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    <p>If you mean <em>design patterns</em> literally (i.e. of the GoF variety) then these would be a few relevant options:</p> <ul> <li>Command: pass a callback to the <code>C</code> items (directly or indirectly through B) so that when they want to talk back to <code>A</code> they can simply invoke this callback -- which can even have parameters</li> <li>Iterator: <code>B</code> exposes a view of its aggregate collection directly to <code>A</code>; communication between <code>A</code> and <code>C</code> is then made directly</li> <li>Mediator: Exposes notifications to which <code>A</code> and <code>C</code> might subscribe to; communication is done by posting events</li> <li>Observer: What you already suggested as the first option</li> </ul> <p>If on the other hand you really mean <em>architectural</em> patterns, then typical options are:</p> <ul> <li>Your first option, <code>A</code> subscribing to <code>C</code> events. At first sight this doesn't look like an all-around good idea unless the event is extremely useful all the time, because it requires <code>n</code> objects to aggregate a pointer back to the callback which in the worst case they could even use just once.</li> <li>Passing references to <code>A</code> is another option, but not a good one if you are going to pollute the public interface of <code>A</code> with methods just so that <code>C</code> can call back to it in very specific scenarios. It can be very effective if <code>A</code> already exposes a suitable interface, but be aware that you might need an adapter class between <code>C</code> calling back to <code>A</code> in order to not tightly couple <code>C</code> to <code>A</code>'s interface.</li> <li>A third option would be <code>A</code> iterating over (a view of) the collection provided by <code>B</code> directly and supplying callbacks to <code>C</code> instances; this has the advantages of being quite loosely coupled and that it will use the least amount of memory, but it might be a bit trickier to code.</li> </ul>
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