carto - how to enable interactivity on named map in cartodb

I have created a named map that gets data from a private table. I can load it into a Leaflet map and can change its marker-fill color placeholder using layer.setParams.

However, I cannot get interactivity to work. I want to be able to hover or click on a feature and do something useful.[1]

What do I need to do to make hover or click events fire?

Here is the named map json file

{
  "version": "0.0.1",
  "name": "mynamedmap",
  "auth": {
   "method": "open" 
  },
  "placeholders": {
      "color": {

        "type":"css_color",
        "default":"blue"
      }
  },
  "layergroup": {
    "version": "1.0.1",
    "layers": [{
      "type": "cartodb",
      "options": {
        "cartocss_version": "2.1.1", 

        "cartocss": "#layer { marker-fill: <%= color %>; }",
        "sql": "select * from mytable",
        "interactivity": ["cartodb_id"]
      }
    }]
  } 
}

Here is the javascript to create the layer:

cartodb.createLayer(map, {

        type: 'namedmap',
        user_name: 'myusername',
        options: {
          named_map: {
            name: 'myusername@mynamedmap'
          }
}})            
.addTo(map)
.done(function(layer) {
    console.log(layer.getSubLayerCount()); //prints '0'        

    layer.setParams('color', 'green'); // works to change marker color

    layer.on('featureClick', function(e, latlng, pos, data) {
      console.log(e, latlng, pos, data); //this never gets called
    });

  });

[1] "Something useful" could be as simple as open infowindow and display cartodb_id and feature name. Or it could mean "get cartodb_id for feature, pass it to my proxy where I can use curl to run some custom sql against my private table, then return json to the map and update display"

Answer

I feel a bit of an idiot for not figuring this out sooner. Here is the solution that worked for me.

Add the following line to the namedmap definition: layers: [{}]

The call to createLayer then looks like this:

cartodb.createLayer(this.map.baseMap, {
      type: 'namedmap',
      user_name: 'myusername',
      options: {
        named_map: {
          name: namedMap.name,
          layers: [{}]

})
.done(function (layer) {
  var subLayer = layer.getSubLayer(0);
  subLayer.setInteraction(true);

});

After doing so, you can add event handling like:

      subLayer.on('featureOver', function(e, latlon, pos, data, subLayerIndex) {
        console.log("hovered over");

        console.log(data);
      }); 

It would have been very helpful for me, as I have commented in another post on this forum, for more thorough documentation - a response there indicated that it is coming, which is great news, as CartoDb is fantastic.