September 93 - MANAGING COMPONENT REGISTRATION
MANAGING COMPONENT REGISTRATION
GARY WOODCOCK
One of the many design problems a component developer may face is how to register
interdependent components in a predetermined fashion so that any given component
is registered before the components that depend on it. This article and the sample code
that accompanies it show you how to do just that.
The Component Manager is an effective mechanism for providing extended functionality to the
Macintosh platform. Although a single component can perform impressive tasks, often it's a
hierarchy of components, cooperating with one another, that provides the most powerful capabilities.
An example of such a hierarchy is found in QuickTime movie playback using the movie controller
component (see Figure 1). This component uses the services of many other components, all of which
cooperate together, to make interaction with QuickTime movies very simple yet very powerful.
There are distinct advantages to partitioning functionality in this manner. First, by creating
components that perform simpler processing, you increase the likelihood that you can leverage the
investment you've made in your code by using it in more than one place. Second, it's easier to debug
smaller components than a gigantic everything-and-the-kitchen-sink component. Finally, a
component that provides very elementary functionality is easier to override or update (via component
replacement or capture) than a large, complex component.
This situation -- a component depending on the presence of several lower-level components to
perform its function -- is very commonplace. In such cases, it's important to take steps to ensure that
supporting components are available when your component needs them. There are two obvious
choices for when to go looking for the components you depend on: when your component is being
registered (in its register routine), or when your component is first opened (in its open routine). Most
software-dependent components don't need to worry much about managing component registration.
Generally such a component should just auto-register, and then check for any required components
in the open routine; if the required components aren't available, your open routine can return an
error. The caller of your component can then handle the error in whatever way is most appropriate.
There is a case, however, where checking at registration time might be necessary; that's what this
article is about.
Figure 1 The Movie Controller Component Hierarchy
DO I REALLY NEED TO WORRY ABOUT THIS?
One potential problem occurs in situations where the Component Manager's registration list is used
to build some user interface element, such as a pop-up menu or a list. In this case, the general
assumption is that because a component's name is displayed in a user interface element, a user can
select it and it will do whatever it's supposed to do -- after all, if the component couldn't perform its
function, it wouldn't be displayed as an option for the user, right? Well, that depends.
Let's look at an example. The SuperOps company builds the WhizBang video digitizer card and
supplies two software components with it -- the WhizBang video digitizer component and the
WhizBang sequence-grabber panel component (which is used to control features specific to the
WhizBang hardware). The component files are named WhizBang Video Digitizer and WhizBang
Panel. In its register routine, the WhizBang video digitizer component checks for its hardware and
registers with the Component Manager only if the hardware is present (this is normal behavior for
components that encapsulate hardware functionality). The WhizBang sequence-grabber panel
component checks for the availability of the WhizBang video digitizer component when it receives
either an open message or a "panel can run" message -- it doesn't get a register message, and
therefore it always registers successfully with the Component Manager.
Now let's say I've got a Macintosh Quadra 950 with multiple sound and video digitizers installed (I
can dream, can't I?), one of which is the WhizBang card. I remove the WhizBang card from my
computer, but I leave the two WhizBang components installed. I then start up my Macintosh Quadra
and run my favorite movie capture application. I display the sequence-grabber video settings dialog
box, and I see a dimmed item in the panel pop-up menu -- "WhizBang panel." The dimmed name
indicates one of two things: another application has the WhizBang video digitizer open, so it's not
available, or the WhizBang video digitizer component isn't registered at all, so the panel can't run.
In this case, we already know that the WhizBang card isn't installed, so there's no way this panel canever
be enabled, given the current hardware configuration. Rather than confuse users by displaying
the panel name in the pop-up menu (even if it is dimmed), it would be nicer if it weren't displayed atall.
To do that, we need to ensure the following order of events at startup: the video digitizer
component must attempt to register first, and then the panel component must attempt to register
(this implies that the panel component must implement a register routine), checking for the presence
of the video digitizer component before it does so. Further, this sequence of events must not be
influenced by the alphabetic order of the component filenames. Guess what? We can realize this goal
by managing component registration.
This article and the sample code on this issue's CD demonstrate various ways of managing
component registration. We start with the easiest, most obvious approach and work our way up to a
more sophisticated solution, pointing out the pros and cons of each along the way. If you just want
the "answer" without any fanfare, skip ahead to the section "Mo' Better: Use a Loader Component to
Manage Registration."
I assume that you're familiar with the Component Manager and that you know something about how
components are written. For more information on these topics, seeInside Macintosh: More Macintosh
Toolbox and "Techniques for Writing and Debugging Components" indevelop Issue 12.
THE SYSTEM VERSION AND THE COMPONENT MANAGER
The Component Manager behaves slightly differently depending on the version of system software
it's running under and how the Component Manager was installed. It's important to know about
these subtleties in order to understand how to work with the Component Manager to install your
components properly.
In system software version 6.0.7, the Component Manager is installed as part of an INIT (usually the
QuickTime INIT). During the INIT installation, the Component Manager examines the contents of
the System Folder and its subfolders for files of type 'thng'; in each 'thng' file, it looks for resources
of type 'thng', which it then uses to register the corresponding components. The important point
here is that the Component Manager is not available until after the INIT has been installed.
Like system software version 6.0.7, versions 7.0 and 7.0.1 pick up the Component Manager via an
INIT, and so again the Component Manager isn't around until after the INIT has been installed.
The main difference in System 7 is that in addition to searching the System Folder and its subfolders
for component files, the Component Manager will also examine the contents of any subfolders that
are in the Extensions folder.
Examples of INITs in system software versions 7.0 and 7.0.1 that install the Component Manager are
QuickTime, AppleScript, and Macintosh Easy Open. Note that your component can't assume that
just because the Component Manager is installed, QuickTime is installed -- always use the Gestalt
selectors to determine what functionality is available.
The Component Manager is actually part of System 7.1 and, as a consequence, is available before the
INIT process is started.
METHODS FOR MANAGING COMPONENT REGISTRATION
Now that we have a good idea of when the Component Manager is installed and where it's searching
for components, let's see what we can do to make sure that our components get registered in the
order we want them to be registered.
We'll use some simple components to illustrate the various methods we might use to manage
component registration. In the sample code provided on the CD are three components -- Moe,
Larry, and Curly -- that together establish a functional component hierarchy (see Figure 2). The
hierarchy is such that Moe doesn't depend on any other components, Larry depends on Moe, and
Curly depends on both Larry and Moe. To enforce these dependencies, we use register routines in
Larry and Curly to make sure that the components they need are present before they actually allow
themselves to be registered with the Component Manager. To let us know when each of these
components is actually registered, Moe's register routine calls SysBeep once, Larry's calls SysBeep
twice, and Curly's calls SysBeep three times. By the way, these components really don't do anything
useful at all, but you probably figured that out already.
Figure 2 The Moe, Larry, and Curly Component Hierarchy
RISKY: LET THE COMPONENT MANAGER TAKE CARE OF IT
We can always simply let the Component Manager do whatever comes naturally -- in this case, auto-
registration. This method works only as long as you aren't picky about the order in which your
components are registered. (Obviously, if your component doesn't depend in any way on the
presence of other components, you're golden.) In our example scenario, though, we can't count on
the Component Manager recognizing our constraints and doing the right thing. The Component
Manager doesn't have enough information to know that our components have an ordering
dependency (kinda reminds you of INITs, doesn't it?).
Nonetheless, let's look at what happens. The following is what occurs on my Macintosh Quadra 700
running System 7.1 and QuickTime 1.6, but you shouldn't infer that this is how the Component
Manager will behave from now until eternity -- there is no documentation whatsoever that provides
this kind of detailed information on component registration behavior, so itcan change.
We start with each component in a separate file. We might expect that the Component Manager
would register component files in alphabetic order, and in fact this is exactly what happens. The first
component that the Component Manager tries to register is Curly. However, Curly needs both Moe
and Larry before it can be registered, and neither of them is present, so Curly bails. Larry comes
next, and because Larry needs Moe, and Moe isn't around yet, Larry bails. Moe is last, and Moe
doesn't depend on any components at all, so Moe is registered successfully. One out of three's not
too good, though.
We might further expect that if we put all of our components in a single file, the Component
Manager would walk the component resources from lowest resource ID to highest resource ID. If
that were true, all we'd have to do is give our components ascending resource IDs in the order in
which we want them to be registered (say, 200 for Moe, 300 for Larry, and 400 for Curly), and we'd
be done! Well, I know we all long for the day that the omniscient System will always figure out the
right thing to do regardless of how we've specified that it be done, but that day's not here yet -- or,
to quote KON, "It's just a computer."
The Component Manager calls Count1Resources to find out how many 'thng' resources are in a file.
It then iterates through these resources, using the Get1IndResources call. Unfortunately, there's no
guarantee that the Resource Manager will index resources in the same numeric order as their
corresponding resource IDs; that is, even if Moe's 'thng' resource ID is lowest (200), Moe's resource
index (as maintained by the Resource Manager) may or may not be 1.
If we actually go ahead and try this (you can try this yourself with the Moe, Larry, and Curly
component file on the CD, which I created by simply Rezzing the three components into a single
'thng' file), we find that we get exactly the same behavior we observed with the separate component
files -- first Curly fails, then Larry fails, and only Moe registers successfully. This approach just isn't
reliable enough for our purposes, and we need a better mousetrap.
BETTER: USE AN INIT TO MANAGE REGISTRATION
Here's an idea -- we can use an INIT to manage the registration order of our components! We'll
create a resource that describes the order in which to register our components, and then the INIT
can read this resource, registering the component resources in the specified order. The registration
order is simply defined as the position in the component list; that is, the first component in the list is
registered first, the second component in the list is registered second, and so on.
The component load order resource. We use a custom resource, called a component load order
resource, to indicate to our INIT the order in which the components in the INIT file should be
registered. The resource type is defined as 'thld' (for "thing load") and the resource is a 1-based list
of structures of type ComponentLoadSpec, as defined below:
#define kComponentLoadOrderResType 'thld'
typedef struct ComponentLoadSpec {
ResType componentResType;
short componentResID;
} ComponentLoadSpec, *ComponentLoadSpecPtr, **ComponentLoadSpecHdl;
typedef struct ComponentLoadList {
short count;
ComponentLoadSpec spec[1];
} ComponentLoadList, *ComponentLoadListPtr, **ComponentLoadListHdl;
The componentResType field contains the component resource type, in this case 'thng', and the
componentResID field contains the component resource ID.
The loader INIT. Our INIT -- called, surprisingly enough, LoaderINIT -- doesn't really do much
work. When the INIT is executed, it checks to see whether the Shift key or mouse button is held
down; if so, it quits. If not, it then checks for the presence of the Component Manager, and if the
Component Manager is installed, it tries to load our components with a call to the LoadComponents
routine.
main (void)
{
KeyMap keys;
// INIT setup for THINK C (these routines are defined in
// <SetupA4.h>)
RememberA0();
SetUpA4();
// If mouse or Shift key down, don't bother.
GetKeys (keys);
if (!Button() && !(1 & keys[1])) {
OSErr result = noErr;
// Is the Component Manager available?
if (HasComponentMgr()) {
// Load the components!
result = LoadComponents (kComponentLoadListResType,
kLoaderBaseResID);
}
}
// INIT cleanup for THINK C (this routine is defined in
// <SetupA4.h>)
RestoreA4();
}
The LoadComponents routine. LoadComponents does the job of reading the component load order
resource and loading each of the components it points to; this routine is shown below.
static OSErr
LoadComponents (ResType loadListResType, short loadListResID)
{
OSErr result = noErr;
ComponentLoadListHdl componentLoadList =
(ComponentLoadListHdl) Get1Resource (loadListResType,
loadListResID);
// Did we get the component load list?
if (componentLoadList != nil) {
ComponentLoadSpec componentLoadSpec;
ComponentResourceHandle componentResHdl;
Component componentID;
short numComponentsToLoad = (**componentLoadList).count;
short i;
for (i = 0; i < numComponentsToLoad; i++) {
// Get the component load spec.
componentLoadSpec = (**componentLoadList).spec[i];
// Get the component resource pointed to by this spec.
componentResHdl = (ComponentResourceHandle) Get1Resource
( componentLoadSpec.componentResType,
componentLoadSpec.componentResID);
// Did we get it?
if (componentResHdl != nil) {
// Register it.
componentID = RegisterComponentResource
(componentResHdl, kRegisterGlobally);
if (componentID == 0L) {
// RegisterComponentResource failed.
result = -1L; // Return anonymous error
}
}
else {
// Get1Resource failed.
result = ResError();
}
}
}
else {
// Couldn't get component loader resource.
result = ResError();
}
return (result);
}
Why it's too good to be true. LoaderINIT works fine if we're running System 7.1 or later (the
Component Manager is installedbefore the INIT 31 process begins) or we name LoaderINIT
something alphabetically greater than the name of the INIT that's installing the Component
Manager (assuming we know this somehow). If neither of these conditions is met, LoaderINIT will
execute before the Component Manager is installed, and none of our components will be registered.
Bummer.
We could do something sneaky like patch a trap that we've observed being called right before the
Finder comes up, and then execute our INIT code. In effect, this defers our normal INIT execution
until after all other INITs load (provided they aren't pulling the same sneaky trick). However, we'd
rather be more elegant and, dare I say, more compatible. We could also name our INIT
~LoaderINIT (or something similar) to guarantee that we run last in the INIT sequence (a
somewhat naive hope), but we'd rather not become participants in the latest chapter of the ongoing
saga of INIT Wars (Chapter XX: MacsBug Strikes Back). So what's a component developer to do?
MO' BETTER: USE A LOADER COMPONENT TO MANAGE REGISTRATION
Fortunately, we don't have to give up yet. We can avoid the shortcomings of the INIT approach by
using a component to load our components -- a component we'll call, oh, I don't know, something
original; how about . . . a loader component.
The loader component. The loader component is a very simple component. It implements only the
open, close, can do, version, and register selectors, and has no unique selectors of its own. It resides
in a file of type 'thng', so the Component Manager will auto-register it. Also, the
cmpWantsRegisterMessage flag is set in the componentFlags field of its component resource so the
Component Manager will send it a register message at auto-register time. Our other three
components (Moe, Larry, and Curly) are also included in the loader component file.
When the loader component is registered, it receives three messages from the Component Manager
-- open, register, and close. The register routine does all the work. It performs basically the same
checks that are performed in LoaderINIT and calls the same LoadComponents routine described
earlier to manually register Moe, Larry, and Curly. The loader component's register routine is shown
below.
pascal ComponentResult
_LoaderRegister (Handle storage)
{
KeyMap keys;
LoaderPrivateGlobalsHdl globals =
(LoaderPrivateGlobalsHdl) storage;
OSErr result = noErr;
#ifndef BUILD_LINKED
short savedResRefNum = CurResFile();
short compResRefNum = OpenComponentResFile ((**globals).self);
// Use the component's resource file (not the THINK project
// resource file) if we're running standalone.
UseResFile (compResRefNum);
#endif BUILD_LINKED
// If mouse or Shift key down, don't bother.
GetKeys (keys);
if (!Button() && !(1 & keys[1])) {
// Load the components!
result = LoadComponents (kComponentLoadListResType,
kLoaderBaseResID);
}
#ifndef BUILD_LINKED
// Restore the resource file (if running standalone).
CloseComponentResFile (compResRefNum);
UseResFile (savedResRefNum);
#endif BUILD_LINKED
return ((result == noErr) ? 0L : 1L);
}
The 'gnht' resource. Everything's pretty cool up to this point, except for one minor detail -- we can't
keep the component resources for Moe, Larry, and Curly as 'thng' resources in our loadercomponent file.
Why? Well, if theyare kept as 'thng' resources, they'll be auto-registered along with
the loader component, and our carefully constructed mechanism for managing registration goes right
out the window! Worse, we end up trying to load our components twice -- once via the Component
Manager's auto-registration mechanism, and once by our own loader component!
So, we need to mildly fake out the Component Manager. We do this by keeping Moe, Larry, and
Curly's component resources around as 'gnht' resources instead of 'thng' resources. The 'gnht'
resource is identical to the 'thng' resource, but the Component Manager doesn't know to look for it,
so Moe, Larry and Curly aren't auto-registered. The loader component (whose component resource
is of type 'thng')does get auto-registered, and it knows where to find the component resources for
Moe, Larry, and Curly because the component load order resource provides this information. Recall
that in LoaderINIT, the component load specs in the component load order resource all point to
resources of type 'thng'. We simply change these fields to point to resources of type 'gnht', and we're
set!
PRACTICE SAFE REGISTRATION
In this article, we've looked at several approaches to installing components in a predetermined order.
While you're encouraged to adapt these methods freely to fit your particular problem, keep in mind
that your solution should strive to be as compatible as possible with other system extensions -- your
users will thank you for sparing them the frustration of renaming and removing extensions just to get
your software running!
GARY WOODCOCK, an optically challenged, melanin-impoverished male who lives with his feline-American companion
Phaser, hopes someday soon to be able to spend a few motivationally deficient days enjoying a reduced state of
awareness without becoming terminally inconvenienced. He feels P. J. O'Rourke's observation that "Giving money and
power to government is like giving whiskey and car keys to teenage boys" carries far too much truth. *
For more information on overriding components, see Bill Guschwan's "Somewhere in QuickTime" column in this issue. *
For more information on QuickTime components, see Inside Macintosh: QuickTime and Inside Macintosh: QuickTime
Components (included in the QuickTime Developer's Kit v. 1.5) and "Inside QuickTime and Component-Based Managers"
in develop Issue 13.*
KON's pithy quote was immortalized in the Puzzle Page in develop Issue 9.*
THANKS TO OUR TECHNICAL REVIEWERS Bill Guschwan, Peter Hoddie, Casey King *