Monday, 10 July 2006

Programming idioms #1: Mixin classes

I've decided to devote a few blog posts to programming idioms. These aren't as fancy as design patterns but they are useful and often skipped over by text books. At the very least, you rarely see a whole bunch of them collected together. So... in the next few weeks expect to see posts on point-free-programming and all sorts of other goodness.

Firstly, though: Mixin classes. This is an idiom object-oriented programming. A mixin class is one which implements a bunch of different behaviours but doesn't hold any state. The idea is that you use the mixin class as a superclass so that it's methods are "mixed in" with the methods of the sub class.

An obvious use for mixin classes is GUI programming. There's a variety of behaviours which are useful in most applications written with any given widget set, such as setting text in a status bar, popping up a modal dialog, and so on. Each of these tasks requires some logic and resource handling and because the operations are common to many different applications they can usefully be factored out. Here's an example stub from a GUI I recently wrote (in Python):

class GUIMixin:
def about(self):
"""About dialog."""
def error(self, str):
"""Modal dialog box to display error messages."""
def onQuit(self, event=None):
"""Handle resources and close GUI."""
def help(self, title, helpfile):
"""Display a help file."""

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