Pree's Metapatterns

• Pree's metapatterns are an expansion of the Template Method pattern
• Encompasses several other patterns from Gamma et al
• Metapatterns are directed at framework class libraries:
  • Library "structures" the application or algorithm
  • Library is "in control"
  • Application developer overrides details of library's algothims
• Metapatterns may be viewed (but aren't in the book) as the cross of 3 binary attributes:
  • unification vs connection
  • single reference vs multiple reference
  • non-recursive vs recursive

  Pree's metapatterns may be viewed as the cross of 3 binary attributes

  	non-recursive		recursive
  	single	multiple	single	multiple
  Unification	Unification		1:1 Recursive Unification	1:N Recursive Unification
  Connection	1:1 Connection	1:N Connection	1:1 Recursive Connection	1:N Recursive Connection

• All the single versions are represented by the patterns in Gamma et al (if you account for 1:1 Recursive Unification being the degenerate case of 1:1 Recursive Connection)
• The multiple versions don't appear in Gamma et al except for 1:N Recursive Connection, which is very similar to Composite

Unification

• Same as the Template Method pattern
• Both the template and hook methods are in a single class
• User subclasses to override the hook function
• This is the least flexible approach as behavior is fixed at run time
• Does not allow for behavior change at run time

1:1 Connection

• Same as the Strategy pattern
• The template class refers to one instance of the hook class
• There is no inheritance relationship between the template and hook classes
• Allows for behavior change at run time

1:N Connection

• The template class refers to any number of instances of the hook class
• There is no inheritance relationship between the template and hook classes
• Allows for behavior change at run time

1:1 Recursive Connection

• Same as the Chain of Responsibiltiy pattern and similar to the Decorator pattern
• The template class refers to one instance of the hook class
• The hook class is an ancestor of the template class.
• Can model chains of instances
• Allows for behavior change at run time

1:1 Recursive Unification

• The degenerate version of the 1:1 Recursive Connection
• The template and hook classes are the same class
• Allows for behavior change at run time

1:N Recursive Connection

• Similar to the Composite pattern
• The template class refers to any number of instances of the hook class
• The hook class is an ancestor of the template class
• Can model trees of instances
• Allows for behavior change at run time

1:NRecursive Unification

• The degenerate version of the 1:N Recursive Connection
• The template and hook classes are the same class
• Allows for behavior change at run time