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| ////////////////////////////////////////// |
| |
| = Composite Pattern |
| |
| The http://en.wikipedia.org/wiki/Composite_pattern[Composite Pattern] allows you to treat single instances of an object the same way as a group of objects. The pattern is often used with hierarchies of objects. Typically, one or more methods should be callable in the same way for either __leaf__ or __composite__ nodes within the hierarchy. In such a case, composite nodes typically invoke the same named method for each of their children nodes. |
| |
| == Example |
| |
| Consider this usage of the composite pattern where we want to call `toString()` on either `Leaf` or `Composite` objects. |
| |
| image::assets/img/CompositeClasses.gif[] |
| |
| In Java, the `Component` class is essential as it provides the type used for both leaf and composite nodes. In Groovy, because of duck-typing, we don't need it for that purpose, however, it can still serve as a useful place to place common behaviour between the leaf and composite nodes. |
| |
| For our purposes, we will assemble the following hierarchy of components. |
| |
| image::assets/img/CompositeComponents.gif[] |
| |
| Here is the code: |
| |
| [source,groovy] |
| ---- |
| include::{projectdir}/src/spec/test/DesignPatternsTest.groovy[tags=composite_code,indent=0] |
| ---- |
| |
| Here is the resulting output: |
| |
| ---- |
| root |
| -leaf A |
| -comp B |
| --leaf B1 |
| --leaf B2 |
| -leaf C |
| ---- |