Recap: Observer Pattern

The subject keeps a list and calls it; whoever subscribes must also unsubscribe.

Observer lets any number of objects subscribe to one, and be called when it changes.

Intent. Define a one-to-many dependency so that when one object changes, all its dependants are notified.
Participants. The subject with attach, detach and notify; the observer interface with update; their concrete classes.
Push or pull. update carries the new data, or the observer asks the subject for it.
In the wild. Node's EventEmitter with on, off and emit, and the browser's addEventListener.
In Python. A list of callbacks is a subject; keep a reference to any callback you will remove.
The trap. A lapsed listener keeps its object alive and still reacts; pair every attach with a detach.
When not. One fixed dependant, reactions whose order matters, or chains of notifications nobody can follow.

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