An axiomatic approach to defining user interface elements: building complex elements from simple ones

Just as geometry builds up complex results from simple axioms, and programming languages build up complex constructs from simple primitives, it should be possible to create complex user interface elements from simple elements. But the lack of great building blocks for web user interface components causes people to waste a colossal amount of time reproducing common behaviors or, worse, forces them to settle for something quick but suboptimal.

Take something as basic as tabs. Every web UI package includes a widget or component that produces a set of tabs, such as the typical example from jQuery UI:

While a tab set may seem to be an irreducible unit of user interface complexity, we can actually decompose its behavior into smaller, simpler chunks:

1. Ensuring a single element within a set is “active” at any given time. Here, only one of the tab buttons is in the active state. There are many other manifestations of this behavior. Single-selection list boxes, for example, also have a notion that a single item in the list is active.
2. Showing a single element a time. The main region of the tab set shows a single page which corresponds to the active tab button. The active page is the only one that’s shown; the non-active pages are hidden. This behavior comes up in situations other than tabs. For example, photo “carousel” controls let a user page through photos one at a time, generally with previous/next buttons instead of a tab strip.
3. Showing a set of identical elements that correspond to items in a list. The strip of tab buttons across the top has an internal consistency: every tab button is represented with the same type of button.
4. Positioning one collection of elements directly above another. Here, the strip of tab buttons is stacked on top of the tabbed pages. This kind of layout seems so simple as to not deserve consideration. However, in current web browsers, this can be frustratingly difficult to achieve in the common cases where the size of the tab set is flexible. Suppose you want the tab set to fill the viewport, or a region of the viewport. The tab strip should consume a given height (which for a variety of reasons should not be fixed beforehand in, say, pixels), and the remainder of the space given over to the tabbed pages. This type of layout can be achieved with a CSS flexbox, but at least for a little while longer, many app developers will need to support older browsers (i.e., IE).
5. Giving UI elements a description which can shown elsewhere. The pages shown within the tab set are rectangular regions, but the name of the tab is shown outside. It’s fairly common to want to give a UI element a user-friendly name like this.
6. Letting a user tap or click a button to achieve a result. That is, the elements in the tab strip behave like buttons.

It should be possible to create UI classes that implement each of these more fundamental behaviors or aspects. It should then be possible to exploit these behaviors on their own, or recombine them with other behaviors to produce other recognizable user interface controls. In effect, we should be able to arrive at fundamental behaviors that behave like the axioms in a mathematical domain or, alternatively, like atoms in a physical system of elements.

The domain of computer science has much to say on the topic of axiomatic design. Programming languages are often predicated on the notion that you can boil down everything you’d want to do in the language to a tiny set of primitive functions. It’s only this small set of primitives which must be written in a lower-level language (e.g., a machine language). Everything else can be built up in the language itself. This not only keeps things clean, it ensures the language’s popularity and survival by facilitating the porting of the language to new platforms — only the primitives must be rewritten, and all the remaining code built on top of the primitives can be used as is. The original example of this axiomatic principle in language design was Lisp, whose story Paul Graham recounts in his article The Roots of Lisp. (The full article is available on his site in the original Postscript version, or in various converted PDF versions.) From his article:

In 1960, John McCarthy… showed how, given a handful of simple operators and a notation for functions, you can build a whole programming language.

[McCarthy’s] ideas are still the semantic core of Lisp today. It’s not something that McCarthy designed so much as something he discovered. It’s not intrinsically a language for AI [artificial intelligence] or for rapid prototyping, or for any other task at that level. It’s what you get (or one thing you get) when you try to axiomatize computation. … By understanding [Lisp] you’re understand what will probably the main model of computation well into the future.

Can we determine a similar axiomatic deconstruction of user interface elements? That’s a topic I’m acutely interested in, and I believe the answer is yes. Even through graphical user interfaces span a range of devices, platforms, and frameworks, the underlying collection of distinct user interface behaviors is quite consistent: clicking one thing something makes something else appear; items in lists are given consistent representations and behavior; modes (for both better and worse) constrain the user’s attention and powers; and so on. It should be possible to boil those consistent behaviors into reusable code.

The result of this decomposition is a set of UI primitives which is significantly bigger than the canonical tiny set of user interface controls: a push button, a radio button, a check box, a text box. Of all the aspects numbered above, only #6 (push buttons) are available as a native browser control. Web developers are generally forced to recreate all the other aspects through combinations of CSS and JavaScript. That's inefficient and error-prone. As noted above, even something as seemingly straightforward as stacking two regions on top of one another can prove unexpectedly complex.

The actual set of web UI primitives is probably an order of magnitude larger than what browsers expose as interactive UI controls. At the same time, the set of really general purpose contemporary UI (see this article for a breakdown of UI elements by context-specificity) is not so large it can't be enumerated or understood. For today’s typical mobile or web application, I believe a reasonably comprehensive collection of UI primitives would number in the 100 – 200 range.

What would those primitives be? My work on the QuickUI Catalog is an attempt this question. It’s a work in progress, and is by no means complete. It currently includes controls which shouldn’t be there (they’re really just sample uses of an underlying component), and on the other hand doesn’t (yet) include enough controls for common situations like mobile. Nor is the set of controls completely stable yet. I occasionally realize two controls exhibit similar behavior whose implementation should (or shouldn’t) be shared, which results in both minor and major refactorings. Nevertheless, the Catalog already represents a highly useful starting point for creating application UIs.

Let’s return to the tab set example above. The QuickUI Catalog includes a Tabs control for this purpose, which can be used as is. But that Tabs control is simply a combination of lower-level components corresponding to the attributes listed above:

1. A Sequence base class. A Sequence control keeps track of which one (and only one) of its children is currently active.
2. A Modes control. Extends the Sequence class to hide everything but the active child.
3. A List control. Maps an array of internal data items to an array of user-visible controls.
4. A VerticalPanels control. Stacks things vertically. This inherits from SimpleFlexBox, a user interface polyfill which uses a CSS flexbox for layout on modern browsers, and a manual engine for layout on older browsers.
5. A Tab control. Associates a description property with an arbitrary block of content. It's this description the Tabs control displays in a List of buttons across the top.
6. A BasicButton control. This wraps the browser’s native <button> as a component. Among other things, this allows a BasicButton to be used to render items in the List (above) to create the strip of tab buttons.

All these derive from a common Control base class.

We can show the relationships between all these classes in a graph, where a solid line represents an “is a” relationship (one class derives from another) and a dotted line shows a “has a” relationship (one class makes use of instances of another class):

This arrangement entails a lot more pieces than a typical web user interface platform. The browser itself only provides a native button. Most existing web user interface frameworks provide some button class wrapper (such as BasicButton here) and a tab set class (Tabs). They may or may not expose a general purpose UI component base class (here, Control). The tab set class is typically fixed in a monolithic implementation, and can only be modified via parameters the framework designers have anticipated beforehand.

Traditional client-side UI frameworks (e.g., Windows Presentation Foundation) do have rich class hierarchies, although even their UI primitives tend to be too course-grained. And contemporary web UI frameworks rarely have good building blocks. (Some people claim the Sencha framework does, but it's unfortunately encumbered with developer licensing fees, and requires you to build your app on top of a proprietary substrate. To me, that's moving in the exact opposite direction of web development trends.)

The main obstacles to UI like this on the web may have multiple causes, including the fact that the web's primary client-side programming language JavaScript, still has no native support for traditional object-oriented classes. Moreover, the browser doesn't yet expose a model for modular component composition, which creates a lot of work for a UI framework's creators.

In the above implementation of a tab set, all the lower-level pieces are directly available to the user interface designer and developer. These can be used on their own, or combined with other types of elements to create other user interface elements. And, significantly, new elements constructed with this approach are, by default, extensible and recombinable in their own right. In a subsequent post, I plan to show some of the other sorts of UI controls which can be created by combining some of the pieces above in different ways.

As noted above, this Catalog implementation isn’t perfect. Among other things, there are inherent limitations on what you can achieve with a classic single inheritance hierarchy. But, overall, this feels like a promising direction, and in practice is a highly efficient way to create web apps. Above all, this axiomatic approach feels like the right paradigm for building UI.

McCarthy's big advance with Lisp wasn't to create programming language primitives — all programming langauges have primitives. His insight was that the primitives in programming languages of the time weren't primitive enough. Instead, you should break a language into irreducible axioms, and let people combine those axioms to create any new language functions they need. The functions you create with those Lisp primitives are just as powerful as any pre-packaged functions created with those same primitives by the language's designers. That is, there's nothing special the language designer can do you which you cannot also do.

Similarly, a UI platform should give you a powerful set of axiomatic appearances and behaviors and a means to combine them to create new elements which are every bit as powerful as those elements that come bundled with the platform. This is why attempts to build a tiny handful of new controls into web browsers  is almost completely uninteresting to me. A new date picker in the browser, to take just one example, is just never going to solve your date picker needs. It's like the FORTRAN committee adding yet another hard-baked statement to the language. What's infinitely more interesting is a UI platform that gives you the building blocks you need to build a date picker of your own that's as powerful as anything in the browser itself.