Understanding the Composite Pattern
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by David Simmonds
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I suggest that you apply the following order and principles when working on composite projects.

·         Allow the client to control invocation of the addition and removal logic.

·         Get the additions and then the removals right.

Then deal with the each Implementation-specific functionality, one at a time.

So, if we are dealing with find-accomodation, deal with find-accomodation across component, composite and leaf. Then move on to deal with bookings across component, composite and leaf. Finally, move on to unbooking across component, composite and leaf

Deal with subsidiary functions first. So for example, deal with the find-accomodation first. This is used before you can do the booking, so in the same way that we would be guided by general software development principles to deal with it first (composing bigger functionality from primitive functions), similarly, would we do it here.

Test the solution thoroughly. Remember you are not done coding until you have tested and this holds true for every software project.

Now you will notice that Composite can be a real pain to test, since you have to repeatedly create composites and leaves before you can test operations on them. And depending on the number of functionalities you have to test, this can be horrendous.

My suggestion is to create the object-hierarchy of test-data and include a routine which uses some persistence mechanism to persist-to-disk and restore the hierarchy (possibly with an option to restore from several backups). You can then make changes to it, tweak it, add test cases and remove extraneous stuff. In-between tests, you persist the object-hierarchy. This not only saves you time and frustration (plus allowing other developers to code and test the solution simultaneously), but it also helps you to ensure consistency in the tests.

Composite/Treeview/XML: a natural fit

Composite is one of the most elegant and intuitive patterns. Later on when we look at the Interpreter pattern, you will see where it can save you from knowing a lot of XPATH. I load the XML file straight into a composite object-hierarchy which is linked to a treeview, do all the processing in the Composite hierarchy (including searching) and then store the results back into XML.


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