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

PopChar 7.1 - Floating window shows avai...
We're also selling a 5-license family pack for only $25.99! PopChar helps you get the most out of your font collection. With its crystal-clear interface, PopChar X provides a frustration-free way to... Read more
BBEdit 11.1.1 - Powerful text and HTML e...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more
Picasa 3.9.139 - Organize, edit, and sha...
Picasa and Picasa Web Albums allows you to organize, edit, and upload your photos to the Web from your computer in quick, simple steps. Arrange your photos into folders and albums and erase their... Read more
Mac DVDRipper Pro 5.0.5 - Copy, backup,...
Mac DVDRipper Pro is the DVD backup solution that lets you protect your DVDs from scratches, save your batteries by reading your movies from your hard disk, manage your collection with just a few... Read more
NetShade 6.2 - Browse privately using an...
This promotion is for NetShade and 1 year of Proxy and VPN services NetShade is an anonymous proxy and VPN app+service for Mac. Unblock your Internet through NetShade's high-speed proxy and VPN... Read more
CrossOver 14.1.3 - Run Windows apps on y...
CrossOver can get your Windows productivity applications and PC games up and running on your Mac quickly and easily. CrossOver runs the Windows software that you need on Mac at home, in the office,... Read more
Little Snitch 3.5.3 - Alerts you about o...
Little Snitch gives you control over your private outgoing data. Track background activity As soon as your computer connects to the Internet, applications often have permission to send any... Read more
OmniGraffle Pro 6.2.3 - Create diagrams,...
OmniGraffle Pro helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use... Read more
OmniFocus 2.2 - GTD task manager with iO...
OmniFocus helps you manage your tasks the way that you want, freeing you to focus your attention on the things that matter to you most. Capturing tasks and ideas is always a keyboard shortcut away in... Read more
1Password 5.3.2 - Powerful password mana...
1Password is a password manager that uniquely brings you both security and convenience. It is the only program that provides anti-phishing protection and goes beyond password management by adding Web... Read more

MooVee - Your Movies Guru (Entertainmen...
MooVee - Your Movies Guru 1.0 Device: iOS iPhone Category: Entertainment Price: $1.99, Version: 1.0 (iTunes) Description: MooVee helps you effortlessly manage your movies, on your iPhone. | Read more »
Geometry Wars 3: Dimensions (Games)
Geometry Wars 3: Dimensions 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: Enjoy the next chapter in the award-winning Geometry Wars franchise and enjoy stunning, console-quality... | Read more »
CHAOS RINGS Ⅲ (Games)
CHAOS RINGS Ⅲ 1.0.0 Device: iOS Universal Category: Games Price: $19.99, Version: 1.0.0 (iTunes) Description: The newest addition to the popular smartphone RPG series is finally here! ・CHAOS RINGS Overview | Read more »
The Popular Insight Series of Travel Gui...
Getting around in a country when you can't understand the primary language can be tough. Fortunately there are several options available to help wold travellers with the important stuff like giving directions to a cab driver or asking where the... | Read more »
Desktop Dungeons is Now on the iPad Desp...
Desktop Dungeons has been a well-loved roguelike on PC for quite some time, and now it's finally available for the iPad! Just the iPad, though. Sorry iPhone users. [Read more] | Read more »
Moleskine Timepage – Calendar for iCloud...
Moleskine Timepage – Calendar for iCloud, Google & Exchange 1.0 Device: iOS iPhone Category: Productivity Price: $4.99, Version: 1.0 (iTunes) Description: The most elegant calendar for your pocket and wrist, Timepage is a... | Read more »
QuizUp Gets Social in its New Update
Plain Vanilla Corp has released a new and improved version of their popular trivia game, QuizUp. The app now emphasizes social play so you can challenge friends from all over the world. [Read more] | Read more »
The Deep (Games)
The Deep 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Swipe Controls Delve into the deep in this retro rogue-like! Swipe to move your diver around and keep away from the enemies as you... | Read more »
Sproggiwood (Games)
Sproggiwood 1.2.8 Device: iOS Universal Category: Games Price: $9.99, Version: 1.2.8 (iTunes) Description: Sproggiwood was developed for devices with at least 1GB of RAM. We recommend you only download Sproggiwood if your device... | Read more »
Battle of Gods: Ascension (Games)
Battle of Gods: Ascension 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: TURN-BASED TACTICAL COMBATFight tactical battles against the forces of Hades! In Battle of Gods: Ascension you play... | Read more »

Price Scanner via MacPrices.net

Apple refurbished 2014 13-inch Retina MacBook...
The Apple Store has Apple Certified Refurbished 2014 13″ Retina MacBook Pros available for up to $400 off original MSRP, starting at $979. An Apple one-year warranty is included with each model, and... Read more
What Would the ideal Apple Productivity Platf...
For the past four years I’ve kept a foot in both the Mac and iPad camps respectively. my daily computing hours divided about 50/50 between the two devices with remarkable consistency. However, there’... Read more
PageMeUp 1.2.1 Ten Dollar Page Layout Applica...
Paris, France-based Softobe, an OS X software development company, has announced that their PageMeUp v. 1.2.1, is available on the Mac App Store for $9.99. The license can be installed on up to 5... Read more
Eight New Products For USB Type-C Application...
Fresco Logic, specialists in advanced connectivity technologies and ICs, has introduced two new product families targeting the Type-C connector recently introduced across a number of consumer... Read more
Scripps National Spelling Bee Launches Buzzwo...
Scripps National Spelling Bee fans can monitor the action at the 2015 Spelling Bee with the new Buzzworthy app for iOS, Android and Windows mobile devices. The free Buzzworthy app provides friendly... Read more
13-inch 2.5GHz MacBook Pro on sale for $120 o...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $979 including free shipping plus NY sales tax only. Their price is $120 off MSRP, and it’s the lowest price for this model (except for Apple’... Read more
27-inch 3.3GHz 5K iMac on sale for $1899, $10...
B&H Photo has the new 27″ 3.3GHz 5K iMac on sale for $1899.99 including free shipping plus NY tax only. Their price is $100 off MSRP. Read more
Save up to $50 on iPad Air 2, NY tax only, fr...
B&H Photo has iPad Air 2s on sale for up to $50 off MSRP including free shipping plus NY sales tax only: - 16GB iPad Air 2 WiFi: $469 $30 off - 64GB iPad Air 2 WiFi: $549.99 $50 off - 128GB iPad... Read more
Updated Mac Price Trackers
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers: - 15″ MacBook Pros - 13″ MacBook... Read more
New 13-inch 2.9GHz Retina MacBook Pro on sale...
B&H Photo has the 13″ 2.9GHz/512GB Retina MacBook Pro on sale for $1699.99 including free shipping plus NY tax only. Their price is $100 off MSRP, and it’s the lowest price for this model from... Read more

Jobs Board

Program Manager, *Apple* Community Support...
**Job Summary** Apple Support Communities ( discussions. apple .com) helps customers get the most from their Apple products and services by providing access to Read more
Senior Data Scientist, *Apple* Retail - Onl...
**Job Summary** Apple Retail - Online sells Apple products to customers around the world. In addition to selling Apple products with unique services such as iPad Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
*Apple* Watch SW Application Project Manager...
**Job Summary** The Apple Watch software team is looking for an Application Engineering Project Manager to work on new projects for Apple . The successful candidate Read more
Engineering Manager for *Apple* Maps on the...
…the Maps App Team get to take part in just about any new feature in Apple Maps, often contributing a majority of the feature work. In our day-to-day engineering work, we Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.