Template method pattern: Difference between revisions

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In [[object-oriented programming]], the '''template method''' is one of the [[behavioral pattern|behavioral]] [[Software design pattern|design patterns]] identified by Gamma et al.<ref name=":0">{{cite book|title=Design Patterns|last1=Gamma|first1=Erich|last2=Helm|first2=Richard|last3=Johnson|first3=Ralph|last4=Vlissides|first4=John|publisher=Addison-Wesley|year=1994|isbn=0-201-63361-2|pages=[https://archive.org/details/designpatternsel00gamm/page/325 325–330]|chapter=Template Method|author-link1=Erich Gamma|author-link2=Richard Helm|author-link3=Ralph Johnson (computer scientist)|author-link4=John Vlissides|title-link=Design Patterns}}</ref> in the book ''[[Design Patterns]]''. The template method is a method in a superclass, usually an abstract superclass, and defines the skeleton of an operation in terms of a number of high-level steps. These steps are themselves implemented by additional ''helper methods'' in the same class as the ''template method''.

The ''helper methods'' may be either ''[[Abstract method|abstract methods]]'', in which case subclasses are required to provide concrete implementations, or ''[[hook methods]],'' which have empty bodies in the superclass. [[Subclass (computer science)|Subclass]]es can (but are not required to) customize the operation by [[Method overriding|overriding]] the hook methods. The intent of the template method is to define the overall structure of the operation, while allowing subclasses to refine, or redefine, certain steps.<ref name=":2">{{cite book|url=http://shop.oreilly.com/product/9780596007126.do|title=Head First Design Patterns|last2=Freeman|first2=Elisabeth|last3=Sierra|first3=Kathy|last4=Bates|first4=Bert|publisher=O'REILLY|year=2004|isbn=978-0-596-00712-6|editor-last=Hendrickson|editor-first=Mike|volume=1|pages=289, 311|format=paperback|editor-last2=Loukides|editor-first2=Mike|last1=Freeman|first1=Eric|access-date=2012-09-12}}</ref>

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* The "template method" is implemented as a method in a [[base class]] (usually an [[abstract class]]). This method contains code for the parts of the overall algorithm that are invariant. The template ensures that the overarching algorithm is always followed.<ref name=":0" /> In the template method, portions of the algorithm that may ''vary'' are implemented by sending self messages that request the execution of additional ''helper'' methods. In the base class, these helper methods are given a default implementation, or none at all (that is, they may be abstract methods).

* Subclasses of the base class "fill in" the empty or "variant" parts of the "template" with specific algorithms that vary from one subclass to another.<ref name=":1" /> It is important that subclasses do ''not'' override the ''template method'' itself.

At run-time, the algorithm represented by the template method is executed by sending the template message to an instance of one of the concrete subclasses. Through inheritance, the template method in the base class starts to execute. When the template method sends a message to self requesting one of the helper methods, the message will be received by the concrete sub-instance. If the helper method has been overridden, the overriding implementation in the sub-instance will execute; if it has not been overridden, the inherited implementation in the base class will execute. This mechanism ensures that the overall algorithm follows the same steps every time, while allowing the details of some steps to depend on which instance received the original request to execute the algorithm.

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==Usage==

The ''template method'' is used in frameworks, where each implements the invariant parts of a ___domain's architecture, while providing hook methods for customization. This is an example of [[inversion of control]]. The template method is used for the following reasons.<ref name=":1">{{cite web | url = http://sourcemaking.com/design_patterns/template_method | title = Template Method Design Pattern | publisher = Source Making - teaching IT professional | quote = Template Method is used prominently in frameworks. | access-date = 2012-09-12}}</ref>

* It lets subclasses implement varying behavior (through [[Method overriding (programming)|overriding]] of the hook methods).<ref name=":3">{{Cite book|title=Pro Objective-C Design Patterns for iOS|last=Chung|first=Carlo|publisher=Apress|year=2011|isbn=978-1-4302-3331-2|___location=Berkeley, CA|pages=266}}</ref>

* It avoids duplication in the code: the general workflow of the algorithm is implemented once in the abstract class's template method, and necessary variations are implemented in the subclasses.<ref name=":3" />