Stricter OOP without AOP! Sort of ;)

I've been playing around with the idea of forcing my components not to add or remove variables from their 'variables' scope, other than when instantiated. Its something I'd like not to be possible and I'd like a ColdFusion error if it happens. I blogged my first effort at cracking it here:

Stricter OOP using AOP!

The biggest downside to this was complexity of implementation. While I don't mind hidden complexity, using AOP meant that I would have to explicitly declare that each of my objects was wanting to use this feature/'aspect' - the effort in doing so using ColdSpring far outweighed any benefit. This was a 'global aspect' that would be best placed in a global base class if possible.

So, how to wrap all of a component's methods automatically using a base class that it extended? The concept is similar to AOP but it would lack the hassle of having to apply AOP to my model. Of course, the hassle would be figuring out how to do it!

A very long and steep-learning-curved night later and I have a working version. It borrows from the concept used in ColdSpring's AOP implementation of creating a temporary file in order to create component methods on the fly (mixins). The use is slightly different however. The following is an attempt to explain the base class constructor as concicely as possible:

1. Store copy of self in a state variable.


2. Create temporary cfc file with methods of the same name as those in this component. The body of these methods have a single line that calls a 'CallMethod()' method, passing the method name and any args as arguments.


3. Instantiate temporary cfc and delete the file.


4. Overwrite the original component methods with those in the temporary component


5. Removed methods used in this process from the component

The result is a component that appears unchanged (from the outside) but that has each of its methods replaced with a wrapper that invokes the samed named method on a copy of the original component.

Once this component has been written, implementation couldn't be much easier. Simply extend the component and call it's constructor from within the extended component and you're done.

Rather than have all that code be used specifically for monitoring a component's variables, I first created an uber-base class which for now I have called selfproxy. This does what I have described above but does nothing useful in the 'self proxied' methods. To do something useful (such as monitor the variables scope), I just extend the component and override its method interceptor.

This 'self proxying' mullarky lacks the precision and flexibility of AOP, but if you're painting with a broad brush it might be a useful thing.

A working example (tested on ColdFusion 8 running on Windows XP).

I'm just scatting on a theme here and would be interested to hear any constructive criticism ;)

Anyways, time for bed!

Stricter OOP using AOP!

An application I wrote recently was exhibiting a strange bug. When a user added contacts to the contacts database, details from previously entered contacts were being inserted in fields that were left blank for the new contact. The client had entered over a hundred contacts before they realised that this was happening and were somewhat worried. After some investigation, I tracked down the cause of the problem: an accidentally unscoped variable in a CFC method.

In Object Oriented languages, an object's properties are explicitly declared in its definition; they are integral to the make-up of the object. To add or remove an object's properties during its life could be seen as mutating the object and an undesirable behaviour in a strict OO design (I do not believe it is possible to do this in C++ but I may be mistaken).

Due to the way we emulate OO in ColdFusion, such mutation is exactly what happened when I clumsily used an unscoped variable in my CFC; the variable became part of my object's 'integral make-up' resulting in the sneaky bug.

This made me wonder: how could I restrict my objects from ever modifiying their definitions by deleting or creating new variables in the variables or this scope during their existance?

The first, and so far only, useable answer I have arrived at is through using Aspect Oriented Programming (AOP). My implementation uses the ColdSpring framework although it could be done without it. For the AOP savvy, I simply created an around advice object that checks the target object's properties before and after any method execution - if there is an inconsistency, an appropriate error is thrown. The pseudo code looks like this:

BEGIN
SET variablesBefore = Get object's variables
EXECUTE object's method
SET variablesAfter = Get object's variables

IF variablesBefore <> variablesAfter THEN throw error
END

If anyone is interested in the Source code I can post it, just a PITA to put code up using blogger! Better solutions on a postcard.

Dominic