September 95 - Implementing Shared Internet Preferences With Internet Config
Quinn "the Eskimo !"
Having
to enter the same Internet preferences, such as e-mail address and news server,
into multiple applications is bothersome not just for users, but also for
developers who must create the user interface associated with them. The
Internet Configuration System (IC) provides a simple user application for
setting preferences, and an API for getting the preferences from a database
that's shared by all applications. It's easy to add IC support to your
application and take advantage of the flexibility gained by IC's use of the
Component Manager -- a valuable technique in itself.
Preferences, like nuclear weapons, proliferate. At times it seems that the
major developers are engaged in a "preferences race," where each one tries to
gain the upper hand by adding a dozen new preferences in each new release. Like
the arms race, the preferences race is obviously counterproductive, even
dangerous, and yet no one knows how to stop it.
Some of the worst offenders are Internet-related applications. How many times
have you had to enter your e-mail address into a configuration window? And what
about your preferred type and creator for JPEG files? Doesn't this just seem
like a waste of your time? The Internet Configuration System, or Internet
Config for short, spares everyone this trouble. And it spares developers the
complexities of implementing these preferences in each application.
This article takes you inside Internet Config. Take a good look at the design:
IC implements its shared library as a component, and uses switch glue to
provide a default implementation if the component is absent. Using the
Component Manager to implement shared libraries is a helpful technique not just
for IC, but for other APIs as well. Note too that Internet Config is useful for
more than its name implies. For example, the extension-to-file-type mapping
database is useful for any program that deals with "foreign" file systems.
Indeed, IC is a perfectly valid mechanism for storing private preferences that
have nothing to do with the Internet.
Although IC is intended as an abstract API, all its source code is placed in
the public domain -- a condition of its development. This lets me illustrate
the text with snippets from the actual implementation and gives you full access
to the source code. Both the IC user's kit and the IC developer's kit, which
contain code and documentation, are included on this issue's CD. Note that
Internet Config was developed independently and is not supported by Apple.
The latest versions of the kits are always available from the ftp sites
ftp://ftp.share.com/pub/internet-configuration/ and
ftp://redback.cs.uwa.edu.au/Others/Quinn/Config/. In addition, the user kit is
available from UMich and Info-Mac mirrors around the world.*
As with any new piece of software intended to be widely adopted, Internet
Config needs developer support in order to be successful. I hope this article
raises the awareness of IC in the developer community and prompts some of you
to support it.
Before going inside Internet Config, it's important to know how the system
works as a whole. The best way to do this is to get a copy of the Internet
Config application and run it (there's a copy on this issue's CD), but if
you're too relaxed to do that right now, keep reading for a description of the
basics. We'll look at IC first from the user's perspective and then from the
programmer's point of view.
To the user, Internet Config is a proper Macintosh application. It supports the
standard menu commands New, Open, Save, Save As, and so on. The only difference
is that the files it operates on are preferences files. Figure 1 shows Internet
Config and its related files.
The first time the Internet Config application is run, it installs the Internet
Config Extension into the Extensions folder and creates a new, blank Internet
Preferences file in the Preferences folder. It then displays the main window,
shown in Figure 2, which allows the user to edit the preferences.
Each of the buttons in the main window displays another window containing a
group of related preferences. For example, the Personal button brings up the
window shown in Figure 3. The user enters preferences into each of these
windows and then quits and saves the preferences.
From this point on, the user never has to enter those preferences again. Any
IC-aware program the user runs simply accesses the preferred settings without
requiring them to be reentered. This makes the user very happy (we presume).
Users can even run IC-aware applications "out of the box" -- they don't have to
run Internet Config first. If the Internet Config Extension isn't installed,
IC-aware client applications access the Internet Preferences file directly
instead of through the extension (as shown by the black arrows in Figure 1).
The way this is done is described later in the section "The Inner Workings of
an API Routine."
To programmers, Internet Config consists of a set of interface files that
define the API, and a library to be statically linked to their programs. IC can
be used from all of the common Macintosh development environments: MPW, THINK,
and Metrowerks; Pascal and C; and 680x0 and PowerPC. The examples in this
article, like IC itself, were written in THINK Pascal.
What's
in an IC preference. Before getting to the details of the API, you need to know
more about IC preferences. In IC, a preference is an item of information that's
useful to the client application program. Each preference has three components:
its key, its data, and its attributes.
- The key is a Str255 that identifies the preference. You can use the key to
fetch the data and attributes.
- The data is an untyped sequence of bytes that represents the value of
the preference. The data's structure is determined by the client program. The
structures of the common preferences are defined in the IC programming
documentation.
- The attributes represent information about the preference that's
supplementary to the preference data, such as whether the preference is
read/write or read-only.
In the e-mail address preference, for example, the
key is the string "Email". If you pass this string into IC, it returns the
preference's data and attributes. By convention, the data for the key "Email"
is interpreted as a Pascal string containing the user's preferred e-mail
address.
IC's
core API routines. Internet Config has the following core API routines.
Although the API has a lot more depth, these four routines are all you need to
program with IC.
FUNCTION ICStart (VAR inst: ICInstance; creator: OSType): ICError;
FUNCTION ICStop (inst: ICInstance): ICError;
FUNCTION ICFindConfigFile (inst: ICInstance; count: Integer;
folders: ICDirSpecArrayPtr): ICError;
FUNCTION ICGetPref (inst: ICInstance; key: Str255; VAR attr: ICAttr; buf: Ptr; VAR size: LongInt): ICError;
The
ICStart routine is always called first. Here you pass in your application's
creator code so that future versions of IC can support application-dependent
preferences. ICStart returns a value of type ICInstance; this is an opaque type
that must be passed to every other API call. ICStop is called at the
termination of your application to dispose of the ICInstance you obtained with
ICStart.
ICFindConfigFile is called immediately after ICStart. IC uses this routine to
support applications with double-clickable user configuration files, a common
phenomenon among Internet applications. If you need to support these files, see
the IC programming documentation; otherwise, just pass in 0 for the count
parameter and nil for the folders parameter.
The ICGetPref routine takes a preference key and returns the preference's
attributes in attr and its data in the buffer pointed to by buf. The maximum
size of the buffer is passed in as size, which is adjusted to the actual number
of bytes of preference data.
The
simplest example. The program in Listing 1 demonstrates the simplest possible
use of IC technology. All it does is write the user's e-mail address to the
standard output. This program calls the four core API routines: it begins by
calling ICStart and terminates with an ICStop call; it calls ICFindConfigFile
with the default parameters and uses ICGetPref to fetch the value of a specific
preference -- in this case the user's e-mail address.
Listing
1. The simplest IC-aware program
PROGRAM ICEmailAddress;
{ The simplest IC-aware program. It simply outputs the user's }
{ preferred e-mail address. }
USES
ICTypes, ICAPI, ICKeys; { standard IC interfaces }
VAR
instance: ICInstance; { opaque reference to IC session }
str: Str255; { buffer to read e-mail address into }
str_size: LongInt; { size of above buffer }
junk: ICError; { place to throw away error results }
junk_attr: ICAttr; { place to throw away attributes }
BEGIN
{ Start IC. }
IF ICStart(instance, '????') = noErr THEN BEGIN
{ Specify a database, in this case the default one. }
IF ICFindConfigFile(instance, 0, NIL) = noErr THEN BEGIN
{ Read the real name preferences. }
str_size := sizeof(str);{ 256 bytes -- a similar construct }
{ wouldn't work in C }
IF ICGetPref(instance, kICEmail, junk_attr, @str, str_size)
= noErr THEN BEGIN
writeln(str);
END; { IF }
END; { IF }
{ Shut down IC. }
junk := ICStop(instance);
END; { IF }
END. { ICEmailAddress }
The IC API just described is really all you need to know to make your program
IC-aware; now we'll get into the guts of Internet Config to see how it achieves
its magic. We'll look first at its underlying design and then at how its
internal structures work together.
The design requirements for Internet Config evolved during early discussions of
what an Internet configuration system might look like (see "How Internet Config
Came to Be"). These requirements guided the development process and form the
basic structure of Internet Config -- an efficient, expandable system that's
easy to use and easy to support.
Designing Internet Config was a complicated business. The process began in
March 1994 with a discussion on the Usenet newsgroup comp.sys.mac.comm. Many
people thought simplifying Internet configuration was a good idea, but few
agreed how best to achieve the goal, or indeed what the goal was.
We set up a mailing list to swap ideas, and discussion continued apace for
weeks. One of the biggest issues was the disparity between the problems we
wanted to solve and the ones we could solve given our limited resources.
After a week or two of thrashing out the requirements, Peter N. Lewis, Marcus
Jager, and I proposed the first API. A few weeks later we shipped the first
implementation of the Internet Config Extension.
The problem IC solves is actually quite simple, so it didn't take long to
implement the design. As usual, however, it took some time to go from a working
implementation to a final product -- we shipped Internet Config 1.0 in December
1994. Though we've made minor additions and changes, the initial design
survives to this day.
Internet Config can accept sweeping changes while maintaining API
compatibility, and it allows for patches to support future extensions and bug
fixes. We couldn't achieve such expandability with a simple shared preferences
implementation, and the consequent loss of simplicity caused a lot of debate
during the development process.
The need for simplicity was implicit from the beginning. To add support for
Internet Config, application developers have to revise their code. Developers
tend to be lazy -- hey, I mean that as a compliment -- and generally prefer
simple systems to complicated ones. Developer support is critical for success,
so we kept the system simple. Still, it isn't so simple as to compromise the
need for expandability.
As we've already seen, IC has several other interesting design features. The
API supports applications with double-clickable user configuration files. The
Internet Config user application accesses all the Internet preferences through
the API, and is thereby isolated from the implementation details. IC-aware
applications work even if the Internet Config Extension isn't installed. We
even included support for System 6 (much as we resented it).
As you can see in Figure 4, the Internet Config application and IC-aware client
programs have very similar internal structures. In fact, except for a few
artifacts caused by implementing "safe saving," the Internet Config application
uses the standard API to modify the Internet Preferences file. The Internet
Config component, which the user sees as the Internet Config Extension, is
basically a shared library of routines implemented as a component (see "The IC
Component and Shared Libraries on the Macintosh").
The Internet Config component is essentially a shared library of routines. So
why implement it as a component? The answer lies in the confused state of
shared libraries on the Macintosh.
When we started writing IC we knew we'd need a shared library. The problem was
not that the system didn't have a shared library mechanism, but that it had too
many. At the time there were four Apple shared library solutions, each with its
unique drawbacks: the Component Manager wasn't a "real" shared library system;
the Apple Shared Library Manager (ASLM) had limited availability and lacked
PowerPC support and developer tools; the Code Fragment Manager (CFM) lacked
680x0 support; and the System Object Model (SOM) lacked any availability.
These days life is a little better. ASLM now works on the PowerPC platform, CFM
is being ported to the 680x0 platform, SOM is imminent, and Apple has issued a
clear statement of direction on shared libraries, centered on CFM.
But statements of direction don't solve problems -- they just clear up
confusion. The shared library problem persists. When I was writing this article
someone asked me for advice about which shared library mechanism to use. My
recommendation today is the same as at the start of the IC project: use the
Component Manager. It's still the only solution that has the developer tools,
has 680x0 and PowerPC support, and is already installed on most users'
machines.
The switch glue is a common interface that applications use to call IC. This
glue decides whether the Internet Config component is available and, if it is,
routes all calls through to it. If the component isn't present, the calls are
routed through to the link-in implementation, which then does the work.
This switching mechanism satisfies two design requirements. It allows the API
to be patched by replacing or overriding the Internet Config component. It also
allows IC-aware programs to work even if the component isn't installed; they
simply fall back to using the link-in implementation.
Now we'll look more closely at how the Start and GetPref routines are
implemented in each part of the Internet Config system. We'll trace these two
calls from the top level, where they're called by the client program, all the
way down to the link-in implementation, where the real action takes place.
This section is quite technical; if you're not interested in the implementation
details, you might want to just skim through it. Many of the details are
provided for illustrative purposes only. Take heed! If you write client
programs that rely on these details, they will break in future revisions of IC.
The public interface to IC is defined in the IC programming documentation.
We'll start with the switch glue and proceed through the standard call path. On
the way we'll examine the component glue, wrapper, and "smarts," and finally,
the link-in implementation. The path is convoluted but rewards you with both
data and code abstraction.
Start and GetPref appear in each part of the system, and each appearance has a
specific purpose, as we'll see in a moment. To keep things straight, various
instances of the same routine are prefixed to denote which part of the system
they're in. The prefixes are listed in Table 1, which shows the various
specifications for the GetPref routine as an example. (Note that these
specifications vary only in the name's prefix and the type of the first
parameter. The "R" in the ICR prefix indicates that these routines actually use
the Resource Manager to modify the preferences; all the other routines are
glue.)
The switch glue relies on ICRRecord, the central data structure of IC, shown in
Listing 2. The first field of ICRRecord, instance, is a ComponentInstance,
which normally holds the connection to the Internet Config component. If the
component is installed, the instance field holds the connection to it; the rest
of the fields are ignored because the component has a separate ICRRecord in its
global variables. If the component isn't installed, the instance field is nil,
and the link-in implementation uses the rest of the fields to hold the
necessary state (as we'll see later).
Listing 2. ICRRecord
TYPE
ICRRecord = RECORD
{ This entire record is completely private to the }
{ implementation!!! Your code will break if you depend }
{ on the details here. You have been warned. }
instance: ComponentInstance;
{ nil if no component available; if not nil, }
{ then rest of record is junk }
... { other fields to be discussed later }
END;
ICRRecordPtr = ^ICRRecord;
The
switch glue for the application's Start routine, ICStart, is shown in Listing
3. The first thing ICStart does is attempt to allocate an ICRRecord; if it
succeeds, it then tries to open a connection to the component with the
component glue routine ICCStart. ICCStart either succeeds, setting the internal
instance field to the connection to the component, or fails and returns an
error. If ICCStart returns an error, ICStart falls back to using the link-in
implementation by calling ICRStart. If ICRStart fails, Internet Config fails to
start up; ICStart sets inst to nil and returns an error.
Listing 3. The switch glue for Start
FUNCTION ICStart (VAR inst: ICInstance; creator: OSType): ICError;
VAR
err: ICError;
BEGIN
inst := NewPtr(sizeof(ICRRecord));
err := MemError;
IF err = noErr THEN BEGIN
err := ICCStart(ICRRecordPtr(inst)^.instance, creator);
IF err <> noErr THEN BEGIN
err := ICRStart(ICRRecordPtr(inst)^, creator);
END; { IF }
IF err <> noErr THEN BEGIN
DisposePtr(inst);
inst := NIL;
END; { IF }
END; { IF }
ICStart := err;
END; { ICStart }
The
switch glue for GetPref, and all the other API routines for that matter, is
very simple. All it does is consult the internal instance field to determine
whether ICStart successfully connected to the component. If so, it calls
through to the component glue routine ICCGetPref; otherwise, it calls through
to the link-in implementation routine ICRGetPref. This is shown in Listing 4.
Listing 4. The switch glue for GetPref
FUNCTION ICGetPref (inst: ICInstance; key: Str255; VAR attr: ICAttr;
buf: Ptr; VAR size: LongInt): ICError;
BEGIN
IF ICRRecordPtr(inst)^.instance <> NIL THEN BEGIN
ICGetPref := ICCGetPref(ICRRecordPtr(inst)^.instance,
key, attr, buf, size);
END
ELSE BEGIN
ICGetPref := ICRGetPref(ICRRecordPtr(inst)^, key, attr, buf,
size);
END; { IF }
END; { ICGetPref }
The
switch glue implementations of both Start and GetPref do a lot of casting
between ICInstance and ICRRecordPtr, because the ICRRecordPtr type describes
details of the implementation that shouldn't "leak out" to the client's view of
IC. The client programs know only of ICInstance, which is an opaque type. The
explicit casts could have been avoided with some preprocessor tricks, but we
decided to include them longhand for clarity.
The component glue calls the Internet Config component. In the component glue
for the Start routine, shown in Listing 5, Internet Config attempts to connect
to the IC component by calling the Component Manager routine
OpenDefaultComponent.
Listing 5. The component glue for Start
FUNCTION ICCStartComponent (inst: ComponentInstance; creator: OSType):
ICError;
INLINE { standard Component Manager glue }
$2F3C, $04, $0, { move.l #$0004_0000,-(sp) }
$7000, { moveq.l #0,d0 }
$A82A; { _ComponentDispatch }
FUNCTION ICCStart (VAR inst: ComponentInstance; creator: OSType):
ICError;
VAR
err, junk: ICError;
response: LongInt;
BEGIN
inst := NIL;
IF Gestalt(gestaltComponentMgr, response) = noErr THEN BEGIN
inst := OpenDefaultComponent(internetConfigurationComponentType,
internetConfigurationComponentSubType);
END; { IF }
IF inst = NIL THEN BEGIN
err := badComponentInstance;
END
ELSE BEGIN
err := ICCStartComponent(inst, creator);
IF err <> noErr THEN BEGIN
junk := CloseComponent(inst);
inst := NIL;
END; { IF }
END; { IF }
ICCStart := err;
END; { ICCStart }
If
the Internet Config component isn't installed or can't be opened for any other
reason, the routine sets inst to nil and fails with a badComponentInstance
error. Remember that the calling code, ICStart, will notice this error code and
fall back to the link-in implementation, as shown in Listing 4.
If the routine successfully opens a connection to the Internet Config
component, it calls the ICCStartComponent routine, which is standard Component
Manager glue that calls the component's initialization routine.
The component glue version of GetPref is a lot simpler. It's just a standard
piece of Component Manager glue, as shown in Listing 6. The inline instructions
of the component glue for GetPref translate into the piece of assembly code
shown in Listing 7.
Listing 6. The component glue for GetPref
FUNCTION ICCGetPref (inst: ComponentInstance; key: Str255;
VAR attr: ICAttr; buf: Ptr;
VAR size: LongInt): ICError;
INLINE { standard Component Manager glue }
$2F3C, $10, $6, { move.l #$0010_0006,-(sp) }
$7000, { moveq.l #0,d0 }
$A82A; { _ComponentDispatch }
Listing
7. Disassembling the component glue
move.l #$0010_0006,-(sp); push the routine selector (6) and the
; number of bytes of parameters (16)
moveq.l #0,d0 ; _ComponentDispatch routine selector to
; call a component function
_ComponentDispatch ; call the component through the Component
; Manager
You
can read more about the Component Manager and its dispatch mechanism in
Inside
Macintosh: More Macintosh Toolbox.
Calling components from PowerPC code is not described in this article or in
Inside Macintosh: More Macintosh Toolbox. You can find out how to do this by
reading the Macintosh Technical Note "Component Manager Version 3.0" (QT
5).*
Now let's look inside the Internet Config component at the component wrapper
(Listing 8). The component wrapper's basic function is to dispatch all of the
IC component's routines based on the selector in params.what; it uses a big
CASE statement to determine the routine's address and then calls the routine
with the Component Manager function CallComponentFunctionWithStorage. The
Component Manager is smart enough to sort out the parameters at this stage.
Listing 8. Sections of IC's component wrapper
FUNCTION Main (VAR params: ComponentParameters; storage: Handle):
ComponentResult;
{ Inside Macintosh has params as a value parameter when it should be }
{ a VAR parameter. Don't make this mistake. }
VAR
proc: ProcPtr;
s: SignedByte;
BEGIN
proc := NIL;
CASE params.what OF
{ Dispatch the routines required by the Component Manager. }
... { routines omitted for brevity }
{ Dispatch the routines that make up the IC API. }
kICCStart:
proc := @ICCIStart;
kICCGetPref:
proc := @ICCIGetPref;
... { remaining IC API routines omitted for brevity }
OTHERWISE
Main := badComponentSelector;
END; { case }
IF proc <> NIL THEN BEGIN
IF storage <> NIL THEN BEGIN
s := HGetState(storage);
HLock(storage);
END; { IF }
Main := CallComponentFunctionWithStorage(storage, params, proc);
IF (storage <> NIL) AND
(params.what <> kComponentCloseSelect) THEN BEGIN
HSetState(storage, s);
END; { IF }
END; { IF }
END; { Main }
Most
of the API routines are immediately dispatched by the component wrapper to an
internal routine that simply calls the link-in implementation to do the work.
For example, the ICCIGetPref routine, shown in Listing 9, calls through to
ICRGetPref, changing only the first parameter.
Listing 9. The component wrapper for GetPref
FUNCTION ICCIGetPref (globals: globalsHandle; key: Str255; VAR attr:
ICAttr; buf: Ptr; VAR size: LongInt): ICError;
BEGIN
ICCIGetPref := ICRGetPref(globals^^.inst, key, attr, buf, size);
END; { ICCIGetPref }
So
you can see that there are two ways to call ICRGetPref, either from the
component's internal routine ICCIGetPref or from the switch glue's ICGetPref.
This is consistent with the design outlined in Figure 4. Of course, these
routines call two different copies of the code, one linked into the program and
one linked into the component.
The component "smarts" are wedged between the component wrapper and the link-in
implementation. Most component wrapper routines don't have smarts; they call
straight through to the link-in implementation. Adding smarts to a routine
allows it to work better than its link-in cousin without the need to maintain
two versions of the routine.
A good example of a smart routine is the component wrapper version of the Start
routine, ICCIStart (Listing 10). This fixes a potential localization problem
associated with the link-in implementation with a clever sleight of hand.
ICCIStart is basically the same as ICCIGetPref in that it immediately calls
through to its link-in implementation equivalent. But then it does something
tricky: the component calls itself to get the default filename for the Internet
Preferences file. For the gory details of why this is "smart," see "Smart
Components for Smart People."
Listing 10. A smart component wrapper
FUNCTION ICCIStart (globals: globalsHandle; creator: OSType): ICError;
{ Handle the start request, which is basically a replacement for the }
{ open because we need another parameter, the calling application's }
{ creator code. }
VAR
err: OSErr;
BEGIN
err := ICRStart(globals^^.inst, creator);
IF err = noErr THEN BEGIN
err := ICCDefaultFileName(globals^^.current_target,
globals^^.inst.default_filename);
END; { IF }
ICCIStart := err;
END; { ICCIStart }
Because Internet Config needs to know the default filename of the Internet
Preferences file when it creates a new preferences file, and because all
filenames should be stored in resources so that they can be localized, the
default filename should be stored in a resource. This approach is fine for the
component, which can get at its resource file with OpenComponentResFile, but
doesn't work for the link-in implementation since it can be linked in to a
variety of applications.
We considered working around this by requiring all applications to add a
resource specifying the name, but this would force all of our developers to add
resources to their applications, and the resource ID might clash with their
existing resources. The biggest disadvantage, however, is that IC clients are
not necessarily applications and may not even have resource files associated
with them.
So we solved this problem by making the component version of IC smarter than
the link-in version. The link-in version sets default_filename to "Internet
Preferences" and leaves it at that, while the component version calls itself to
get the correct filename from the resource file.
One thing to note is that when ICCIStart calls the component to get the default
filename, it doesn't do so directly, but instead uses the component glue to
call its current_target global variable. Targeting is cool Component Manager
technology that allows you to write override components (more on this later in
"Override Components").
With each new version of Internet Config, the component implementation gets
smarter than the link-in implementation. Component smarts are used in IC 1.0 to
improve ease of localization; in IC 1.1, they're also used to improve
targetability. In a future version of IC, component smarts may be used to
implement a preference cache.
It may be hard to imagine, but everything you've seen so far is glue. The code
that does the real work in IC is the link-in implementation. The link-in
implementation sees a different view of the ICRRecord, one that contains enough
fields to store all the data that the implementation requires. This extended
view of the ICRRecord is shown in Listing 11.
Listing 11. The full ICRRecord in the link-in implementation
TYPE
ICRRecord = RECORD
{ This entire record is completely private to the }
{ implementation!!! Your code will break if you depend }
{ on the details here. You have been warned. }
instance: ComponentInstance;
{ nil if no component available; if not nil, then rest }
{ of record is junk }
have_config_file: Boolean;
{ determines whether any file specification calls, that }
{ is, ICFindConfigFile or ICSpecifyConfigFile, have been }
{ made yet; determines whether the next field is valid }
config_file: FSSpec;
{ our chosen database file }
config_refnum: Integer;
{ a place to store the resource refnum }
perm: ICPerm;
{ the permissions the user opened the file with }
inside_begin: Boolean;
{ determines if config_refnum is valid }
default_filename: Str63;
{ the default IC filename }
END;
ICRRecordPtr = ^ICRRecord;
The
instance field is still there but the link-in implementation ignores it. It's
the subsequent fields that are of interest. Most of them are easy to understand
with the help of their comments.
The link-in implementation for the Start routine initializes the remaining
ICRRecord fields, as shown in Listing 12.
Listing 12. The link-in implementation for Start
FUNCTION ICRStart (VAR inst: ICRRecord; creator: OSType): ICError;
VAR
junk: ICError;
BEGIN
inst.have_config_file := false;
inst.config_file.vRefNum := 0;
inst.config_file.parID := 0;
inst.config_file.name := '';
inst.config_refnum := 0;
inst.perm := icNoPerm;
junk := ICRDefaultFileName(inst, inst.default_filename);
ICRStart := noErr;
END; { ICRStart }
FUNCTION ICRDefaultFileName (VAR inst: ICRRecord; VAR name: Str63):
ICError;
BEGIN
name := ICdefault_file_name;
ICRDefaultFileName := noErr;
END; { ICRDefaultFileName }
Finally,
there's the link-in implementation for GetPref, portions of which are shown in
Listing 13. The actual implementation is a bit long, so the listing leaves out
a lot of messing around with resources, bytes, pointers, attributes, and so on.
The basic operation of the routine is simple, however: it checks its
parameters, opens the preferences file (by calling ICRForceInside), gets the
preference, closes the preferences file, and returns.
Listing 13. The link-in implementation for GetPref
FUNCTION ICRGetPref (VAR inst: ICRRecord; key: Str255;
VAR attr: ICAttr; buf: Ptr; VAR size: LongInt)
:ICError;
VAR
err, err2: ICError;
max_size, true_size: LongInt;
old_refnum: Integer;
prefh: Handle;
force_info: Boolean;
BEGIN
max_size := size;
size := 0;
attr := ICattr_no_change;
prefh := NIL;
err := ICRForceInside(inst, icReadOnlyPerm, force_info);
IF (err = noErr) AND (inst.config_refnum = 0) THEN BEGIN
err := icPrefNotFoundErr;
END; { IF }
IF (err = noErr) AND ((key = '') OR
((max_size < 0) AND (buf <> nil))) THEN BEGIN
err := paramErr;
END; { IF }
IF err = noErr THEN BEGIN
old_refnum := CurResFile;
UseResFile(inst.config_refnum);
err := ResError;
IF err = noErr THEN BEGIN
... { lots of resource hacking here }
UseResFile(old_refnum);
END; { IF }
END; { IF }
IF prefh <> NIL THEN BEGIN
ReleaseResource(prefh);
END; { IF }
err2 := ICRReleaseInside(inst, force_info);
IF err = noErr THEN BEGIN
err := err2;
END; { IF }
ICRGetPref := err;
END; { ICRGetPref }
The future . . . where Macintosh applications glide along the information
superhighway, seamlessly perceiving the user's every preference. You'd better
hope your applications are IC aware!
Internet Config is a very flexible system that can expand in several
dimensions. Indeed, some are already being explored -- in particular, the use
of components to maintain and extend the system. And we're looking forward to
seeing IC extended in ways we never anticipated.
One of the coolest features of the Component Manager is targeting -- one
component can capture another and override it. This effectively prevents
external programs from using the captured component, while still allowing it to
be called by the override component. Very much like inheritance in
object-oriented design, this technology lets you write a very simple component
that captures the Internet Config component so that you can patch just one
routine. For example, the Internet Config RandomSignature extension overrides
the ICGetPref routine. If an IC client requests the signature preference, the
extension randomly chooses one from a collection of signatures.
The possibilities for override components are endless. Let's say your
organization wants to preconfigure all news clients to access a central news
server. You can do this by writing a simple override component that watches for
programs getting the NNTPHost preference and returns a fixed read-only
preference value. This way, all IC-aware news readers use the correct host but
can't change it. As we say in the system software business, it's a wonderful
third-party developer opportunity.
Because all client programs call Internet Config through a well-defined API,
it's possible to write a replacement for IC and gain complete control of the
system. Imagine that you're tired of having the same preferences in all your
IC-aware applications. You can change them by writing a replacement that
conforms to the existing API. First, replace the Internet Config component with
a smarter one that's capable of storing a set of preferences for each
application and returning the right preferences to the right application. Then
replace the Internet Config application with a much more sophisticated
application that can manage multiple sets of preferences, and your job is done.
All IC-aware programs will automatically benefit without recompilation.
Or suppose you want to store your user preferences on a central server and
access them through some network protocol. Again, IC lets you do it. You could
replace the Internet Config component with a network-aware one, and establish
the user's identity in some way, perhaps by requiring the user to log on before
using any IC-aware programs. You could then choose to use either a Macintosh
application to administer the server or tools from the server's native
environment.
No program is ever finished, nor is any program ever 100% bug free. Internet
Config is getting better all the time, and you can update to the newest,
improved version with a minimum of fuss. When the application detects that its
version of the Internet Config Extension is out of date, it simply installs the
new one. Because all IC-aware programs are dynamically linked to the component
contained within this extension, they automatically receive the update without
having to be recompiled.
By the time you read this article, IC 1.1 should be released and busily
updating old versions of the Internet Config Extension around the globe. IC 1.1
offers many improvements and bug fixes, including an extended API and a shell
for writing override components easily. Share and enjoy!
If you want to find out more about Internet Config itself, the following
documents may be of interest:
- "Using the Internet Configuration System" by Quinn, MacTech Magazine,
April 1995.
- Internet Configuration System: User Documentation and Internet
Configuration System: Programming Documentation by Quinn, in the IC User's Kit
and IC Developer's Kit, respectively (1994). These kits are provided on this
issue's CD.
- "Internet Config FAQ" by Quinn (1994-1995). Available from the ftp site
ftp://redback.cs.uwa.edu.au/Others/Quinn/Config/IC_FAQ.txt.
Here's where
you can find out more about components, the technology Internet Config is based
on:
- Inside Macintosh: More Macintosh Toolbox (Addison-Wesley, 1993).
- Macintosh Technical Note "Component Manager Version 3.0" (QT 5).
- "Be Our Guest: Components and C++ Classes Compared" by David Van Brink,
develop Issue 12.
- "Inside QuickTime and Component-Based Managers" by Bill Guschwan,
develop Issue 13.
- "Somewhere in QuickTime: Derived Media Handlers" by John Wang, develop
Issue 14.
- "Managing Component Registration" by Gary Woodcock, develop Issue
15.
Finally, if you're interested in the mindset of Internet Config's
authors, you can do no better than to read the following:
- He Died With a Felafel in His Hand by John Birmingham (The Yellow Press,
1994).
- The UNIX-HATERS Handbook by Simson Garfinkel, Daniel Weise, and Steven
Strassmann (IDG Books, 1994).
- http://www.cm.cf.ac.uk/Movies/
QUINN "THE ESKIMO!" (quinn@cs.uwa.edu.au) has a first name but, when asked
about it, his usual response is "I could tell you but then I'd have to kill
you!" He programs for a living with the Department of Computer Science at the
University of Western Australia, but on weekends he gets together with Peter N.
Lewis and programs for fun. The Internet Configuration System is a product of
these misspent recreational hours. Quinn writes in Pascal using a Dvorak
keyboard on a Macintosh Duo that he carries around on his bicycle, and he's
still trying to figure out how to use this minority status to his economic
advantage.*
Thanks to our technical reviewers Peter Hoddie, Peter N. Lewis, Jim Reekes, and
Greg Robbins. Internet Config is a joint development by Peter N. Lewis and
Quinn, with design input from Marcus Jager. We'd like to thank all of those on
the Internet Config mailing list and the developers who are supporting the
system.*
The Internet Config mailing list is dedicated to discussing the technical
details of Internet Config. You can subscribe by sending mail to
listserv@list.peter.com.au with the body of the message containing "subscribe
config Your Real Name."*
