.viewer.removeEventListener(Autodesk.Viewing.GEOMETRY_LOADED_EVENT, this.onGeometryLoadedBinded);
  this.onGeometryLoadedBinded = null;

  this.viewer.removeEventListener(Autodesk.Viewing.OBJECT_TREE_CREATED_EVENT, this.onObjectTreeCreatedBinded);
  this.onObjectTreeCreatedBinded = null;

  return true;

When the events fire, the following functions are called to allow us to handle the event however we want:

// Event handler for Autodesk.Viewing.SELECTION_CHANGED_EVENT
NBSExtension.prototype.onSelectionEvent = function (event) {
  var currSelection = this.viewer.getSelection();

  // Do more work with current selection

// Event handler for Autodesk.Viewing.GEOMETRY_LOADED_EVENT
NBSExtension.prototype.onGeometryLoadedEvent = function (event) {

// Event handler for Autodesk.Viewing.OBJECT_TREE_CREATED_EVENT
NBSExtension.prototype.onObjectTreeCreatedEvent = function (event) {


2. Get object properties

Once we have the selected item, we can call getProperties on the viewer to get an array of all of the property key/value pairs for that object.

var currSelection = this.viewer.getSelection();

// Do more work with current selection
var dbId = currSelection[0];

this.viewer.getProperties(dbId, function (data) {
  // Find the property NBSReference 
  var nbsRef = _.find(, function (item) {
    return (item.displayName === 'NBSReference');

  // If we have found NBSReference, get the display value
  if (nbsRef && nbsRef.displayValue) {
    console.log('NBS Reference found: ' + nbsRef.displayValue);
}, function () {
  console.log('Error getting properties');

The call to this.viewer.getSelection() returns an array of dbId’s (Database ID’s). Each Id can be passed to the getProperties function to get the properties for that dbId. My extension then looks through the array of properties for an “NBSReference” property which can be used to display the associated specification for that object.

Notice that I use Underscore.js’s _.find() function to search the array of properties. I opted for this as I found IE11 didn’t support Javascript’s native Array.prototype.find(). I like the readability of the function and Underscore.js provides the necessary polyfill for IE11.

3. Getting area and volume information

Once the geometry is loaded from the model and the internal object tree create, it’s possible to query the properties in the model that relate to area and volume. For my prototype, I wanted to sum the area and volume of types of a objects the user has selected in the model.

In order to do this, I needed to:

  1. Get the dbId of the selection item
  2. Find that dbID in the object tree
  3. Move to the object’s parent and get all of it’s children (in other words, get the siblings of the selected item)
  4. Sum the area and volume properties of the children

The first step is to build our own representation of the model tree in memory (this must effectively be how the Forge viewer displays the model tree). My code is based on this blog post by Philippe Leefsma.

var viewer = viewerApp.getCurrentViewer();
var model = viewer.model;

if (!modelTree && model.getData().instanceTree) {
  modelTree = buildModelTree(viewer.model);

var buildModelTree = function (model) {
  // builds model tree recursively
  function _buildModelTreeRec(node) {
    instanceTree.enumNodeChildren(node.dbId, function (childId) {
      node.children = node.children || [];

      var childNode = {
        dbId: childId,
        name: instanceTree.getNodeName(childId)


  // get model instance tree and root component
  var instanceTree = model.getData().instanceTree;
  var rootId = instanceTree.getRootId();
  var rootNode = {
    dbId: rootId,
    name: instanceTree.getNodeName(rootId)

  return rootNode;

This gives us a representation of the model tree. Once we’ve located all of the siblings, we can use the dbId of each sibling to get it’s area and volume properties.

The code I wrote was based on this sample, originally written be Jim Awe I have to admit, my code is a little bit messy. There are a lot of asynchronous operations going on, which use quite a few callbacks and you do end up close to a pyramid of doom. The code was good for my needs, but I think if I was doing anything more complicated I’d look in to using Promises to tidy the code up a bit.

function _getReportData(items, callback) {
  var results = { "areaSum": 0.0, "areaSumLabel": "", "areaProps": [], "volumeSum": 0.0, "volumeSumLabel": "", volumeProps: [], "instanceCount": 0, "friendlyNotationWithSuffix": friendlyNotationWithSuffix.trim() };

  var viewer = viewerApp.getCurrentViewer();
  var nodes = items;

  nodes.forEach(function (dbId, nodeIndex, nodeArray) {
    // Find node 
    var leafNodes = getLeafNodes(dbId, modelTree);
    if (!leafNodes) return;
    results.instanceCount += leafNodes.length;

    leafNodes.forEach(function (node, leafNodeIndex, leafNodeArray) {
      viewer.getProperties(node.dbId, function (propObj) {
        for (var i = 0; i <; ++i) {
          var prop =[i];
          var propValue;
          var propFormat;

          if (prop.displayName === "Area") {
            propValue = parseFloat(prop.displayValue);

            results.areaSum += propValue;
            results.areaSumLabel = Autodesk.Viewing.Private.formatValueWithUnits(results.areaSum.toFixed(2), prop.units, prop.type);

            propFormat = Autodesk.Viewing.Private.formatValueWithUnits(prop.displayValue, prop.units, prop.type);
            results.areaProps.push({ "dbId": dbId, "val": propValue, "label": propFormat, "units": prop.units });
          } else if (prop.displayName === "Volume") {
            propValue = parseFloat(prop.displayValue);

            results.volumeSum += propValue;
            results.volumeSumLabel = Autodesk.Viewing.Private.formatValueWithUnits(results.volumeSum.toFixed(2), prop.units, prop.type);

            propFormat = Autodesk.Viewing.Private.formatValueWithUnits(prop.displayValue, prop.units, prop.type);
            results.volumeProps.push({ "dbId": dbId, "val": propValue, "label": propFormat, "units": prop.units });

        // Callback when we've processed everything
        if (callback && nodeIndex === nodeArray.length - 1 && leafNodeIndex === leafNodeArray.length - 1) {

var getLeafNodes = function (parentNodeDbId, parentNode) {
  var result = null;

  function _getLeafNodesRec(parentNodeDbId, node) {
    // Have we found the node we're looking for?
    if (node.dbId === parentNodeDbId) {
      // We return the children (or the node itself if there are no children)
      result = node.children || [node];
    } else {
      if (node.children) {
        node.children.forEach(function (childNode, index, array) {
          if (result) return;
          _getLeafNodesRec(parentNodeDbId, childNode);

 _getLeafNodesRec(parentNodeDbId, parentNode);
 return result;

A couple of things to call out from the above code – The function getLeafNodes is used to get the siblings of the selected item. And the Autodesk Forge viewer has a method to nicely format volumes and areas with the appropriate units:

Autodesk.Viewing.Private.formatValueWithUnits(prop.displayValue, prop.units, prop.type);

I couldn’t actually find this documented in the API though – it was only in the samples on GitHub. But it’s a nice way of getting a nicely formatted string of values with the appropriate units.

This has been another fairly lengthy blog post – so it deserves a few screenshots of the functionality that has been implemented:

And a big shout out to Kirsty Hudson for her awesome UX work!