onShow.js

ItemView = Marionette.ItemView.extend({
  // ...

  onShow: function(){
    // I am guaranteed to be called from the CollectionView
    // because the CollectionView registers me in a promise
  }
});

CollectionView = Marionette.CollectionView.extend({

  initialize: function(){
    this.onShowCallbacks = new Marionette.Callbacks();
  },

  onShow: function(){
    this.onShowCallbacks.run();
  },

  appendHtml: function(cv, iv){
    cv.append(iv.el);

    // promise to run 'onShow' if it exists
    if (iv.hasOwnProperty("onShow")){
      this.onShowCallbacks.add(iv.onShow);
    }
  }
});