Revents

Testing

• Given
  • Manipulating the time
• Then

Testing the command dispatch process and the business logic inside our aggregates can be easily done with AggregateTestHelper.

It follows the Given/When/Then approach:

• Given: initializing the aggregate with replaying some events, simulating the current state
• When: firing a command
• Then: expecting 1 or more events emitted by the system

OrderId orderId = OrderId.create();
AggregateTestHelper.testing(Order.class)
    .given(new OrderCreated(orderId))
    .when(new ApproveOrder(orderId))
    .thenExpectSingle(OrderApproved.class, event -> assertThat(event.orderId())
        .isEqualTo(orderId))

Given

The .given() method can be called without argument, which means we are testing the creation of a new aggregate. Multiple arguments can be passed, the order of the events is important as it is kept during the aggregate initialization process. Calling further .andGiven(event) method is also possible.

Manipulating the time

Revents uses ReventsClock.system() as a system clock, which is Clock.system(ZoneId.of("UTC")) by default. Overriding this clock on system level can be done via ReventsClock.overrideSystemClock(newClock). However if we only want to manipulate the time during testing then the related methods in AggregateTestHelper sould rather be used. We can override the Clock, set an Instant or a ZonedDateTime to simulate the current date/time (a.k.a. now).

Then

We also have several options to validate the aggregate’s behavior. Returning all the emitted events, or executing assertions against 1, or all events is achievable with then*() methods. Expected errors can also be validated. Using AssertJ is not mandatory, but recommended.