Testing ViewModels

Create a fresh testMediator and ViewModel inside each test. Register the handler body the test needs, drive the ViewModel, then assert:

@Test
fun `createOrder success updates stateFlow`() = runTest {
    val mediator = testMediator {
        handle<CreateOrderCommand, OrderResult> { OrderResult(orderId = "ORD-1", responseTime = 10) }
    }
    val vm = OrderViewModel(mediator)

    vm.createOrder("ORD-1", 99.0)
    advanceUntilIdle()

    val state = vm.stateFlow.value
    assertEquals("ORD-1", state.orderResult?.orderId)
    assertFalse(state.isLoading)
    assertNull(state.error)
}

@Test
fun `createOrder failure sets error`() = runTest {
    val mediator = testMediator {
        handle<CreateOrderCommand, OrderResult> { throw RuntimeException("Network unavailable") }
    }
    val vm = OrderViewModel(mediator)

    vm.createOrder("1", 99.0)
    advanceUntilIdle()

    assertEquals("Network unavailable", vm.stateFlow.value.error)
    assertNull(vm.stateFlow.value.orderResult)
}

@Test
fun `initial state is empty and not loading`() {
    val vm = OrderViewModel(testMediator { }) // real empty mediator — the test never calls send
    assertEquals(OrderUiState(), vm.stateFlow.value)
}

Need to change a handler's behaviour between calls in the same test? Build a fresh testMediator { } for each phase, or keep a mutable response the handler reads:

@Test
fun `error is cleared on next call`() = runTest {
    var fail = true
    val mediator = testMediator {
        handle<CreateOrderCommand, OrderResult> {
            if (fail) throw RuntimeException("first failure") else OrderResult(orderId = "ORD-2")
        }
    }
    val vm = OrderViewModel(mediator)

    vm.createOrder("1", 10.0); advanceUntilIdle()
    assertNotNull(vm.stateFlow.value.error)

    fail = false
    vm.createOrder("2", 20.0); advanceUntilIdle()
    assertNull(vm.stateFlow.value.error)
    assertEquals("ORD-2", vm.stateFlow.value.orderResult?.orderId)
}

To assert which commands the ViewModel dispatched, read the recording — mediator.sentOf<CreateOrderCommand>(). See testMediator.

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