How to handle tvOS MenuBarTemplate selection?

I have a basic MenuBarTemplate set up and displaying.

How do I react to a user's Menu selection and load an appropriate content template?

In the menuItem tag include a template attribute pointing to the template to load and a presentation attribute set to menuBarItemPresenter.

<menuItem template="${this.BASEURL}templates/Explore.xml.js" 
  presentation="menuBarItemPresenter">
    <title>Explore</title>
</menuItem>

You can then use the menu bar's MenuBarDocument feature to associate a document to each menu bar item.

menuBarItemPresenter: function(xml, ele) {
  var feature = ele.parentNode.getFeature("MenuBarDocument");
  if (feature) {
    var currentDoc = feature.getDocument(ele);
    if (!currentDoc) {
      feature.setDocument(xml, ele);
    }
  }

This assumes you're using a Presenter.js file like the one in Apple's "TVML Catalog" sample. The load function specified there is what calls the function specified in the menuItem's presentation attribute.