In the Composite pattern, every participant is referred to
as a component. By providing a common interface which components adhere to, we
ensure that we can interact with all participants in a uniform way.
The need for uniform interaction is due to the fact that we
also have composite-participants and leaf-participants. A leaf is the smallest
indivisible component in the pattern. You could think of leaves as being
A Composite is simply a component which knows how to
aggregate other components by adding and removing other components to and from
itself. It knows how to account for them and also knows how to forward messages
to each of its components. Any message to a composite will end up with
delegation to its aggregated components.
We do not care how each component gets its job done. We
simply pass messages to a component at any level of the hierarchy we choose.
Most times the message is passed to a component at the top which can be a leaf
or composite. If it is a composite, then it automatically forwards the
instructions to be carried out to each of its member components. Each component
will in turn figure out whether it is a composite or a leaf. If it is a leaf,
then it carries out basic operations on itself and returns the result if
necessary or simply updates itself. If it is a composite, then it forwards
messages to each of its member components in turn. All this transparency is
achieved because we treat each component equally. In effect, we are saying to
each component, “Take responsibility for yourself. You must figure out whether
you are an individual (Leaf) or a group (Composite).” Leaves operate on
themselves and represent themselves. Composites pass on the instructions coming
from the top to their components (which again might be leaves or composites).
In this way, we have a self traversing structure which internally takes
responsibility for ensuring that every member is properly instructed or
updated. At the top, we do not care who we are talking to. We know our
instructions will be carried out properly regardless.
In the electoral example, each component gets an instruction
to return the vote-count for candidates in the constituency. The ballot paper
is a leaf and represents the smallest indivisible unit of information in the
voting process. In pattern-talk, each vote will get an instruction to return a
vote-count. It returns 1 for the person who got that vote and 0 for other
candidates. An instruction to the constituency to count itself is forwarded to
its aggregated polling stations. The polling station also knows that it is a
composite, not a leaf. It therefore forwards vote-count requests to its
aggregates, the ballot-papers.
Notice one thing though. In the electoral example, we have a
leafy-composite situation. The component (constituency) which aggregates
polling stations is itself a leaf. This is necessary since the determination of
which party has won and so will provide the cabinet and Prime minister is decided
not by a popular vote count, but by a seat count (in the US an Electoral College count). The PM can get 48% of the popular vote-count, but because her party
wins the most seats (at the constituency level) she wins the elections and her
party goes into power.
The example has been simplified somewhat. Ballots are
counted in boxes and sometimes they are spoiled and count for nothing. In large
constituencies, there are polling stations within polling divisions. In some
countries there are more than two parties and coalitions are formed between
several parties. Also, on a ballot where you are able to vote for several
positions and propositions at the same time, the ballot itself becomes a
composite and the vote on each issue becomes the leaf. But let us keep the
example simple for now.