Meteor Iron-Router: When is 'data' called?

Question

Here's the route in question:

this.route('meeting', { 
  path: '/meeting/:_id',
  template: 'meeting',
  waitOn: function(){
    console.log("iron-router: meeting waitOn");
    return Meteor.subscribe('meetings');
  },
  data: function() {
    console.log("iron-router: meeting data");
    return Meetings.findOne({"uuid":this.params._id});
  },
  onBeforeAction: OnBeforeActions.loginRequired,
  action: function () {
    console.log("iron-router: meeting action");
    if (this.ready()) {
      this.render();
    }
  }
});

The meeting template has a nested calendar template which contains 28 day templates. I'm trying to figure out when/why the iron-router data function is called, because I see this when I load the page:

iron-router: meeting waitOn
iron-router: meeting data
iron-router: meeting data
iron-router: meeting waitOn
iron-router: loginRequired
iron-router: meeting action
iron-router: meeting data
iron-router: meeting data
(28x) iron-router: meeting data

That's a lot of repeated calls to the same thing! Why?

Answer 1

Data is called whenever there is a change to any reactive variable in it. In your case the Meetings collection.

When your app loads it will re-run whenever each document in your collection is added to the client. It's also run once initially to register reactive dependencies.

It will run multiple times and if there are changes they will be painted to the DOM. If not it will not proceed further to update the DOM. The re-runs are to check for changes.