src/spec/doc/design-pattern-template-method.adoc

= Template Method Pattern


The http://en.wikipedia.org/wiki/Template_method_pattern[Template Method Pattern] abstracts away the details of several algorithms. The generic part of an algorithm is contained within a base class. Particular implementation details are captured within base classes. The generic pattern of classes involved looks like this:

image::assets/img/TemplateMethodClasses.gif[]

== Example

In this example, ++Accumulator++ captures the essence of the accumulation algorithm. The base classes ++Sum++ and ++Product++ provide particular customised ways to use the generic accumulation algorithm.

[source,groovy]
----
include::{projectdir}/src/spec/test/DesignPatternsTest.groovy[tags=template_method_example,indent=0]
----

The resulting output is:

----
10
24
----

In this particular case, you could use Groovy's inject method to achieve a similar result using Closures:

[source,groovy]
----
include::{projectdir}/src/spec/test/DesignPatternsTest.groovy[tags=template_method_example2,indent=0]
----

Thanks to duck-typing, this would also work with other objects which support an add (plus() in Groovy) method, e.g.:

In this particular case, you could use Groovy's inject method to achieve a similar result using Closures:

[source,groovy]
----
include::{projectdir}/src/spec/test/DesignPatternsTest.groovy[tags=template_method_example3,indent=0]
----

We could also do the multiplication case as follows:

[source,groovy]
----
include::{projectdir}/src/spec/test/DesignPatternsTest.groovy[tags=template_method_example4,indent=0]
----

Using closures this way looks more like the <<_strategy_pattern,Strategy Pattern>> but if we realise that the built-in ++inject++ method is the generic part of the algorithm for our template method, then the Closures become the customised parts of the template method pattern.