A Refactoring for Perl.

You need to subclass a class that contains factory methods,
and also the class that the factory methods return.

Change

  sub factory {
      my $self = shift;
      ...
      my $newInstance = new Bar(...);
      ...
      return $newInstance;
  }
[download]

to

  sub factory {
      my $self = shift;
      ...
      my newInstance = $self->factoryClass()->new(...);
      ...
      return newInstance;
  }

  sub factoryClass {
      "Bar"
  }
[download]

Motivation

Factory methods return instances of some class known to the factory method. Typically the class name is hard-coded. If the class that holds the factory method is subclassed, the factory method is inherited, but will continue to return instances of the same hard-coded class. This isn't always desirable. A set of cooperating base classes, one of which uses a factory method to create instances of the other, may both need to be subclassed. If a factory method uses a hard-coded class name, that factory method must be copied into a subclass and modified. If the factory method performs other work (e.g., bookkeeping to account for the newly created instance), this can result in redundant code in the class hierarchy.

The solution is to "soft code" the class name that the factory method will create new instances of, by creating a new method that returns the class name, and using the new method in place of the hard-coded class name. The factory method can then be inherited by subclasses, which merely need to override the method that returns the class name.

Mechanics

• Identify a factory method that hard-codes a class name.
• Create a new method that returns the class name.
• In the factory class, replace the hard-coded class name with an invocation of the new method.
• Test.

Discussion

I first ran into the need for this when attempting to subclass pieces of GIFGraph, and finding myself initially thwarted by hard-coded class names in its factory methods. (I had to refactor methods to create some factory methods, but that's a separate story.)

This refactoring is hardly original thinking. It's been done in the Smalltalk world for years. If you know of an existing writeup of this refactoring for Perl, please provide a reference.

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