instrumentation-testing.md

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31. Instrumentation Tests & UI Testing

Compose UI Testing

Tip

Use composeRule.setContent {} to render UI, .onNodeWithText() to find nodes, .performClick() for interactions, .assertIsDisplayed() for assertions.

composeRule · Semantics tree queries · Scroll actions · Assertions

💻 Code Example

@RunWith(AndroidJUnit4::class)
class MyScreenTest {
    @get:Rule val composeRule = createComposeRule()

    @Test
    fun testButtonClick() {
        composeRule.setContent {
            var count by remember { mutableIntStateOf(0) }
            Column {
                Text("Count: $count")
                Button(onClick = { count++ }) { Text("Click me") }
            }
        }
        composeRule.onNodeWithText("Click me").performClick()
        composeRule.onNodeWithText("Count: 1").assertIsDisplayed()
    }
}

🔩 Under the Hood

Compose Test API

composeRule.setContent(): Renders composable in test environment, not emulator activity. No Android context overhead (unit-like speed).

onNodeWithText(): Queries semantics tree (not visual). Finds nodes by contentDescription, text, or test tags.

performClick(): Sends click event to semantics node. Triggers recomposition if state changes.

User vs Understander

A user knows	An understander also knows
"Find nodes by text"	Searches semantics tree, not visual hierarchy. Text nodes tagged in semantics during composition.
"performClick triggers recomposition"	Click event posted → state update → recomposition → new tree. Test continues (no Thread.sleep needed).
"No activity context needed"	ComposeTestRule creates synthetic test context. Avoids full activity startup (faster than Espresso).

Espresso for Legacy Views

Tip

Use ActivityScenarioRule to launch activity. onView(withId()) to find views. perform(click()) for actions. check(matches()) for assertions.

ActivityScenarioRule · View matchers · ViewActions · Idling

💻 Code Example

@RunWith(AndroidJUnit4::class)
class ActivityTest {
    @get:Rule val activityRule = ActivityScenarioRule(MainActivity::class.java)

    @Test
    fun testButtonClick() {
        onView(withId(R.id.button)).perform(click())
        onView(withText("Success")).check(matches(isDisplayed()))
    }

    @Test
    fun testRecyclerView() {
        onView(withId(R.id.recycler)).perform(
            RecyclerViewActions.scrollToPosition<UserViewHolder>(20)
        )
        onView(withText("User 20")).check(matches(isDisplayed()))
    }
}

🔩 Under the Hood

Espresso Idling & Synchronization

Espresso waits for app idle before performing actions (waits for background tasks, animations).

Custom IdlingResource: If Espresso can't detect idleness (custom thread pool), register IdlingResource.

User vs Understander

A user knows	An understander also knows
"Espresso waits for idle"	Espresso checks for background tasks/animations. If idle, proceeds. If not, retries for ~5s before failing.
"withId/withText are matchers"	Matchers are predicates. onView finds first matching view in hierarchy. Multiple matches = ambiguous, fails.
"RecyclerViewActions scrolls to position"	Scrolls RecyclerView until item at position visible (or throws exception). Different from manual scroll.

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