Structuring A Compose Test

import androidx.compose.ui.test.junit4.v2.createComposeRule
import kotlinx.coroutines.test.StandardTestDispatcher

@MediumTest
@RunWith(AndroidJUnit4::class)
class MyButtonTest {
    @get:Rule val rule = createComposeRule(StandardTestDispatcher())
}

import androidx.compose.ui.test.junit4.v2.createAndroidComposeRule
@get:Rule val rule = createAndroidComposeRule<FragmentActivity>(StandardTestDispatcher())

@Test
fun switch_stateChange_movesThumb() {
    var checked by mutableStateOf(false)                          // STATE (hoisted)
    rule.setMaterialContent(lightColorScheme()) {                  // SET CONTENT (in test body)
        val spacer = @Composable { Spacer(Modifier.size(16.dp).testTag("spacer")) }
        Switch(
            modifier = Modifier.testTag(defaultSwitchTag),
            checked = checked,
            thumbContent = spacer,
            onCheckedChange = { checked = it },
        )
    }

    rule.onNodeWithTag("spacer", useUnmergedTree = true)           // FIND + ASSERT
        .assertLeftPositionInRootIsEqualTo(8.dp)

    rule.runOnIdle { checked = true }                              // ACT (mutate from outside)

    rule.onNodeWithTag("spacer", useUnmergedTree = true)           // RE-ASSERT
        .assertLeftPositionInRootIsEqualTo(28.dp)

    rule.runOnIdle { checked = false }
    rule.onNodeWithTag("spacer", useUnmergedTree = true).assertLeftPositionInRootIsEqualTo(8.dp)
}

val (index, offset) = rule.runOnIdle {
    state.firstVisibleItemIndex to state.firstVisibleItemScrollOffset
}

rule.runOnIdle { count = 5 }

// WRONG
@Test
fun toggle() {
    rule.setContent {
        var checked by remember { mutableStateOf(false) }   // <-- trapped inside composition
        Switch(checked = checked, onCheckedChange = { checked = it })
    }
    rule.runOnIdle { checked = true }                       // does not compile / has no reference
}
// WRONG because: the test thread has no reference to `checked`. The state cannot be driven
// from outside, and the test can only verify the very first frame.

// RIGHT
@Test
fun toggle() {
    var checked by mutableStateOf(false)                    // <-- hoisted to test scope
    rule.setContent {
        Switch(checked = checked, onCheckedChange = { checked = it })
    }
    rule.onNodeWithTag("switch").assertIsOff()
    rule.runOnIdle { checked = true }                       // mutate via runOnIdle
    rule.onNodeWithTag("switch").assertIsOn()
}

// WRONG
@Before
fun setUp() {
    rule.setContent { MyScreen(state) }                     // <-- shared across all tests
}

@Test fun stateA() { /* mutate `state` here, but it was already composed without this case */ }

// RIGHT
@Test
fun stateA() {
    val state = ScreenState(initial = "A")
    rule.setContent { MyScreen(state) }                     // <-- per-test composition
    rule.onNodeWithTag("title").assertTextEquals("A")
}

// WRONG
class MyTest {
    @get:Rule val rule = createComposeRule(StandardTestDispatcher())

    @Test fun a() = runComposeUiTest {                      // <-- second test environment
        setContent { /* … */ }
    }
}
// WRONG because: createComposeRule and runComposeUiTest each manage an independent test
// environment (composition + MainTestClock + IdlingResourceRegistry). Mixing them produces
// undefined behavior. KDoc warns explicitly: see the ComposeUiTest.android.kt KDoc on
// runComposeUiTest, runAndroidComposeUiTest, runEmptyComposeUiTest, and the v2 variants.

// RIGHT — pick exactly one entry point per class
class MyRuleTest {
    @get:Rule val rule = createComposeRule(StandardTestDispatcher())
    @Test fun a() { rule.setContent { /* … */ } }
}

// or
class MyFunctionTest {
    @Test fun a() = runComposeUiTest { setContent { /* … */ } }
}

// RIGHT — ComponentActivity comes free from ui-test-manifest
@get:Rule val rule = createComposeRule(StandardTestDispatcher())

// RIGHT — a custom Activity must be declared in src/androidTest/AndroidManifest.xml
@get:Rule val rule = createAndroidComposeRule<MyActivity>(StandardTestDispatcher())

<!-- src/androidTest/AndroidManifest.xml -->
<manifest xmlns:android="http://schemas.android.com/apk/res/android">
    <application>
        <activity android:name=".MyActivity" />
    </application>
</manifest>

@MediumTest                     // default — sub-second tests
class SwitchTest { /* … */ }

@LargeTest                      // long-clicks, multi-second animations
class CombinedClickableTest { /* … */ }