On 2/17/07, Mark Thurman <[EMAIL PROTECTED]> wrote:
> I'm a young programmer (just finishing high school) who has done a fair
> amount of programming with PHP, MySQL, and other web technologies.
> However, being self trained, I was wondering if the members could help
> me with some questions. I am looking at projects such as PHPNuke,
> Wikipedia, phpBB2, etc and they all have modules, extensions etc.
>
> How does one go about designing a program so it can be extended later
> with plugins, apis, and modules?
>
> For example, in Wikipedia, to add a "Special" page, you include that
> page's source code in the main file, and then voila, it appears in
> lists, links, and adds its own functions and links to the code? How
> does one think and design a program so that functionality is possible?
>
> I've tried reading source code from some of these projects and I never
> can just follow along myself. Therefore, if anyone has some simple code
> they'd be willing to share, links to only books or howtos, or just
> personal experiences, please reply on or off list. If I haven't made
> myself clear, please ask for clarification.
Mark, if i understand you correctly, you are seeking how to design a
general purpose program that can be extended by others. It would help
us to know what your target environment is. Especially if it is a
dynamic language like Perl, Ruby or Python.Or a compiled language like
Java or C/C++. The difference lies in linking others source code with
yours, interpreted languages are easier in this respect.
That said, the general techniques are well-established. For purposes
of illustration, I'll call the code you are wanting to write the host
(application) and the external modules, the guest (module.) The basic
idea is to use various callbacks into the guest module from the host
application. But first the guest application must register itself with
the host (see it is like a hotel checkin...) This registration
process can take many forms and is usually dictated by the programming
environment. Anyway, the host maintains a list of registered guests.
Each time a new guest registers, he is appended to said list.
Next, the host will then use the handle that represents the main
object of the guest, and call an initialize routine in the guest. That
routine sets stuff and gets a handle to the host so it can call things
in the framework API to open windows, etc.
So the basic steps are:
Devise a registration process
Maintain a list of registered guest modules
when starting, loop over your registered guests and call their
initialize routines
when a guest's initialize routine is called, it calls pre-defined
host API calls to open windows, or other things.
These might cause the framework (in the host) to callback to the
guest to display the window, and paint the contents of the windows.
You want to make your plugin callback interface as narrow as possible.
And you want your host API to be simple to create widgets, windows,
whatever in a few easy steps. If using a O-O language like Java or C#,
use interfaces for both the IPlugin (guest) and IPluginHost (host) and
guest module writes will inherit from or implement those interfaces.
Ideally, the minimal IPlugin interface could be as small as init() and
destroy() (if destroy is needed.)
Finally, if starting fresh, you might think about designing your
entire application to nothing but the framework and your own pieces
will simply be plugins.
The hard part is the registration process. Do you allow files to be
uploaded to a webserver? Does it write and re-read a config file
listing plugins? I haven't looked at DotNuke or PHPNuke or Typo,
WordPress or any of the other ones. But the answer is in there.
Ruby on Rails has a built-in plugin architecture, but not one that you
can upload files to, at least not w/o restarting the RoR app iteself,
IIRC. It looks in a subdirectory for plugin subdirs for a file called
init.rb. It just executes whatever is in that tile.
http://en.wikipedia.org/wiki/Plugin
http://codex.wordpress.org/Writing_a_Plugin
http://www.codeguru.com/Cpp/misc/misc/plug-insadd-ins/article.php/c3879/
HTH, somewhat.
Ed
>
> Thanks for your help,
> Mark
--
Ed Howland
http://greenprogrammer.blogspot.com
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