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= Chain of Responsibility Pattern

In the Chain of Responsibility Pattern, objects using and implementing an interface (one or more methods) are intentionally loosely coupled. A set of objects that __implement__ the interface are organised in a list (or in rare cases a tree). Objects using the interface make requests from the first __implementor__ object. It will decide whether to perform any action itself and whether to pass the request further down the line in the list (or tree). Sometimes a default implementation for some request is also coded into the pattern if none of the implementors respond to the request.

== Example

In this example, the script sends requests to the `lister` object. The `lister` points to a `UnixLister` object. If it can't handle the request, it sends the request to the `WindowsLister`. If it can't handle the request, it sends the request to the `DefaultLister`.

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

The output will be a list of files (with slightly different format depending on the operating system).

Here is a UML representation:

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

Variations to this pattern:

* we could have an explicit interface, e.g. `Lister`, to statically type the implementations but because of _duck-typing_ this is optional
* we could use a chain tree instead of a list, e.g. `if (animal.hasBackbone())` delegate to `VertebrateHandler` else delegate to `InvertebrateHandler`
* we could always pass down the chain even if we processed a request
* we could decide at some point to not respond and not pass down the chain
* we could use Groovy’s meta-programming capabilities to pass unknown methods down the chain