Making a one-page app using Dancer2::Plugin::Ajax

It's possible to make bits of DOM on a Dancer2 template behave like a one-page application, auto-updating themselves using some clever JavaScript and Dancer2::Plugin::Ajax. In this article, I'll give you a quick runthrough of how to do that.

The example: messaging

In this example, I want the app to check every few minutes for messages coming from some other system. I'll have Dancer2 look for the actual messages, and the frontend of the app will call a route using .ajax, and if there's a message, update some DOM. I'll be using jQuery for this, though other tookits have similar sorts of constructs.

Here's the HTML needed for the message block:

<div id='messageBlock' style='min-height: 50px; padding:10px; margin: 20px; border: 1px solid black'>
    <h2>Messages from Beyond:</h2>
    <div id='messageResults'></div>    <!-- Nothing in this div.  Yet. -->

And here's the route we'll be calling. In this example, I'm randomly generating the messages, but in a real app, you might be fetching them from a message box, a file, a database, or whatever. Because it's what I'm used to, I set the application-type in my config.yml to 'application/json', but this can do xml, or whatever you need; just make sure that's what you're returning!

use Dancer2::Plugin::Ajax;

ajax '/messages' => sub {
    my @messages = (
        'Boo! I betcha I scared you, ha-ha!',
        'A girl has no name, but she still has a list.',
        'Space, the final frontier.',
        '<h1>Really big text</h1>',
    send_as JSON => { message => @messages[int(rand(5))] };

Now, here's where the wizardry comes in. Call a function that repeats an AJAX request every ten seconds, and on success, update the DOM in that empty div we created for these messages. If you've got the style right, the larger div with the border box will even enlarge and shrink in height as needed.

$(document).ready( function() {
  var timer = setInterval(
    function() {
        url: '/messages',
        type: 'GET',
        success: function(data) {
           // Update the DOM
        error: function(event, stat, error) {
          // throw a message into the browser's developer console
          var str = 'Request failed. Status: ' + stat + ' Error: ' + error;
 10000);    // in milliseconds.

Nice, but what can you do with it?

Whatever you like, really; some obvious things for this little example would be to append the messages to a list until dismissed, or css treachery to make them fade in and out. On the Perl/Dancer2 side, you could have the route check for some event to happen--an okay from a manager, some outside processing finishing, you-name-it, and then letting the user know the success or failure. You could also cause growl or other notification blocks to appear as needed with a pattern like this; just change the JavaScript to do what you need.

We've used patterns like this for auto-updating blocks of HTML based on the results of a request in a few places at my current job, and I'm using it on a pet project I'm tinkering with. It's easy to use and maintain, and works like a dream!


This article has been written by D Ruth Holloway (GeekRuthie) for the Perl Dancer Advent Calendar 2020.

Copyright & License

Copyright (C) 2020 by D Ruth Holloway (GeekRuthie). This work is licensed under a Creative Commons Attribution 4.0 International License.