Question

In his book Extreme Programming Applied, Ken Auer casually mentions an Enabler pattern. Kent Beck also mentions it (at the very least in an email dated November 08, 2004), but I haven’t been able to find any details in the usual places (Google, the wiki at c2.com, etc.)

Where can I find out more?

Answer 1

The Enabler pattern is really just a variation of an Observer pattern. You set up observers on interesting parts of a system, creating Conditions. Enablers can then observe the conditions and enable/disable widgets.

E.g. when you load up a Window, you create Conditions that watch stuff like whether a list or combo box (or a model underneath the list or combo-box) has something selected. You can call it something like “somethingSelectedCondition”. You then hook up enablers to widgets that are tied to one or more conditions (which you can and/or together… though typically you just want to and them together in practice).

I usually implement them by having a widget factory such that one of the parameters you pass in is an array of named conditions that, when anded together, enable/disable a widget. E.g.

widgetFactory.createButton(String buttonLabel, String methodActionName, String[] conditionNames)

The conditionNames will look up Conditions that are available in a some sort of HashMap, creat a CompositeCondition (which ANDs all of the observed boolean values of the conditions) and creates an Enabler which listens to the CompositeCondition and tells the created widget to enable/disable based on the underlying condition.

It sounds a bit complicated, but once you create the underlying infrastructure, a couple of lines of code wire up your conditions, and then the user interface is always in sync with the conditions.

I don’t think I ever officially wrote up the Enabler pattern, or I’d just point you to a URL.

HTH,

Ken