Displaying loader while meteor collection loads

I have a template, task_list, that looks like this:

{{#each tasks}}
    {{> task}}
{{/each}}

Template.task_list.tasks returns a collection and in the ui, it seems to take a bit of time to load.

While the collection is loading, I'd like to show a loading indicator.

Any ideas on how I might be able to do that?

BTW, I did try the templates' rendered event on task_list template, however it gets fired before the list is actually loaded. I also tried using rendered on the task template but it seems to never get called.

Meteor 1.0.4 update: Now that template-level subscriptions are available and the preferred pattern to using iron:router or standalone subscriptions,

There is a complementary function Template.instance().subscriptionsReady() which returns true when all of the subscriptions called with this.subscribe are ready.

Inside the template's HTML, you can use the built-in helper Template.subscriptionsReady, which is an easy pattern for showing loading indicators in your templates when they depend on data loaded from subscriptions.

Example:

Template.notifications.onCreated(function () {
  // Use this.subscribe inside onCreated callback
  this.subscribe("notifications");
});
<template name="notifications">
  {{#if Template.subscriptionsReady}}
    <!-- This is displayed when all data is ready. -->
    {{#each notifications}}
      {{> notification}}
    {{/each}}
  {{else}}
    Loading...
  {{/if}}
</template>

This is better than having a generic loading template for the whole page, because the loading section is localized to the part of the page that actually has new data.

Pre-Meteor 1.0.4:

The idea is to pass an onReady function to Meteor.subscribe:

Meteor.subscribe('tasks', function onReady() {
  Session.set('tasksLoaded', true);
});

Then, make your template depend on the tasksLoaded session variable. In the client JavaScript:

Template.task_list.helpers({
  tasksLoaded: function () {
    return Session.get('tasksLoaded');
  }
});

In your template:

<template name="task_list">
  {{#if tasksLoaded}}
    {{#each tasks}}
      {{> task}}
    {{/each}}
  {{else}}
    <img src="http://viewvc.svn.mozilla.org/vc/addons/trunk/bandwagon/skin/images/spinner.gif?revision=18591&view=co&pathrev=18591">
  {{/if}}

UPDATE: With iron-router you have a direct option to specify a loading template which will be displayed while the subscription is loading.

Dan's answer was definitely spot on, but I want to remind that I belive autopublish package has to be removed for it to actually work.

meteor remove autopublish

Plus, I recommend spin package for a nice looking spinner.

There have been some nice packages released in the meantime. Check out these two:

1. Spin - displays a spinning wheel. With Iron Router, you can specify a loading template which shows the spinning wheel.
2. Iron Router Progress - shows a progress bar on the top of the page (Youtube style)

They both work pretty much out of the box, have a look at their documentation for more advanced options.