Last night (May 13, 2009), I gave a short presentation on meta-programming based on watching some code evolve in a project I worked on last year that involved a lot of such shenanigans. Each of the folders in this repository represents one phase of the project. Most of the action is in app/models/project.rb and eventually in lib/task_master.rb. Below is an description of each phase.
This is the starting point. It's just a super basic application. The projects show page (i.e. project/1) list the tasks that are hardcoded in Project::TASKS along with the output those methods. It's lame, so we start improving it.
Looking in the project model, we do away with the hardcoding and switch to using method_added so that any method on Project with the work "task" in it gets added to Project::TASKS automatically and shows up in the interface.
Having naming requirements for methods is lame, so we switch to using a basic task accumulator method and start defining our tasks using syntax like
task :foo do "bar" end
The project.rb file is getting a little large - imagine these tasks actually doing real stuff - so we move the task
method and the various tasks into modules. We still have the Project and Project::TASKS constants hard coded into the
modules, so it's not really very generic.
And, of course, we want it to be generic. We change the syntax in project.rb so it just includes the TestTask and OtherTasks modules and make them do all the magic: automatically making a class accessor on project and accumulating the relevant tasks into it as they are included.