NOTE
Observer Pattern
Notifying registered observers about subject changes and managing coupling, ordering, and lifecycle.
This is a historical learning note and may contain outdated or incomplete understanding.
Observer lets a subject notify interested observers when something changes. In-process event listeners are a common form; distributed publish–subscribe extends the idea across process boundaries with different delivery semantics.
Define subscription lifecycle, error isolation, ordering, and whether notifications are synchronous. Hidden observer side effects can make control flow difficult to trace, so use events for meaningful decoupling rather than routine direct calls.