Testing Notifications

captureNotifications

captureNotifications<T>() is the quickest way to assert that a notification was published. Call it on a FakeMediator — it registers the handler and returns a live list that fills itself as notifications arrive.

@Test
fun `order placed event is published`() = runTest {
    val mediator = FakeMediator {
        +CreateOrderHandler()
    }
    val events = mediator.captureNotifications<OrderPlacedEvent>()

    mediator.send(CreateOrderCommand(id = "ORD-1", amount = 99.0))

    assertEquals(1, events.size)
    assertEquals("ORD-1", events.first().orderId)
}

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fakeNotificationHandler

When you need full control over what happens inside the handler — side effects, conditional logic, custom assertions — use fakeNotificationHandler directly:

@Test
fun `analytics is tracked on order placed`() = runTest {
    val mediator = FakeMediator {
        +CreateOrderHandler()
    }
    val tracked = mutableListOf<String>()
    mediator.registry.registerNotification(
        fakeNotificationHandler<OrderPlacedEvent> { event ->
            tracked += "tracked:${event.orderId}"
        }
    )

    mediator.send(CreateOrderCommand(id = "ORD-1", amount = 99.0))

    assertEquals("tracked:ORD-1", tracked.first())
}

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Multiple handlers for the same notification

registerNotification appends — you can register several handlers for the same type and all of them fire:

@Test
fun `all listeners receive the event`() = runTest {
    val mediator = FakeMediator {
        +CreateOrderHandler()
    }
    val emails = mediator.captureNotifications<OrderPlacedEvent>()
    val sms    = mediator.captureNotifications<OrderPlacedEvent>()

    mediator.send(CreateOrderCommand(id = "ORD-1", amount = 99.0))

    assertEquals(1, emails.size)
    assertEquals(1, sms.size)
}

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Next

→ Testing Handlers