AngularJS for Digital Cultural Collections

In the previous post I introduced our Discover the Queenslander project for the SLQ, and mentioned that we used the AngularJS web framework. That process has got me thinking about some of the technical challenges in creating rich collection interfaces, and the different approaches in play, and I'll report on these in the next two posts. In this one I'll focus on AngularJS, and in the next, some broader questions on working with collection data on the client side.

AngularJS is a Javascript-based framework that focuses on extending HTML to deal with dynamic content. Angular "binds" data to HTML elements; so change the data, and the HTML updates. Even better, the bindings are two-way: interacting with an HTML element can also change its bound data. Angular implements a MVC (Model View Controller) architecture, where the data structure is the Model, the HTML document is the View, and a Javascript Controller links the two together.

Our previous web-based collections projects (TroveMosaic, Manly Images, Prints and Printmaking) were built in plain JS and jQuery. The general approach is pretty straightforward: load and manipulate some collection metadata (either from an API or a static JSON file), then build the HTML dynamically (adding and styling elements according to the data). jQuery makes handling interactions with the HTML pretty straightforward. It also (in my experience) makes for a verbose mess. Because all the HTML is built dynamically there's a lot of code devoted to creating elements, setting attributes, then stuffing them into the DOM. Code that loads and munges data gets tangled with code that builds the document and code handling interactions. Some elements get styled with static CSS, others are styled with hard-coded attributes. It all works fine - jQuery is very robust - but under the surface, it's bad code.

AngularJS tidies this process up quite a bit. Here's a quick example showing how straightforward it is to bind some collection data to some HTML. Say we have a JSON array items where each item looks something like:
{ "id":"702692-19340823-s002b",
 "title":"Illustrated advertisement from The Queenslander, 23 August, 1934",
 "description":["Caption: Practical garments","An Advertisement for women's clothing sewing patterns acquired through mail order from The Queenslander Pattern Service."],
 "subjects":["women's clothing & accessories","advertisements"],
To create a HTML list where each item appears as a list element:
 <li ng-repeat="i in items"> 
  <img ng-src="{{i.thumbURL}}"/>

Angular lets us iterate over a list of elements with the ng-repeat directive; it will simply generate a <li> for each element in the items array. Attributes of each item i are easily bound to the HTML using the {{moustache}} notation - so the item title will appear inside the h1. Apart from the compact, HTML-based rendering syntax, the killer feature here is that the HTML stays bound to the data: in order to change the display, we simply change the contents of items. No jQuery-style DOM manipulation; the data drives the document.

So rendering items in a list is trivially easy; but what about more complex displays? It's a matter of creating the data structures you need, then binding them to HTML in the same way. The Queenslander grid interface (above) includes a histogram showing items per year. In HTML this is simply another list, where each column is a list element. To create the data structure we sort the items into a new array where each element contains both the year, and a count of its items. Then as in the example, we run through the array with Angular building an element (this time a column) for each year. Angular's ng-style directive lets us create a custom height for each element, based on the number of items in the year list. With an array yearTable, where each year y has a totalCount
     <li ng-repeat="y in yearTable">
           <div ng-style="{height: y.totalCount+'px'}"
           ng-click="setYearFilter(y.year);" >
Here Angular is doing some rudimentary data vis, linking variables in the data to the dimensions of each HTML element. Note also that each column element has an ng-click directive, calling a function that filters the items displayed. The term clouds for subjects and creators work the same way.

Hopefully this gives a hint of how AngularJS can be applied to cultural collection interfaces. From a developer's perspective, there are a number of big advantages. Compared to our previous jQuery process, Angular simplifies the page-building process immensely; the templating approach encourages a separation of concerns and more organised, maintainable code. Angular's data-centric binding also provides some big wins. Data structures (models) become more important; Angular requires that you get your data organised before binding it to the DOM. Coming from the free-wheeling procedural world of jQuery, this data-centric approach was the biggest conceptual challenge. The bottom line is: manipulate the data, not the HTML. The payoff is that the work of keeping the HTML and the data coordinated just disappears. Angular's modular architecture and active developer community also bring benefits: in the Queenslander project for example we used ngStorage, a module that made the favourites incredibly easy to build.

Compared to standard web interfaces, the big difference here is that all the collection data (in this case some 1000 items worth) is in the browser, on the client side. No server calls, pulling down a few items at a time - instead we load the whole set up front, and build the interface dynamically based on that data. The biggest payoff for this approach is responsiveness - filtering and exploration are lightning fast - but there are problems too; search engines can't index this dynamic content, and it requires modern browsers with fast JS engines. Some would argue that this approach is just plain wrong; abusing the client/server architecture of the web. I'm more of a pragmatist, but there are certainly some technical issues to consider, and in the next post I'll go a bit deeper into this notion of client-side data for digital cultural collections.


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