Lecture_15_-_Template_Method

					Software Design and Architecture
Behavioral Design Patterns


                             Muhammad Nasir
                               m.nasir@iiu.edu.pk
Template Method

   Intent
       Define the skeleton of an algorithm in an
        operation, deferring some steps to subclasses.
       Template Method lets subclasses redefine
        certain steps of an algorithm without changing
        the algorithm's structure.
Template Method - Motivation
   Consider an application framework that provides
    Application and Document classes.
   The Application class is responsible for opening
    existing documents such as a file.
   A Document object represents the information in a
    document once it's read from the file.
   Applications built with the framework can subclass
    Application and Document to suit specific needs.
Template Method
   For example, a drawing application defines Draw-
    Application and Draw-Document subclasses.
   A spread sheet application defines Spread-sheet-
    Application and Spread-sheet-Document
    subclasses.
   We call OpenDocument a template method. A
    template method defines an algorithm in terms of
    abstract operations that subclasses override to
    provide concrete behavior.
Template Method - Motivation
Template - Applicability
   To implement the invariant parts of an algorithm
    once and leave it upto subclasses to implement the
    behavior that can vary
   When common behavior among subclasses should
    be factored and localized in a common class to
    avoid code duplication
   To control subclasses extensions using the
    template method. Template method calls Hook
    operations there by permitting extensions at those
    points
   The Hollywood Principle –”Don’t call us, we will call
    you”
Template – Hollywood Principle

   The Hollywood Principle allows
    low-level components to hook
    themselves into a system, but the
    high-level components determine
    when they are needed, and how.
       In other words, the high-level
        components tell the low-level
        components “don’t call us, we’ll call
        you”.
Template Method - Structure
Template Method - Participants

   AbstractClass (Application)
       defines abstract primitive operations that
        concrete subclasses define to implement steps
        of an algorithm.
       Implements a template method defining the
        skeleton of an algorithm.
   ConcreteClass (MyApplication)
       Implements the primitive operations to carry out
        subclass-specific steps of the algorithm.
Template Method - Collaboration

   ConcreteClass relies on AbstractClass to
    implement the invariant steps of the algorithm.
Template Method – Sample Code
Template Method – Sample Code
Template Method – Sample Code
Template Method - Consequences

   Template methods are a fundamental technique for
    code reuse.
   Avoid code duplication
   Control Sub-Classing
Template Method – Related Patterns

   Factory Methods are often called by template
    methods. In the Motivation example, the factory
    method CreateDocument is called by the template
    method OpenDocument.
   Strategy: Template methods use inheritance to
    vary part of an algorithm. Strategies use delegation
    to vary the entire algorithm.
The End

   Thanks for listening
   Questions would be appreciated…

				
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posted:4/26/2012
language:English
pages:16
Description: software design and architecture