Volume Number: 14 (1998)
Issue Number: 6
Column Tag: MacTech Online
by Jeff Clites, online@mactech.com
A plug-in architecture can be a clever way to add power and value to your application. It allows third parties to extend the capabilities of your product, and allows you to do the same without the need to redistribute the entire application to end users, and without the danger of adding bugs to the core code as you add features.
It is natural to think of an application plug-in as analogous to a system extension, but there is a crucial difference: extensible applications are specifically designed to work with plug-ins, and have a well-defined interface through which the plug-ins operate, while INITs usually do their work despite the lack of such an interface, relying on low-level mechanisms such as trap patching. As a result, plug-ins don't bring the instability often associated with system extensions, and they tend to provide an application with a specific service or capability, rather than modifying global behavior.
Despite this distinction, certain parts of the Mac OS are designed to accommodate plug-ins, and they do so using a powerful tool that you can use to give your own applications a plug-in architecture: the Component Manager. Apple likens the Component Manager to a Resource Manager which acts on function objects rather than data objects. If you have ever executed a compiled script from within an application, you have used the Component Manager. Many parts of the Mac OS are implemented via components - AppleScript and Frontier are Scripting Components, QuickTime uses components for image decompression and for video effects, and the Translation Manager uses components to enable Mac OS Easy Open to covert files between formats.
You can write components which will only be used by your application, or which will provide their services globally, to any application which requests them. Start learning about the Component Manager from its chapter in Inside Macintosh: More Macintosh Toolbox, and then visit the QuickTime 3.0 documentation for recent changes. Also be sure to look at the sample code which Apple has provided. There is older code, which shows you how to write a simple component, and newer code on the QuickTime 3.0 web site which demonstrates how you can write components which apply video effects or allow applications to import images stored in a new format. To round things out, there is a Technote describing how the Component Manager handles PowerPC native components.
The most expansive example of using a component to supply services to multiple applications is Internet Config, the source to which is in the public domain. Internet Config has become indispensable as more and more applications become internet-savvy, and it's a great example of using a component to save programmers from the need to code in the same functionality over and over. The author's ftp site has the source, which you can use as an example of good component design, and a programmer's kit which shows you how to use Internet Config from within your application. Dan Crevier has also written IC++, a C++ wrapper class for Internet Config, which illustrates how you can make it even easier for you (or others) to call your component from inside an application.
Probably the most well-known type of application plug-ins are Photoshop plug-ins; in fact, and entire industry has been built around them. You can get in on the fun by downloading the Photoshop SDK from Adobe, which also has SDKs for plug-ins to their other applications, including Illustrator and Acrobat. There are actually several different types of Photoshop plug-ins - the most common are filters, which directly manipulate images, but there are also image acquisition plug-ins, which let you drives devices such as scanners from within the application, and import and export plug-ins.
It's possible to support Photoshop plug-ins in your own application, and many third parties do this to increase, for instance, the image-manipulation power of their applications. NIH Image is one example. It is an image processing and analysis application developed by the National Institutes of Health and is widely used for scientific imaging. Not only is it free, but its source code is in the public domain. It supports some Photoshop plug-ins, so it can serve as a test bed for those developers who want to experiment but don't own Photoshop, or who are designing their plug-ins for the widest range of compatibility. The source code, of course, can help those looking to support Photoshop plug-ins in their own application, or who want a model for a plug-in architecture in general. The NIH Image ftp site also has a directory full of plug-ins, some with source code, to serve as examples to developers and to augment Adobe's SDK.
A plug-in architecture is a Good Thing. It can reduce the size of your application, decrease its memory footprint, and promote good design by encouraging you to factor your application into functionally independent pieces and prepare it for future enhancements. While implementing a new design strategy, it's always good to have some printed material on hand. Plug-ins are code fragments at heart, and the definitive text on these is A Fragment of Your Imagination, by Joe Zobkiw. It has chapters on the Component Manager and on Photoshop filters, as well as covering system extensions and most other related topics, and there is even an accompanying web site.
These and other links are available from the MacTech Online web pages at www.mactech.com/online/.
