Match the context details with the pattern that applies. An object contains other objects (elements) and must give access to them for clients, without exposing its internal structure.
A subject object generates events and one or more objects need to be notified when an event occurs.
An object could use different variations of an algorithm and the client of that object wants to supply a custom version of that algorithm.
An object aggregates primitive objects and clients treat the aggregate object as a primitive object, too.
An object's behavior must be enhanced without adding more responsibilities to it.
Observer, iterator, decorator, strategy, composite