The 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.
The 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:
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:
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:
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:
Which adds these two methods to the Person class.
Or if we want all the methods, like this:
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:
Since version 1.6 you can use the built-in delegation mechanism which is based on AST transformation.
This make delegation even easier: