Behavioral patterns

In this module

• Define a family of interchangeable algorithms, put each in its own class, and let the client swap them at runtime, instead of a big if/else picking behaviour.
• Define a one-to-many dependency: when one object changes, all its subscribers are notified automatically, without it knowing who they are.
• Let an object change its behaviour when its internal state changes, so it appears to switch class, by delegating to a State object instead of a wall of if/else.
• Turn a request into a standalone object, so you can parameterize, queue, log, and UNDO actions, and decouple the thing that triggers an action from the thing that performs it.
• Pass a request along a chain of handlers; each either handles it or forwards it to the next, so the sender doesn't need to know who will deal with it.
• Put the fixed skeleton of an algorithm in a base-class method, and let subclasses fill in the specific steps, without changing the overall structure or order.
• Walk through a collection's elements one by one WITHOUT exposing how the collection stores them, and support multiple independent walks at once.
• Stop components from referencing each other directly; route all their interaction through ONE mediator, turning a tangled web (N×N) into a clean hub (N).