src/spec/doc/design-pattern-delegation.adoc

//////////////////////////////////////////

  Licensed to the Apache Software Foundation (ASF) under one
  or more contributor license agreements.  See the NOTICE file
  distributed with this work for additional information
  regarding copyright ownership.  The ASF licenses this file
  to you under the Apache License, Version 2.0 (the
  "License"); you may not use this file except in compliance
  with the License.  You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

  Unless required by applicable law or agreed to in writing,
  software distributed under the License is distributed on an
  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
  KIND, either express or implied.  See the License for the
  specific language governing permissions and limitations
  under the License.

//////////////////////////////////////////

= Delegation Pattern

The http://en.wikipedia.org/wiki/Delegation_pattern[Delegation Pattern] is a technique where an object's behavior (public methods) is implemented by delegating responsibility to one or more associated objects.

Groovy allows the traditional style of applying the delegation pattern, e.g. see <<_replace_inheritance_with_delegation,Replace Inheritance with Delegation>>.

== Implement Delegation Pattern using ExpandoMetaClass

The gapi:groovy.lang.ExpandoMetaClass[] allows usage of this pattern to be encapsulated in a library. This allows Groovy to emulate similar libraries available for the Ruby language.

Consider the following library class:

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

With this in your classpath, you can now apply the delegation pattern dynamically as shown in the following examples. First, consider we have the following classes:

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

We can now use the __delegator__ to automatically borrow methods from the __lender__ object to extend the __Person__ class. We can borrow the methods as is or with a rename:

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

The first line above, adds the __borrowFor__ method to the __Person__ class by delegating to the __lender__ object. The second line adds a __getMoney__ method to the __Person__ class by delegating to the __lender__ object's __borrowAmount__ method.

Alternatively, we could borrow multiple methods like this:

[source,groovy]
----
include::{projectdir}/src/spec/test/DesignPatternsTest.groovy[tags=delegation_usage2,indent=0]
----

Which adds these two methods to the __Person__ class.

Or if we want all the methods, like this:

[source,groovy]
----
include::{projectdir}/src/spec/test/DesignPatternsTest.groovy[tags=delegation_usage3,indent=0]
----

Which will make all the methods in the delegate object available in the __Person__ class.

Alternatively, we can use a map notation to rename multiple methods:

[source,groovy]
----
include::{projectdir}/src/spec/test/DesignPatternsTest.groovy[tags=delegation_usage4,indent=0]
----

== Implement Delegation Pattern using @Delegate annotation

Since version 1.6 you can use the built-in delegation mechanism which is based on AST transformation.

This make delegation even easier:

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