OpenDoc 2
Volume Number: 10
Issue Number: 8
Column Tag: New Technologies
Extending OpenDoc 
The designer’s view on going beyond the initial design
By Kurt Piersol, Apple Computer, Inc.
OpenDoc™ is a component software architecture. It’s been designed to provide a
standard, vendor neutral way to create applications that can work together to create
compound documents and applications. It will work on almost any platform, and
announcements have been made about Macintosh, Windows, OS/2, and UNIX versions of
OpenDoc.
When we first envisioned OpenDoc, the idea of extending it was a central design
goal. We knew that we couldn’t possibly design everything that might be useful in a
component software environment, so we settled for creating a strong core and an
equally strong set of extension mechanisms.
The result is a system that you as a developer can alter significantly. You can add
new services to the OpenDoc environment or alter the behavior of the existing ones. As
you might imagine, there are good ways to do this and not so good ones. We’ll try to
describe the ways that we planned for you to change OpenDoc for the Macintosh.
This article assumes that you already know about writing the OpenDoc equivalent
of existing applications, part editors. There have been a several descriptions of how
you should go about building such editors, including a set of recipes which come as a
part of the OpenDoc seed. If you haven’t seen this documentation, though, then you
should know a little bit of basic information first. If you have seen this before, feel
free to skip the following section.
OpenDoc Basics
First of all, you should know that OpenDoc is basically a scheme for putting
software components together. In its first release OpenDoc focuses on allowing you the
developer to create editors which can work together to form compound documents.
More about this in a moment. However, OpenDoc does not stop with compound
documents. As we’ll see in the rest of the article, OpenDoc is designed to form the
foundation of an extensive component environment.
Compound Documents
The notion of a compound document is not new. People have been working on them
for about 20 years now. A simple document is a collection of some content. It might be
text, it might be a drawing, it might be a movie, it might be a HyperCard™ stack. A
compound document is a document that has more than one kind of content in it. For
example, a document with text and pictures in it counts as a compound document. Any
sort of content can be the basis for a compound document. Contrary to popular opinion,
a compound document is not a “plain paper” document, because even that definition is
too restrictive. Any combination of different kinds of content is possible, whether or
not they could ever be represented on paper.
In OpenDoc, this basic idea is carried a bit further. OpenDoc documents allow all
the kinds of content in the document to be directly edited, in place, in the document.
Further, there is no limit on how many kinds of content can appear together in a single
document. You could combine fifty different kinds of content into an OpenDoc document,
and have them all live and editable.
We call each of these bits of content a “part.” We chose the name because when
we asked users what to call these things, they replied that they were “parts of the
document”. Thus, the term “part.”
As a side note, we understand the natural inclination to refer to the famous
chicken commercial motto: “Parts is parts.” Most members of the OpenDoc team can
manage a tolerant smile when this occurs, although a few snarl and one whimpers.
However, we all pretty much exhausted all the possibilities of this particular joke
about a year ago.
Part Handlers
The way OpenDoc achieves this flexible ability to compose documents is through
the use of special applications called part handlers. There are two kinds of part
handler, editors and viewers. A viewer is nothing more than a specialized editor which
doesn’t allow the user to change the content of the part. A part editor is much like an
existing application: it displays the part, handles events, performs disk I/O, and
accepts scripting commands.
Writing part editors and viewers is by far the most common way to extend
OpenDoc. By writing one of these, you are extending the scope of information that a
user can incorporate into a document.
Component Infrastructure
With the basics out of the way, we can dive into the guts of OpenDoc looking for
places to extend it. First, we should spend a bit of time describing the basic design of
OpenDoc. We intended for OpenDoc to provide a good basic infrastructure on which
various kinds of component systems might be built. To accomplish this, we needed a
way for components to find one another, negotiate about resources, and discover what
other components were capable of doing.
The first requirement, components finding one another, is fairly simple to
implement. In a distributed object space, one must have a method of determining
whether objects are a part of a given interactive session. For example, if a hundred
people are working in a distributed object environment, how does one tell which
objects are associated with which people? The answer in OpenDoc is an object called
the “session” object. The session is the root of the run-time object space of an
OpenDoc process.
The session is the key to reaching other software components. It points to the
major object services which bind the various software components together. OpenDoc
components should always have a pointer to the session object in which they are active,
or a pointer to some object that can reach the session.
There’s a set of core services in OpenDoc which form the basic building blocks on
which other components can be layered. The document architecture of OpenDoc is an
example of the kind of system that can be built. However, the document architecture is
only one point in a large space of possible architectures. You can build your own
architectures using this same basic mechanism.
Name Spaces
One of the most important of the basic services available from the session object
is the “name space manager”. This object gives you access to a table of “name spaces”
global to the session.
A name space is really a hash table. The hash table, of course, has a list of keys
and values. The key is a special kind of string called an ISOString, and the value is any
four byte unsigned quantity.
ISOStrings are unique names which follow the ISO 9070 standard for creating
unique segmented names. They’re really just long 7-bit ASCII strings which have
enough segments to guarantee that they are different from any other name. If you
follow the rules for creating an ISOString, you can be sure you’ve created a name
that’s unique. OpenDoc uses this technique to create names for data types as well as
names for entries in name spaces.
There’s no particular limit, other than memory, governing how large a name
space can be. It could store hundreds or thousands of values.
Getting back to the name space manager, you can see that this is really a name
space itself, a name space holding pointers to other name spaces.
Now, let’s talk about how to use this rather generic feature to extend OpenDoc.
Let’s assume you wish to create a special interface element which is shared by many
part editors. A color palette, for example. You’ll need a way to make sure the palette
exists as well as find it from any editor that wants to use it.
Name spaces are the perfect tool to use. Here’s the list of steps to use to create
your global palette and store it globally.
1. Choose an ISOString to serve as the name of a name space where you will store
your global information.
2. Choose another ISOString to represent the global palette in the name space.
3. Check to see if a name space using the chosen name has already been created. If so,
get a pointer to it. If not, create it yourself and remember the pointer to it.
4. Check to see if the name for the palette already exists in the name space. If so,
get the value and use it as a pointer to the global palette. If the name does not
exist, then create a new entry in the name space, create your palette, and put a
pointer to the palette object into the value of that entry. You can use this
technique to register global information where anyone who knows the two
ISOString names can find it. By publishing this information, you can create
standard name spaces filled with information being shared between OpenDoc
components. If you don’t publish these names, you can still use the mechanism
for private global objects. It’s a completely open architecture for making
services available to other components, one that avoids naming collisions and
still allows either public or private extensions to the OpenDoc object space.
Arbitrator
A second major piece of the component infrastructure can be seen in the
“arbitrator” object. Once again, the arbitrator is obtained from the session object. Its
purpose in life is to allow the various components to negotiate ownership of various
system resources. Each of these resources is called a “focus of arbitration” or just a
“focus”.
The arbitrator comes with a set of these foci pre-defined. There are foci for the
keystroke stream, the menu bar, the selection, and a few other system level resources.
In addition, a special focus called the “modal” focus allows components to seize all
events for modal behavior such as modal dialog boxes or script execution. Every focus
is defined by an ISOString name.
The interesting point about the arbitrator is that the list of foci is not fixed.
Components can create new foci simply by naming them. Once added, any component can
use the arbitrator to negotiate for ownership of the focus.
Even more interestingly, you can add special behavior to the negotiation for a
particular focus. You can do this by creating a special object called a focus module and
registering it with the arbitrator.
So what’s an example of this? Let’s go back to our global palette example. How do
we determine who gets to to set the state of the palette at any given time? If it’s like a
typical Macintosh palette, it will change state based on the state of the window being
edited.
The arbitrator is an excellent tool to use to solve this problem. By creating a
special focus for the palette, the various editors wishing to use the palette can
negotiate about who “owns” the palette, and can thus change its state.
Let’s go even further, and assume that the palette manages a global hardware
resource. Perhaps the palette switches between various ports on a special card to
control a multimedia presentation. By adding a focus module to specialize the behavior
of the arbitrator, you could prevent the transfer of focus ownership until all pending
I/Os are complete.
A last major part of the core infrastructure of OpenDoc is the ability to add
“extensions” to objects. The core of OpenDoc is focused around user interface and
layout tasks. However, there are limitless other modes of interaction between
components.
To account for this, the OpenDoc team designed the extension mechanism for
OpenDoc objects. Almost every kind of OpenDoc object can be extended using this
technique.
The basis for extensions is once again the ISOString naming mechanism. Each
extension an object is willing to publish must be named with this mechanism.
Every OpenDoc object must be willing to say whether it supports any given
extension. If it claims to support an extension, then it must pass out a secondary object
with the appropriate set of member functions for that extension. This object is passed
to the requester, who must release the object when it is no longer needed.
This mechanism can be used for a wide variety of purposes. Groups are already at
work to define standard extensions for OpenDoc part editors. Spelling and grammar
checking extensions are in the works, as well as extensions for extended layout control.
Going back to our global palette example, the extension mechanism is the perfect
way to let the palette talk to the editors which make use of it. The code running the
palette can check the arbitrator to find the owner of its focus, and then ask that editor
for an extension to let it communicate with that editor. For instance, if the palette was
a color picker, it could use the extension to notify the owning editor whenever the user
decided to pick a new color.
Storage System
OpenDoc’s document architecture is based on a sophisticated storage model. This
storage model is designed to be implemented in a number of different ways, by
different organizations. The initial container suite implementation for the Macintosh is
based on Bento, a multimedia storage format designed for cross-platform use. A
number of vendors already use Bento for storage in shipping products.
However, Bento is not the optimal container suite for all possible applications. A
more powerful solution, one which supported more efficient space reuse, automatic
compression, encryption, or automatic indexing features, would be a welcome addition
to OpenDoc. This represents a significant business opportunity for developers of
utility software.
The mechanism for adding a container suite in OpenDoc is straightforward. When
OpenDoc opens a document, it uses a combination of several objects which together
form a “container suite.” Each of these container suites implements a version of the
classes XMPStorageUnit, XMPDraft, XMPDocument, and XMPContainer. Each individual
document can be controlled by a different container suite. The selection of the
appropriate container suite is performed by the binding object, which we’ll discuss in
a moment.
Several different kinds of containers can be open in a single session. These
container suites are all by definition interoperable. This means that when you follow
the OpenDoc API for container suites, OpenDoc itself handles any differences between
implementations when data is moved between containers.
At this point, it’s traditional to say that there’s good news and bad news about
container suites. First, the bad news. It’s not particularly easy to write a container
suite from scratch. In fact, it would be pretty darned hard to do so.
The good news, though, is that you probably don’t need to rewrite from scratch at
all. You can get your hands on both the OpenDoc source and the Bento source, and you
can use them to subclass the existing behavior! This means that you can do a lot of good
stuff without the hassle of rewriting the storage code of OpenDoc.
In particular, you can take advantage of a feature of Bento called “dynamic
values”. To understand it, you must first understand the part of Bento called the I/O
handler. Bento is designed so that all of its I/O goes through a small set of bottleneck
routines called the I/O handler. These routines are pretty simple, just the basic read,
write, position, and truncate calls common to almost all file systems. You needn’t even
talk to a file system through the handler. In fact, a lot of OpenDoc’s data transfer uses
Bento sitting on a special handler which reads and writes to RAM instead of the disk.
Now, this feature would be fairly neat all by itself, but it goes even further.
Bento handlers can be layered. That means you can start with basic I/O, layer in
encryption or compression or both as independent modules! By layering handlers, you
can add significant extra value to Bento without touching the main Bento code.
So now we can get back to the original point. By altering the container suite so
that it layers additional handlers under Bento when documents are opened, you could
create a plug-in utility that encrypts or compresses every OpenDoc document. You
could also create a special version that works directly from a database or document
repository. You can do any of these things without rewriting the OpenDoc storage code,
simply by creating a subclass that opens documents using layered I/O handlers instead
of the standard OpenDoc handler.
Binding
When OpenDoc opens a document, it chooses the appropriate editor based on the
type of information it finds in the document. These instructions are contained in a
single object, called the “binding” object because it binds editors and data together.
The interface is extremely simple: the binding object is passed a data type and returns
a class identifier for the correct editor. The intelligence inside the object makes
sophisticated use of both user and editor based information to choose the correct editor.
The binding object represents another opportunity to add value to OpenDoc. Just
like with storage, you probably don’t want to write one of these objects from scratch.
Just as before, you can create a replacement from our sources, and deliver added value.
What sort of added value? Well, there are several possibilities.
The first of these is adding a method of checking licensing information during the
binding process. Probably this would involve a server request. The result would be a
way for users working in a corporate setting to check that they have licenses for every
editor run in an OpenDoc document.
A second possibility is to check to see if a later version is available from the
group server, and to update the software if needed. This would give IS managers a way
to manage software versions and distribution automatically.
A third possibility is to examine the editor itself before binding it. This might be
used to check for an authorizing digital signature, or look for virus code, before the
editor is loaded. It might even check the editor against a list of approved editors
supplied by the IS department as a part of the object.
Each of these options could be implemented by a simple subclassing or delegation
technique. We suggest delegating to the system provided binding object or inheriting
from it, rather than replacing this object.You could combine any or all of these
techniques in a single binding object. The result would be a valuable addition to
OpenDoc which might find extensive use in scholastic, workgroup or enterprise
settings.
Event System
Possibly the most extensive area available for extension in OpenDoc is a
mechanism for altering or enhancing how events are dispatched. All OS and high-level
events in OpenDoc pass through a single object called the “dispatcher.” The job of the
dispatcher is to make sure the event is delivered to the correct editor.
The dispatcher itself has none of the needed intelligence to perform this task, but
instead calls on “dispatch modules” which are registered to handle events or groups of
events. As with other parts of OpenDoc, this can be a fairly complex task.
The dispatcher is specifically designed to allow you to perform the equivalent of
patching. You can add behavior to dispatching any class of event by getting the dispatch
module for that event from the dispatcher, remembering it, and then installing a new
replacement module of your own that calls the original dispatch module.
The interface to dispatch modules is extremely simple. They have one interesting
member function, “dispatch”, which takes an event record as its parameter. It can
either consume the event or pass it along to the remembered dispatch module. It
returns a Boolean which signifies whether the event was handled.
As you can see, this makes it easy to install special handlers for certain event
combinations. The arbitrator can also provide you with useful information. Remember,
though, that you’re sharing the dispatcher with everyone else, so try to be at least a
bit polite in how you butcher the event stream.
In this spirit of politeness, we added a way for you to “monitor” the event stream
without necessarily interfering with it or being interfered with yourself. By
registering a dispatch module as a monitor for a given class of event, you guarantee
that OpenDoc will always notify you of any event of that type. However, you cannot
decide to stop further processing of the event as a monitor.
In general, we suggest you install monitors and not patch the dispatcher.
However, we recognize that there are some cases where nothing but a patch will do.
This is really up to you as developers to decide, but remember that you are doing
something every bit as dangerous as patching the OS when you patch the dispatcher.
Document extensions
We’ve talked about a number of methods for extending OpenDoc, but we haven’t
talked about how to install these mechanisms. To extend the arbitrator or dispatcher,
you’ll probably want to install code before any part editors are instantiated. OpenDoc
provides a mechanism for this called “document extensions.”
Document extensions are much like the INIT 31 mechanism built into Mac system
software. When a OpenDoc shell starts up, it gives each of these extension objects a
chance to run before it loads any part editors into the environment. They are passed a
pointer to the session object, and can use this opportunity to install special dispatch
handlers, arbitrator focus modules, or create objects and populate name spaces with
them.
This capability will allow you to install new component architectures into the
OpenDoc run-time environment. You can easily add new services or patch OpenDoc in
clean and easy to maintain ways.
Parting thoughts
So this ends our short tour of extension opportunities in OpenDoc. You’ll find that
this list is far from exhaustive. For instance, you can add a completely new graphics
model to OpenDoc, one complementary to existing models like QuickDraw Classic and
GX, including new notions of drawing canvas, transformations, and shapes. There are
opportunities to provide frameworks to help other developers create OpenDoc part
editors. The possibilities are just about endless.
Nonetheless, we think you will find the possibilities presented here are enough to
keep you busy for a while. We welcome the opportunity to work with you to create new
extensions to the environment. As mentioned above, there are already groups working
to provide extensions for spell checking, extended layout, and human interface sharing.
A good way to get in touch with them is to get in touch with CILabs, the vendor neutral
forum for component software systems. You can reach them on the Internet at
cil@cil.org. If you want to talk to some at Apple about an idea or give us a suggestion,
there’s a talk group on AppleLink, or you can send us mail at
OPENDOC@AppleLink. apple.com.