Whereas many document systems support some form of extensibility,
the Multivalent system pushes this idea to the extreme. Almost
everything that is not a tree node is an extension called a behavior.
Behaviors are Java classes that participate in the communication
protocols detailed in subsequent sections.
Programmatically, this means that behaviors subclass the class
Behavior and override methods corresponding to those
Some behaviors happen to be packaged with the basic system, but they have no
privileges over third-party behavior extensions. All user-level
functionality is implemented by behaviors.
The extension language is the implementation language.
Behaviors can be categorized according to primary function,
although a single behavior may participate in several.
- Media adaptors
- Behaviors that
primarily parse some concrete document format and build a runtime document tree
are known as media adaptors.
For example, the UNIX manual page media adaptor reads roff source,
the HTML media adaptor goes to great pains to correct the files of random
bytes found on the Web into a structurally reasonable tree,
and the scanned paper adaptor builds a document tree
hierarchy of region, paragraph, line, word.
In addition to viewing documents, media adaptors can be used
for general purpose access;
for example, a full-text indexer could use media adaptors to decode
PDF and DVI uniformly to and as easily as ASCII and HTML.
Once bridged into the tree, the document of whatever source format
enjoys the array of existing functionality; this contrasts with
single-format viewers, such as
IDVI for TeX DVI,
Ghostscript for PostScript,
xpdf for PDF,
which, however excellent they are for their single format,
must recapitulate a large and growing amound of standard functionality.
To preserve readability of digital documents in the future, according to
"A Universal Tool to Rescue Old Files From Obsolescence",
Raymond Lorie of IBM Almaden proposes a virtual machine that
and definers of document formats support by writing additional
software in the VM language that reads and displays those documents.
How about Java and Multivalent media adaptors?
- Structural behaviors modify protocols over a document subtree.
Such behaviors "register interest" in a particular subtree,
and subsequently each protocol
invokes the behavior's corresponding methods before and after
passing through the subtree rooted at that node.
For example, table sorting rearranges the children of the given parent
to achieve sorted order, clipboard markup generates a representation of the
selected text with markup tags, and one type of search visualization
hooks onto the scrollbar to paint its results on top of the scrollbar
every time it is painted.
- This very common behavior type of behavior
extends from some offset within a start leaf linearly
through leaf nodes to an offset within an end leaf. Examples of span type
behaviors include font change, highlight, hyperlink, and copy editor markup.
- Lenses, such as Magnify and Decypher, control a geometric
portion of the document (described with a movable, resizable window).
Lenses compose effects where they overlap, so
that magnify plus Show OCR yields magnified OCR,
and Show OCR plus decypher yields decyphered OCR.
- Managers provide specialized coordination among behaviors
beyond that provided by the usual means of communication.
For example, Lens coordination of overlapping lenses is very specialized,
to compose effects when lenses overlap, and yet
its coordinating manager behavior has no special
privileges in the system. When a lens is made, it queries the
browser-level attributes for the lens manager, spontaneously creating
one if it is the first, and registers its existence. During document
painting, the lens manager computes intersections and invokes the
- A number of behaviors customize the document.
Annotations add user-authored information.
Manual pages are transformed into outlines
in order to present an overview of and easy access to
the information contained.
The autosearch behavior highlights keywords
that might otherwise be overlooked in long documenets.
Behaviors could be written to strip ads from HTML
or highlight relevant words in pages returned by a search engine.
Writing behaviors ranges in flexibility and difficulty. An
individual behavior may be customizable with attributes in its hub.
that appears in the popup menu on a word and sends that word to a
dictionary or language translation service takes as attributes that title
to show in the menu and the URL of the service. Most span types rely
on a SpanUI behavior to put them in a menu
(this separates functionality from user interface),
and other spans could be
added and their organization in the user interface rearranged. The
SemanticUI behavior sends an arbitrary semantic event in response to
invoking a menu item or button on the toolbar. A number of other
behaviors are probably simple variations on existing behaviors.
A demonstration "FBI Redaction" behavior, which blacks out spans
of text and associates a reason code and comment, was written in
two hours by starting with the hyperlink annotation behavior,
changing the blue underline to black foreground and background,
and changing the dialog box to ask for a comment rather than a URL.
Of course, the wholly original ideas can be satisfied only
by writing a new behavior from scratch, but it seems possible to
accomplish interesting things in just a few hundred lines of code.
Media adaptors can reuse node types already created for flowed and
fixed document types, and current media adaptors range from 167 lines
for ASCII, 180 for Zip, 238 for directory listing and 260 for Perl's
POD among the simple formats, to about 1000 for UNIX manual pages in
At 4000 lines, HTML is the largest media adaptor, yet
this is only 5% as large(!) as the rough equivalent in Mozilla.
Spans are simpler, with most
under 100 lines and the most complex (hyperlink) at 250. Lens range
from 50 to 100 lines. Other behaviors range from 100 to 400 lines.