Jonathan Crossland
Posts: 630
Nickname: jonathanc
Registered: Feb, 2004
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Jonathan Crossland is a software architect for Lucid Ocean Ltd
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Reverse Composite
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Posted: May 22, 2009 5:46 AM
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This post originated from an RSS feed registered with .NET Buzz
by Jonathan Crossland.
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Original Post: Reverse Composite
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Not sure if there is a Design Pattern relating to this kind of need in another pattern catalogue, but it seemed simple enough to reverse the GOF composite. A class with a collection of leaves, that iterate, recurse its way up to the root. This needs work in terms of combining or actually making a variation on the composite. I think that this is perhaps only a .1 degree of separation.
Problem
You have a list of Items, that may or may not be related. There may be 1 or more trees. You have a bag of unsorted or related objects as a signle array. You need to maintain a complete list of leaves, but also need the relationship/heirarchy to become apparent. You want to express the heirarchy in a Composite Pattern, but it would exclude certain leaves has it has no part in the particular root that the composite gets built on.
Context
You need a collection of leaves, from across the composite tree. You need to have access to the parent and/or recurse to the root.
Types are stripped from its branches.
Structure
A class that works with a collection of leaf classes, that each point to it's parent. In effect a composite in reverse. Keep all the leaf objects in a collection and build the heirarchy from the bottom up.
One of the main reasons behind this is multiple paths to a set endpoint. The root becomes the set endpoint where by we can plot a course from a leaf. The Leaf though is where we start from and full knowledge of all leaves should be known at the outset.
By adding a Parent on the Composite (GoF), you can achieve a similar result, but you will not have a flat list of leaves.
Read: Reverse Composite
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