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📜 WAYPOINT LESSON

Checkpoint — Event-Driven Notification System

⭐⭐ intermediate⏳ 22 min read📍 Lesson 100 of 265

Build a publish/subscribe notification system with typed events, ordering, and clean unsubscribes.

The gate (mini-build)

A NotificationCenter with two events:

  1. Published — EventHandler<NotificationEventArgs> raised for every notification with (Id, Channel, Message). Handlers must see args snapshots; raising with no subscribers must not throw.
  2. Filtered — raised when Publish(channel, message, filter) rejects a message; filter is a Predicate<string> the caller supplies per call.

Plus: UnsubscribeAll() clears both lists so a disposed center releases its subscribers. The graded test registers handlers, publishes through a filter, unsubscribes, and asserts the full observable behavior — order included.