Class Extensions (specifically, an Instance Variable)

On 10-Feb-10, at 9:31 AM, Lautaro Fernández wrote:

Hi,
We need to extend a class A (from package P-1) with some behavior in another package (P-2). In the package extension we will add methods and an instance variable for A.
Is there a clean way to put that I.V. in the class extension which is in package P-2?

If not, which will be a dirty way to do it?
-Just put it in the A class package P-1
-Adding a postLoad block to P-2 and then add it programmatically

Well, another way would be to make a subclass with your instance variable, and then use it instead of the superclass. If needed, you could migrate existing instances of the superclass in the postload block using #become:. 

On Feb 10, 2010, at 6:31 AM, Lautaro Fernández wrote:

Hi,
We need to extend a class A (from package P-1) with some behavior in another package (P-2). In the package extension we will add methods and an instance variable for A.
Is there a clean way to put that I.V. in the class extension which is in package P-2?

If not, which will be a dirty way to do it?
-Just put it in the A class package P-1
-Adding a postLoad block to P-2 and then add it programmatically

The problem is more genrally "how can my new library/framework/module/codecomponent add additional state to existing object structures. There are a couple different patterns/techniques for doing this, with varying amounts of code smell and consequences.

1) Use a Class Definition Override. You select your class. You select "Override>>In Package..." from the class menu. You put the overide in your package. You add the instance variable to the definition. One problem (among others) with this technique is that if the original changes, your override masks those new changes back out of the system.

2) Use the subclass trick. Make a subclass of the TargetClass object (where TargetClass means "the class we want to extend"). Add the instance variable. Change TargetClass>>new/basicNew so that it will always return an instance of your new subclass. There's no overrides in this solution. But it only works if TargetClass is a leaf class with no subclasses.

3) Use a "look aside" variable. You create a global EphemeronDictionary and store your state there. So you can add setter and getter methods for your new state that look like:

newStateAccessor
       ^NewStateRegistry at: self ifAbsent: []

newStateSetter: anObject
       NewStateRegistry at: self put: anObject

Requires managing the NewStateRegistry, and may run into performance problems if you have really really high loads of access for this new state.


If you're lucky sometimes... you may find that you're trying to extend a class that already has a "generic storage" mechanism. Examples of these are VisualPart and CodeComponent, both of which have generic "properties" dictionaries.

--
Travis Griggs
Objologist
"Every institution finally perishes by an excess of its own first principle." - Lord Acton