MainDispatcherRule

Swaps Dispatchers.Main for a TestDispatcher for the duration of a test.

Anything that launches into viewModelScope — including the updateStateWith and updateWith helpers in :core:ui — dispatches to Dispatchers.Main, which has no implementation under a plain JVM unit test. Without this rule those tests fail with Module with the Main dispatcher had failed to initialize.

Defaults to UnconfinedTestDispatcher so coroutines run eagerly and assertions can follow the call directly. Pass a StandardTestDispatcher when a test needs to control ordering itself.

Usage

class MyViewModelTest {
    @get:Rule
    val mainDispatcherRule = MainDispatcherRule()

    @Test
    fun `does the thing`() = runTest { /* ... */}
}

Parameters