Event Bus with Emit, Subscribe, and Unsubscribe

Requirements

• Create an interface representing an event listener.
• Build an EventBus class.
• Provide subscribe(event, listener) for registering a listener with an event.
• Provide emit(event, payload) so every listener registered for that event receives the payload.
• Follow-up: allow several listeners to be attached to the same event.
• Follow-up: allow an individual subscription for one event to be cancelled.

A suitable API shape is:

interface Listener {
    onEvent(payload)
}

class EventBus {
    subscribe(event, listener)
    emit(event, payload)
}

Examples

Example 1

bus = EventBus()
bus.subscribe("door.opened", listenerA)
bus.emit("door.opened", {"room": "lab"})

Output: listenerA receives {"room": "lab"}.

The listener is called because it is registered for "door.opened".

Example 2

bus = EventBus()
bus.subscribe("job.done", listenerA)
bus.subscribe("job.done", listenerB)
bus.emit("job.done", 42)

Output: listenerA and listenerB each receive 42.

Both subscriptions match the emitted event, so both listeners run.

Example 3

bus = EventBus()
cancel = bus.subscribe("message", listenerA)
cancel()
bus.emit("message", "hello")

Output: no listener is invoked.

Calling the returned cancellation operation removes listenerA from that event before the emission occurs.

Preparation

• Begin with subscribe, emit, and unsubscribe, making sure one event can retain multiple listeners; afterward, consider changing subscribe to return a cancellation callback or a subscription token.
• Exercise the design with tests for registration order, repeated registration of the same listener, cancellation while an emission is in progress, listener failures, and events that have no listeners.
• Be ready to explain the trade-offs between synchronous and asynchronous delivery and whether an exception from one listener should prevent later listeners from running.