TweetFollow Us on Twitter

A Simple Plug-In Example

Volume Number: 14 (1998)
Issue Number: 6
Column Tag: Plugging In

A Simple Plug-In Example

by Joe Zobkiw

How to add plug-in support to your application

If you've ever used Adobe Photoshop, HyperCard, or even the latest version of Metrowerks' CodeWarrior you've made use of plug-in technology. Plug-ins are simply executable code (and resources) that reside in a file other than that of the application itself. Applications can load plug-ins dynamically at run-time and benefit from the functionality they provide.

For example, Photoshop is a high-end graphics application that allows you to load an image and manipulate it in any of a number of interesting ways. You can add a drop-shadow, change the color balance, produce lighting effects, and more by using special. Much of this functionality is provided by plug-ins, known in this case as Photoshop Filters. When you launch the Photoshop application, it scans a folder in the same folder as itself named "Plug-ins" for files of a specific type. As it finds these files it makes their functionality available from within the program by displaying them in the Filters menu. Selecting items from this menu invoke the appropriate plug-in and extend the capabilities of Photoshop.

You might ask yourself, why would anyone want to write dozens of separate plug-ins if they are just going to be used from within an application anyway? The reasons are simple. By extracting certain functionality into plug-ins, you can easily update or add new features to an application without changing the application itself. For instance, to add a new filter to Photoshop, you simply drag it into the Plug-ins folder and relaunch the application. This not only allows Adobe to easily manage their software, but it also allows hundreds of third-party developers to enhance and customize the Photoshop application by writing Photoshop Filters to the Adobe-published specification, all without having access to the source code of Photoshop itself.

Given these examples, you can see that using plug-ins can not only help ease the burden of development, but it can also help your salespeople by making your application more accessible, more customizable, and more appealing to your customers. Let's look at an example of how you might implement plug-in support in your application.

Basic Plug-in Support

The following example shows you the most basic steps required to implement plug-in support in an application. We have implemented a simple PowerPC application that loads a special plug-in, in our case compiled as a PowerPC shared library, and executes code within it. Once you understand how this application and plug-in work together, you can easily extend the sample and devise your own plug-in architecture for your application.

Figure 1.

For this project we are using CodeWarrior Professional Release 2. Our shared library is written in C and our application is C++. We started out by creating a single project file that contains both targets for this project, the application and the shared library. The project is set up so whenever we build the application, the shared library will be brought up to date if need be. You do not need to have both of your targets in the same project file. However, CodeWarrior Professional Release 2 allows us to do this and it makes it easier for this particular project.

Figure 2.

Before you design an application to call a plug-in you must decide on the calling conventions. In this simple case we have decided to implement a single function in our shared library that will be called from the application. We are calling our function DisplayDialogAndBeep. It is called with one parameter, inBeepTimes, which represents the number of times to make the computer beep while displaying a dialog. It is defined as follows:

Listing 1.

OSErr DisplayDialogAndBeep(long inBeepTimes);

When the project builds both the application and the shared library, it produces two files. One is an application program named "Application" and the other is a shared library named "Shared Library." When the application is launched, it prompts the user to enter a value for inBeepTimes. Upon entering this value, the application attempts to open the shared library by name, find the exported DisplayDialogAndBeep function by name, and call the function. If these steps are completed successfully, the computer will beep and you will see a dialog box as follows:

Figure 3.

Let's look at the shared library to see how it is created, then we can see how the application is used to call this code. First, it is important to understand that a shared library is simply a file that contains a code fragment (PowerPC code) in the data fork. If you are not familiar with code fragments and the Code Fragment Manager, you will want to read about them in Inside Macintosh. You can do so at http://devworld.apple.com/.

The compiler handles most of the details for you but you want to make sure that your project is set up to export (at a minimum) the functions that you expect to be able to call from your application. If the functions are not exported, your application will not be able to access them. Other than that, a shared library is not much more than a bunch of functions without a required main() entry point as you are used to seeing in applications.

Another interested and useful feature of shared libraries (and any code fragment for that matter) is the ability to include a start function, similar to main(), the main entry point of the code fragment. You can also include initialization and termination functions. These are called when the code fragment is first connected to and when the connection is closed, respectively. Our plug-in makes use of the initialization and termination functions to insure that our shared library resource file is available so we can access our dialog box.

You do not need to use the initialization and termination routines in this way. In fact, you can use them in any way you choose, or not at all. I simply found it convenient to locate the plug-in file given the CFragInitBlockPtr that is passed into the initialization routine in this case.

The required code of the shared library, sans comments, is as follows:

Listing 2

#include <CodeFragments.h>
#include <MixedMode.h>
#include <ConditionalMacros.h>
#include <Sound.h>
#include "Shared Library.h"

enum {
   uppDisplayDialogAndBeepProcInfo = kCStackBased
       | RESULT_SIZE(SIZE_CODE(sizeof(OSErr)))
       | STACK_ROUTINE_PARAMETER(1, SIZE_CODE(sizeof(long)))
};

short   gFileRefNum;

__initialize

OSErr __initialize(CFragInitBlockPtr ibp)
{
   OSErr   err = noErr;

   gFileRefNum = -1;
   
   if (ibp->fragLocator.where == kDataForkCFragLocator) {
      gFileRefNum = 
         FSpOpenResFile(ibp->fragLocator.u.onDisk.fileSpec, 
                              fsRdPerm);
      if (gFileRefNum == -1)
         err = ResError();
   }
   
   return err;
}

__terminate

void __terminate(void)
{
   if (gFileRefNum != -1)
      CloseResFile(gFileRefNum);
}

DisplayDialogAndBeep

OSErr DisplayDialogAndBeep(long inBeepTimes)
{
   DialogPtr       d;
   Str32         sBeepTimes;
   long         i;
   unsigned long   someticks;
   short         saveResFile;
   saveResFile = CurResFile();
   UseResFile(gFileRefNum);
   NumToString(inBeepTimes, sBeepTimes);
   ParamText(sBeepTimes, "\p", "\p", "\p");
   d = GetNewDialog(256, nil, (WindowPtr)-1);
   if (d) {
      ShowWindow(d);
      DrawDialog(d);
   }
   
   for (i = 0; i < inBeepTimes; ++i) {
      FlashMenuBar(0);
      SysBeep(0);
      Delay(15, &someticks);
      FlashMenuBar(0);
      Delay(15, &someticks);
   }
   
   if (d)
      DisposeDialog(d);
      UseResFile(saveResFile);
      return noErr;
}

That's all there is to it, believe it or not.

The shared library isn't too useful on its own. It needs the application to call it in order for it to do anything. Basically the application first needs to locate the shared library by name. We call GetSharedLibrary() in order to do this. By passing in the name of the shared library we are looking for, the Code Fragment Manager will automatically look in the same folder as our application first, then proceed to look in the Extensions folder and the System folder until it either finds a shared library with the correct name or it fails. If found, GetSharedLibrary() will automatically open a connection (and execute its initialization routine mentioned earlier) to the shared library.

Once we find the shared library and connect to it we can then query it for an exported symbol named DisplayDialogAndBeep. In this case the symbol is an exported function but it might also be exported data. If found, we can continue by creating a routine descriptor for the function by calling NewRoutineDescriptor(), calling it by using CallUniversalProc(), and ultimately disposing of the routine descriptor using DisposeRoutineDescriptor().

Another way to call your shared library (or any code fragment for that matter) is by means of a main entry point, sometimes simply called main(). Under the 680x0 architecture that was the only way to communicate with a code resource. The code resource had a main entry point that you would call using a selector-based mechanism. That is, the main entry point would expect a selector (a unique identifier) to distinguish the purpose of the call, and then extra data, possibly in another parameter, to act on during that call. This technique allows the caller to not need to know specific exported function names, it can simply call through the main entry point, passing the correct selector and data. A sample main entry point might look like this:

OSErr main(long inSelector, void* ioDataPtr);

Another might use a single parameter block for all of the data, including the selector itself.

OSErr main(MyParameterBlock* ioParamPtr);

During the call to CallUniversalProc() is when the DisplayDialogAndBeep() function in the shared library will be called. It will be passed the parameters we specified, perform its duty, and return a result code. If you specify the universal procedure pointer incorrectly you will undoubtedly crash your computer during this call.

You can find more information about universal procedure pointers and routine descriptors in the Inside Macintosh chapter on the Mixed Mode Manager at http://devworld.apple.com/.

Once the call returns, the connection to the shared library is closed by calling CloseConnection(). At this time is when the termination routine in the shared library is executed.

The required code of the application, sans comments, is as follows:

Listing 3

ExecuteSharedLib
OSErr ExecuteSharedLib(long inBeepTimes)
{
   OSErr                     err = noErr, err2 = noErr;
   CFragConnectionID   connID = 0;
   Ptr                        mainAddr = nil;
   Str255                   errName;
   
   err = GetSharedLibrary("\pShared Library", 
                                    kPowerPCCFragArch, 
                                    kPrivateCFragCopy, 
                                    &connID, &mainAddr, errName);
   if (err == noErr) {
      Ptr         symAddr = nil;
      CFragSymbolClass   symClass;
      
      err = FindSymbol(connID, "\pDisplayDialogAndBeep", 
                              &symAddr, &symClass);
      if (err == noErr) {
         
         UniversalProcPtr upp =          
            NewRoutineDescriptor((ProcPtr)symAddr, 
            uppDisplayDialogAndBeepProcInfo, GetCurrentISA());
         if (upp) {
            err = CallUniversalProc(upp,    
                                       uppDisplayDialogAndBeepProcInfo, 
                                       inBeepTimes);
            DisposeRoutineDescriptor(upp);
         } else err = memFullErr;
      }
      
      err2 = CloseConnection(&connID);
      if (err == noErr) err = err2;
   }
   
   return err;
}

That's all there is to that too, believe it or not.

Enhanced Plug-in Support

Plug-ins Folder

The above example assumes your application knows the name of the plug-in before it is launched. However, in order to implement a Photoshop-style approach to plug-ins you need to be able to search for the plug-ins at run-time. This can be achieved using a very useful source code library called MoreFiles by Jim Luther.

MoreFiles allows you to (amongst numerous other features) easily scan a folder for files and call a specific function as files are found. Using this technique you can quickly locate all plug-ins that your application can use and add their names to a menu for your user to invoke as needed. MoreFiles can be downloaded from the Internet at ftp://dev.apple.com/devworld/Sample_Code/Files/ or http://members.aol.com/jumplong/.

Figure 4.

Callbacks

Something else to try is exporting functions from your application and having your plug-in call them, just as the application calls the functions in the plug-in. These are called callback functions because the plug-in is "calling back" into the application. These types of functions can be very useful in providing information to the plug-in as it is needed. For example, the plug-in can query the application to see if it has enough memory available in an internal buffer to handle a specific task before setting off on the task.

Import Libraries

You can also compile your shared library in the form of an import library. By doing this you can simply include the library in your project much like you would InterfaceLib. This way, you can easily call the exported functions in the import library without having to worry about the details of locating the library file, locating the exported function itself, and creating a universal procedure pointer. This may defeat the purpose of considering plug-ins in the first place, since the library is "linked" to your project. Another option, however, is to use the "weak link" option. Weak linking meets you in the middle of creating a full-fledged plug-in and "strong" linking to a library as described earlier. See your development environment documentation for details.

Fat Plug-ins

Calling a plug-in fat simply means that it can run natively on more than one microprocessor. A fat plug-in might contain code for both 680x0 and PowerPC microprocessors within the same file. This allows users to install just one file on any Macintosh computer and obtain the benefits of optimized code for their specific computer. You can easily compile and merge both 680x0 and PowerPC code in this manner. An informative book written on the subject (if I do say so myself) covers this in great detail. You can learn more about this technique (and other techniques mentioned in this article) from A Fragment of Your Imagination at http://www.triplesoft.com/fragment/.

What About SOM?

SOM is IBM's System Object Model. It was originally introduced on the Macintosh with OpenDoc. Although OpenDoc has moved on, SOM has stuck around. The Mac OS 8 Contextual Menu Manager uses SOM, for example. SOM allows you to use object-oriented techniques in a shared library. You garner all of the advantages of being able to create and override classes (including special SOM base classes) with the advantages of coding a shared library. Depending on your needs, SOM may be something you will want to explore.

For more information on SOM, see the February 1998 issue of MacTech magazine which contains articles on SOM and the Contextual Menu Manager. You can also find information on Apple's developer web site at http://devworld.apple.com/.

What About COM?

COM is Microsoft's Component Object Model. It is a programming model that defines how objects can communicate with one another, similar but different to SOM. ActiveX controls (previously known as OLE controls) are based on COM. For more information on COM and ActiveX read the June 1997 issue of MacTech Magazine.

Conclusion

Once you understand the basic concepts described in this article, you will begin to find new uses for plug-ins in your application. Many applications have areas that can be logically broken out into a plug-in architecture. The key is to understand and then experiment. Don't use this approach if you don't need it, but if you do, you can easily add years of life to your application by opening it up to yourself and third-party developers in this way. I look forward to hearing about how you've used this introductory plug-in architecture.

Special Thanks

Special thanks goes to our technical reviewers: Tantek Celik, Nick DeMello, Eric Gundrum and Marty Wachter.


Joe Zobkiw, zobkiw@triplesoft.com, is a programmer, author, musician and practicing carver of stone. He is the author of A Fragment of Your Imagination, a book about code fragments and code resources for the Mac OS. You can learn more about (and order a copy of) the book at http://www.triplesoft.com/fragment/.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

iClock Pro 3.4.7 - Customize your menuba...
iClock Pro is a menu bar replacement clock for Apple's default clock. iClock Pro is an update, total rewrite and improvement to the popular iClock. Have the day, date and time in different fonts and... Read more
PhotoDesk 4.1.5 - Instagram client for p...
PhotoDesk lets you view, like, comment, and download Instagram pictures/videos. (NO Uploads! / Image Posting! Instagram forbids that! AND you need an existing Instagram account). But you can do so... Read more
Capo 3.5.1 - Slow down and learn to play...
Capo lets you slow down your favorite songs so you can hear the notes and learn how they are played. With Capo, you can quickly tab out your songs atop a highly-detailed OpenCL-powered spectrogram... Read more
HandBrake 1.0.2 - Versatile video encode...
HandBrake is a tool for converting video from nearly any format to a selection of modern, widely supported codecs. Features Supported Sources VIDEO_TS folder, DVD image or real DVD (unencrypted... Read more
Sound Studio 4.8.6 - Robust audio record...
Sound Studio lets you easily record and professionally edit audio on your Mac. Easily rip vinyls and digitize cassette tapes, or record lectures and voice memos. Prepare for live shows with live... Read more
HandBrake 1.0.2 - Versatile video encode...
HandBrake is a tool for converting video from nearly any format to a selection of modern, widely supported codecs. Features Supported Sources VIDEO_TS folder, DVD image or real DVD (unencrypted... Read more
PhotoDesk 4.1.5 - Instagram client for p...
PhotoDesk lets you view, like, comment, and download Instagram pictures/videos. (NO Uploads! / Image Posting! Instagram forbids that! AND you need an existing Instagram account). But you can do so... Read more
Sound Studio 4.8.6 - Robust audio record...
Sound Studio lets you easily record and professionally edit audio on your Mac. Easily rip vinyls and digitize cassette tapes, or record lectures and voice memos. Prepare for live shows with live... Read more
Capo 3.5.1 - Slow down and learn to play...
Capo lets you slow down your favorite songs so you can hear the notes and learn how they are played. With Capo, you can quickly tab out your songs atop a highly-detailed OpenCL-powered spectrogram... Read more
Google Earth 7.1.8.3036 - View and contr...
Google Earth gives you a wealth of imagery and geographic information. Explore destinations like Maui and Paris, or browse content from Wikipedia, National Geographic, and more. Google Earth combines... Read more

Super Mario Run dashes onto Android in M...
Super Mario Run was one of the biggest mobile launches in 2016 before it was met with a lukewarm response by many. While the game itself plays a treat, it's pretty hard to swallow the steep price for the full game. With that said, Android users... | Read more »
WarFriends Beginner's Guide: How to...
Chillingo's new game, WarFriends, is finally available world wide, and so far it's a refreshing change from common mobile game trends. The game's a mix of tower defense, third person shooter, and collectible card game. There's a lot to unpack here... | Read more »
Super Gridland (Entertainment)
Super Gridland 1.0 Device: iOS Universal Category: Entertainment Price: $1.99, Version: 1.0 (iTunes) Description: Match. Build. Survive. "exquisitely tuned" - Rock Paper Shotgun No in-app purches, and no ads! | Read more »
Red's Kingdom (Games)
Red's Kingdom 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Mad King Mac has kidnapped your father and stolen your golden nut! Solve puzzles and battle goons as you explore and battle your... | Read more »
Turbo League Guide: How to tame the cont...
| Read more »
Fire Emblem: Heroes coming to Google Pla...
Nintendo gave us our first look at Fire Emblem: Heroes, the upcoming mobile Fire Emblem game the company hinted at last year. Revealed at the Fire Emblem Direct event held today, the game will condense the series' tactical RPG combat into bite-... | Read more »
ReSlice (Music)
ReSlice 1.0 Device: iOS Universal Category: Music Price: $9.99, Version: 1.0 (iTunes) Description: Audio Slice Machine Slice your audio samples with ReSlice and create flexible musical atoms which can be triggered by MIDI notes or... | Read more »
Stickman Surfer rides in with the tide t...
Stickson is back and this time he's taken up yet another extreme sport - surfing. Stickman Surfer is out this Thursday on both iOS and Android, so if you've been following the other Stickman adventures, you might be interested in picking this one... | Read more »
Z-Exemplar (Games)
Z-Exemplar 1.4 Device: iOS Universal Category: Games Price: $3.99, Version: 1.4 (iTunes) Description: | Read more »
5 dastardly difficult roguelikes like th...
Edmund McMillen's popular roguelike creation The Binding of Isaac: Rebirth has finally crawled onto mobile devices. It's a grotesque dual-stick shooter that tosses you into an endless, procedurally generated basement as you, the pitiable Isaac,... | Read more »

Price Scanner via MacPrices.net

12-inch 1.2GHz Space Gray Retina MacBook on s...
Amazon has the 2016 12″ 1.2GHz/512GB Space Gray Retina MacBook (Apple model #MLH82LL/A) on sale for $1299.99 including free shipping. Their price is $300 off MSRP. Read more
Twelve South Releases RelaxedLeather Cases fo...
Inspired by the laid-back luxury of burnished leather boots and crafted in rich tones of taupe, herb and marsala, RelaxedLeather cases deliver smart, easy protection for the iPhone 7. Each genuine... Read more
Week’s Best Deal: New 2016 13-inch 2.0GHz Mac...
Amazon has the new 2016 13″ 2.0GHz non-Touch Bar MacBook Pros on sale for a limited time for $225 off MSRP including free shipping: - 13″ 2.0GHz MacBook Pro, Space Gray (MLL42LL/A): $1274.99 $225 off... Read more
Back in stock: Apple refurbished Mac minis fr...
Apple has Certified Refurbished Mac minis available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz Mac mini: $419 $80 off MSRP - 2.6GHz Mac... Read more
Apple Ranked ‘Most Intimate Brand’
The top ranked ‘”intimate” brands continued to outperform the S&P and Fortune 500 indices in revenue and profit over the past 10 years, according to MBLM’s Brand Intimacy 2017 Report, the largest... Read more
B-Eng introduces SSD Health Check for Mac OS
Fehraltorf, Switzerland based independant Swiss company- B-Eng has announced the release and immediate availability of SSD Health Check 1.0, the company’s new hard drive utility for Mac OS X. As the... Read more
Apple’s Education discount saves up to $300 o...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free: -... Read more
4-core 3.7GHz Mac Pro on sale for $2290, save...
Guitar Center has the 3.7GHz 4-core Mac Pro (MD253LL/A) on sale for $2289.97 including free shipping or free local store pickup (if available). Their price is a $710 savings over standard MSRP for... Read more
128GB Apple iPad Air 2, refurbished, availabl...
Apple has Certified Refurbished 128GB iPad Air 2s WiFis available for $419 including free shipping. That’s an $80 savings over standard MSRP for this model. A standard Apple one-year warranty is... Read more
13-inch 2.7GHz Retina MacBook Pro on sale for...
B&H Photo has the 2015 13″ 2.7GHz/128GB Retina Apple MacBook Pro on sale for $100 off MSRP. Shipping is free, and B&H charges NY tax only: - 13″ 2.7GHz/128GB Retina MacBook Pro (MF839LL/A): $... Read more

Jobs Board

*Apple* macOS Systems Integration Administra...
…most exceptional support available in the industry. SCI is seeking an Junior Apple macOS systems integration administrator that will be responsible for providing Read more
*Apple* Retail - Multiple Positions- Deer Pa...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Technician - nfrastructure (United S...
Let’s Work Together Apple Technician This position is based in Portland, ME Life at nfrastructure At nfrastructure, we understand that our success results from our Read more
*Apple* Mobile Master - Best Buy (United Sta...
**467692BR** **Job Title:** Apple Mobile Master **Location Number:** 000602-Columbia MO-Store **Job Description:** **What does a Best Buy Apple Mobile Master Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.