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Django Limehouse

A Django app to enhance sites by using pushState and ajax. Limehouse relies on plate, which is a Django template language implementation written in Javascript by Chris Dickinson.

Quick Start

Add limehouse to your INSTALLED_APPS::

INSTALLED_APPS = (
    ...
    'limehouse',
    ...
)

Setup your server to serve your templates as static files. For example, in Nginx with a path to your javascript-safe templates being, /path/to/project/templates/jstemplates -- use something like this configuration::

location /jstemplates/ {
    root path/to/project/templates/;
}

include jquery.js, plate.js, and limehouse.js in your template::

<script src="{{ STATIC_URL }}js/jquery.js"></script>
<script src="{{ STATIC_URL }}js/plate.js"></script>
<script src="{{ STATIC_URL }}js/limehouse.js"></script>

Annotate your links with data-targets. The contents should be in the form::

data-targets="<jquery selector>:<template path>;<jquery selector>:<template path>"

Selector-template pairs are delimited by a colon and multiple pairs are delimited by a semicolon.

For views that you want Limehouse-enabled, include a mixin::

from django.views.generic import View
from limehouse.views import ContextOptionalMixin


class SomeView(ContextOptionalMixin, View):
    ...

    def serialize(self, context):
        # serialize context into something JSON serializeable
        ...

Example Project

Included is an example project to demonstrate a barebones example of what is possible. Simply::

$ cd <virtual env>
$ pip install -e git+https://github.com/sebleier/django-limehouse.git#egg=limehouse
$ cd limehouse/example
$ ./manage.py syncdb
$ ./manage.py runserver

That should be enough to get the example project running. If you navigate the links, there should be no page refreshes. When you hit the back button, the pages should return to their previous state.

The Problem

Previously, it's been hard for django developers to fully embrace using client-side javascript templates because of a few problems.

First, there isn't a well known django template language implementation that has been able to run on javascript. This has forced developers/designers to use different template engines, e.g. mustache or handlebars. This comes with the consequence of having to duplicate your logic if you want to maintain the ability to render templates on the server. This can be avoided if you use a ported-to-python version of a javascript template language, but that doesn't help people who like and want to use the Django template language. plate_ is such an implementation and has most features of the python version.

The Solution

Django Limehouse provides a framework for updating a page to another url, without doing a page refresh. However, if javascript is not enabled or the browser doesn't support any of the html5 features required, it will degrade gracefully.

How does it work?

Let's start with a simple page with a list of links that point to various blog posts::

<ul id="nav">
    {% for entry in entries %}
    <li><a href="{% url blog_entry slug=entry.slug %}">{{ entry.title }}</a></li>
    {% endfor %}
</ul>

<section id="body">
    {% include "blog/entry_body.html" with entry=current_entry %}
</section>

Nothing special here, so let's enable Limehouse to load new entry pages via ajax::

<ul id="nav">
    {% for entry in entries %}
    <li><a data-targets="#body:/blog/entry_body.html" href="{% url blog_entry slug=entry.slug %}">
        {{ entry.title }}
    </a></li>
    {% endfor %}
</ul>

<section id="body">
    {% include "blog/entry_body.html" with entry=current_entry %}
</section>

Here we have the anchor tag annotated with a data-targets attribute, which provides a declarative way to specify what html targets will use what templates. You list out jQuery compatible selectors, paired with template paths and Limehouse will fetch those templates. The href of the anchor tag is left unchanged and will be used to retrieve the context required to render the view.

When the user clicks on the link, Limehouse will intercept the click event and fetch all the templates specified in the data-targets attribute. It also uses ajax to retrieve the context for that view using a special HTTP header. Once the context and all the templates are retrieved, Limehouse takes all the targets and constructs a state object that contains a mapping between the jQuery selectors and their current html. We replace the current history state with our newly created object and then start to construct the new html.

We use plate_ to render the templates, using a serialized form of the context used to render the view server-side. When all the templates are finished rendering, a new state object is created for the page about to be constructed and inserted into the history using pushState. Then, the new page is constructed with the rendered templates that are inserted into their specified targets.

The advantage of this approach is that the "blog/entry_body.html" template is one file, that works both on the front end as well as the back end.

The Django side of things

Limehouse comes with a special class-based view mixin that allows for this functionality to work seamlessly. When the client requests the view from the server, it sends an HTTP request header X-Context-Only, which is used in the mixin during its render_to_response method. Instead of creating and rendering the templates server-side, the context is instead returned in a serialized format. That format is controlled by a method called serialize and is responsible for returning data in a json serializeable format.

That's about it.

Server

Templates are served as static files from a webserver from a templates directory. It can be the same directory as your regular templates or from a separate directory that is designated as your javascript-safe templates. This might help you to make sure you're not including any templates with tags require being on the server.

One of the neat aspects of this approach is that a template can be cached by the browser and prevent the entire template from transferring over the wire. This means that once the templates have been cached, the only thing transferring over the wire will be data!

Caveats

This approach is good at moving between pages, but it may not be a good way to preforming application logic or real-time state changes. For example, you probably can't use a link to delete a resource, because you are probably going to be deleting nodes in the DOM. That can have deeper implications than simply replacing the inner HTML of a node. Similarly, it may not be a good idea to be POSTing data to a view.

Plate's implementation is also incomplete, since it can only handle local data. You can write template tags and filters that retrieve data from the server, but the developer has to be aware that not all tags are going to work out of the box.

Only works with class-based views right now, since it is easy to just to add the mixin into any class-based views.

.. _plate: https://github.com/chrisdickinson/plate .. _plate.js: https://github.com/chrisdickinson/plate .. _Chris Dickinson: http://neversaw.us/ .. _jquery.js: http://code.jquery.com/jquery.min.js