model view controller design pattern

The Model-View-Controller Design Pattern and iPod-like User Interfaces

In my previous blog entry I asked whether the iPod is an example of a

new user interface paradigm, and concluded that it is. This time, I

try to provide more of the nomenclature for describing user interfaces

of this type.

Discussion of desktop user interfaces is much aided by the desktop

metaphor conceptual framework, augmented by the MVC

(model-view-controller) design pattern. The desktop metaphor describes

how overlapping graphics contexts give rise to on-screen windows that

mimic the look of overlapping pieces of paper. MVC provides a design

framework for (potentially) multiple views of the same data to be

present on the screen and to stay in synch as the underlying data

changes.

Together, the desktop metaphor and MVC are the foundation of nearly

every non-trivial desktop user interface. iPod-like user interfaces

discard the desktop metaphor. Overlapping objects are out of place on

the iPod. They have no metaphoric meaning, and the screen is too small

to give overlapping objects enough visual context.

In the desktop metaphor, each window is a viewport into a

two-dimensional document that extends, mostly, above and below the

view the viewport provides. In an iPod-like user interface, the whole

of the screen is a viewport into a document consisting of a network of

nodes. And here is found an interesting connection to the desktop

metaphor: Most document data models are networks of nodes: Web

browsers' data models of Web pages, for example. The iPod, therefore,

achieves an elegant presentation of a document by being one step

closer to the type of data model used to model many documents.

The iPod shows that MVC is not inextricably linked to the desktop

metaphor: The iPod has a model, a view, and a controller, although

each is simplified: The model doesn't change until iTunes changes it,

the view is a full-screen view of just one set of nodes which all are

children of the same parent-node, and the controller provides no means

of editing the content of nodes.

Not every element of the desktop metaphor is a success. The desktop

metaphor is plagued by an innovation known as the dialog box. The

dialog box is familiar to most users, and in its most florid examples

brings no less joy than a long session of filling out tax forms. The

dialog box was a feature, used sparingly, in early desktop metaphor

user interfaces, to put a pressing matter in front of the user, in

situations where the user could not proceed without at least

acknowledging the matter, e.g. a serious error. Since then, user

interfaces have experienced a metastasis of the dialog box as mediocre

designers, armed with visual dialog box layout tools, have attached

numerous multi-tabbed tumors to software. Pre-AJAX Web user interfaces

brought the dialog box to its pinnacle of dominance: If you have ever

filled out a multi-page "registration" for some Web site, you know it

well.

The iPod does without dialog boxes, not least because text entry on an

iPod would be brutally hard, but also because dialog boxes, like other

overlapping elements, have no good visual representation on an

iPod-sized screen. The lesson embodied in the desktop metaphor's

encrustation of dialog boxes compared with the iPods lack of such

goiters is that designing the direct manipulation of the data model is

hard, and widgets, dialogs, and menus are easy. The iPod escapes this

damage by being an environment that is hostile to the concept and

implementation of dialog boxes. The iPod is also a browser, not an

editor, and therefore manipulation means viewing, not modification.

Can this simplified MVC user interface survive being extended into

providing a first party call control user interface? Or a mobile

commerce interface that brings iTunes Music Store into a connected

iPod? While these are not small design challenges, there is reason to

think this simple and elegant UI paradigm can encompass new functions

without losing its qualities: First party call control is not document

manipulation. It has its own very severe challenges, but it should not

require a fundamental shift away from the iPod user interface.

Similarly, shopping can be thought of as a series of database queries

followed by the user browsing the results. This, too, should be

possible to implement in the iPod paradigm without warping it out of

shape. These examples are not randomly chosen: If there is to be a

wirelessly networked iPod that includes a PLMN or VOIP telephony

subsystem, these are the major components that would have to be added

to the iPod user interface.

The iPod can also be categorized as a "one-handed" UI. One-handedness

is also linked to the lack of overlapping elements in the iPod UI: If

you don't have a pointing device to access overlapping elements the

way you would access papers on a desk, you should not have them in the

UI. Failure to heed this distinction between one-handed and two-handed

UIs leads to such kludges as "two level focus" and other severe

compromises that are inherently incomprehensible and inexplicable to

users. For examples, you can look to any mobile handset HTML Web

browser that attempts to map the two-handed PC Web browser human

interface to a one-handed handset interface. Microsoft still struggles

to fully reshape the two-handed Windows CE user interface - which is a

fine user interface if you assume the user is holding a stylus - into

a friendly one-handed UI for mobile phones. Two handed UIs allow rich

and ambitious interfaces, while the iPod's strict observance of what

the user's thumb is capable of leads to an entirely different result.

The iPod, therefore, can considered to be a full-fledged example of a

distinct user interface conceptual framework. It has all the

characteristics of the consistent application of such a conceptual

framework to an implementation. Graph or network traversal,

one-node-at-a-time views, mapping of architectural aspects to the MVC

design pattern, and extensibility to other application domains put the

iPod UI alongside the desktop metaphor as something that we will

likely see more of, in other types of devices, applied to other