Testing Lazy Lists

Testing Lazy Lists — Tag the Container, Scroll by Semantics, Probe layoutInfo

LazyColumn / LazyRow / LazyVerticalGrid only compose items that are or were near the viewport. That breaks the naive "find by text and assert" flow because off-screen items may not exist, may exist as detached semantic nodes, or may exist with wrong bounds. This skill encodes the patterns androidx itself uses — tag the container, scroll via the semantic action or via testTag + a swipe gesture, verify with assertIsDisplayed, and read LazyListState.layoutInfo for ground truth.

When to use this skill

• The developer is writing a test for any Lazy* composable (LazyColumn, LazyRow, LazyVerticalGrid, LazyHorizontalGrid, LazyVerticalStaggeredGrid).
• A test asserts an item is "present" but the developer cannot tell whether it is on-screen.
• The developer wants to programmatically scroll to index 7, key "key_10", or by an offset.
• A test asserts on a per-item index/offset and needs the canonical state.layoutInfo shape.
• The developer is testing Modifier.animateItem() or any item placement animation.
• A reviewer asks why assertExists is wrong for a lazy item.

When NOT to use this skill

• The list under test is a non-lazy Column/Row/FlowRow. Use ../structuring-a-compose-test/SKILL.md plus ../../assertions/asserting-node-state-and-text/SKILL.md.
• The class skeleton itself is wrong; fix ../structuring-a-compose-test/SKILL.md first.
• The scroll action is a custom Modifier.scrollable (not LazyListState-backed); use ../../actions/injecting-touch-gestures/SKILL.md.
• The test is for animation timing; layer ../../synchronization/testing-animations-deterministically/SKILL.md.

Prerequisites

• Class skeleton from ../structuring-a-compose-test/SKILL.md (@MediumTest + @RunWith(AndroidJUnit4::class) + createComposeRule(StandardTestDispatcher())).
• A const tag for the container in production source (skydoves directive #1):

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const val LazyListTag = "LazyListTag"

• Each item carries Modifier.testTag(it) where it is the stable item key (matches compose/foundation/foundation/integration-tests/lazy-tests/src/androidTest/kotlin/androidx/compose/foundation/lazy/list/LazyListTest.kt:285-296).

Workflow

• 1. Tag the container, tag each item by key. Tags belong in production source.

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// production
LazyColumn(modifier = Modifier.testTag(LazyListTag), state = state) {
    items(snacks, key = { it.id }) { snack ->
        Row(Modifier.testTag(snack.id.toString())) { /* … */ }
    }
}

• 2. Scroll to an index via the semantic action. PREFERRED for tests that do not care which Lazy* composable is under test.

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rule.onNode(hasScrollAction()).performScrollToIndex(7)

• 3. Scroll via the container tag. Use this when the test must disambiguate among multiple scrollable nodes.

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rule.onNodeWithTag(LazyListTag).performScrollToIndex(7)

• 4. Scroll to a key. This is the most stable scrolling primitive — items can be added or reordered without breaking the test.

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rule.onNode(hasScrollToKeyAction()).performScrollToKey("key_10")

Cited from compose/ui/ui-test/src/androidDeviceTest/kotlin/androidx/compose/ui/test/actions/ScrollToKeyTest.kt:70-80.

• 5. Scroll by raw gesture when the container has no scroll semantics. Fall back to a touch swipe.

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rule.onRoot().performTouchInput { swipeUp() }
// or, scoped to the list container:
rule.onNodeWithTag(LazyListTag).performTouchInput { swipeUp() }

• 6. Verify visibility with assertIsDisplayed, not assertExists. Lazy items can be composed but off-screen, or removed from composition entirely depending on beyondBoundsItemCount. assertExists only proves the node is in the semantics tree.

• 7. Count visible children with .onChildren().assertCountEquals(N). This counts what the merged tree currently surfaces, NOT every backed item.

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rule.onNodeWithTag(LazyListTag).onChildren().assertCountEquals(3)

• 8. For ground truth, read state.layoutInfo inside runOnIdle. This is the highest-signal probe.

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rule.runOnIdle {
    val keys = state.layoutInfo.visibleItemsInfo.map { it.key }
    assertThat(keys).isEqualTo(listOf(0))
}

Cited from compose/foundation/foundation/integration-tests/lazy-tests/src/androidTest/kotlin/androidx/compose/foundation/lazy/list/LazyListRequestScrollTest.kt:155-184 (the requestScrollToItem test reads state.firstVisibleItemIndex, state.firstVisibleItemScrollOffset, and state.visibleKeys).

• 9. For animated item placement, set mainClock.autoAdvance = false and step frames manually. See skydoves directive #3.

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@Before
fun before() {
    rule.mainClock.autoAdvance = false
}

@Test
fun reorderTwoItems() {
    var list by mutableStateOf(listOf(0, 1))
    rule.setContent { LazyList { items(list, key = { it }) { Item(it) } } }

    assertPositions(0 to 0f, 1 to itemSize)
    rule.runOnUiThread { list = listOf(1, 0) }

    onAnimationFrame { fraction ->
        assertPositions(
            0 to 0 + itemSize * fraction,
            1 to itemSize - itemSize * fraction,
            fraction = fraction,
        )
    }
}

private fun onAnimationFrame(duration: Long = Duration, onFrame: (fraction: Float) -> Unit) {
    require(duration.mod(FrameDuration) == 0L)
    rule.waitForIdle()
    rule.mainClock.advanceTimeByFrame()
    var expectedTime = rule.mainClock.currentTime
    for (i in 0..duration step FrameDuration) {
        val fraction = i / duration.toFloat()
        onFrame(fraction)
        if (i < duration) {
            rule.mainClock.advanceTimeBy(FrameDuration)
            expectedTime += FrameDuration
            assertThat(expectedTime).isEqualTo(rule.mainClock.currentTime)
        }
    }
}

Cited from compose/foundation/foundation/integration-tests/lazy-tests/src/androidTest/kotlin/androidx/compose/foundation/lazy/list/LazyListItemPlacementAnimationTest.kt:120-148, 1724-1738. Detail in ../../synchronization/testing-animations-deterministically/SKILL.md.

• 10. Parameterize vertical vs horizontal with a base class. Mirror BaseLazyListTestWithOrientation to share scrolling helpers between LazyColumn and LazyRow tests.

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@RunWith(Parameterized::class)
class MyLazyTest(orientation: Orientation) : BaseLazyListTestWithOrientation(orientation) {
    companion object {
        @JvmStatic
        @Parameterized.Parameters(name = "{0}")
        fun params() = arrayOf(Orientation.Vertical, Orientation.Horizontal)
    }
}

Cited from compose/foundation/foundation/integration-tests/lazy-tests/src/androidTest/kotlin/androidx/compose/foundation/lazy/list/BaseLazyListTestWithOrientation.kt:55-114.

Patterns

Pattern: assertExists vs assertIsDisplayed

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// WRONG
@Test
fun item7Visible() {
    rule.setContent { LazyColumn(Modifier.testTag(LazyListTag)) { items(100) { Item("item-$it") } } }
    rule.onNodeWithTag(LazyListTag).performScrollToIndex(7)
    rule.onNodeWithTag("item-7").assertExists()
}
// WRONG because: assertExists only proves the node is in the semantics tree. Lazy lists can
// keep prefetched / detached items in semantics; an item composed out of viewport will pass
// assertExists yet not be on-screen.

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// RIGHT
@Test
fun item7Visible() {
    rule.setContent { LazyColumn(Modifier.testTag(LazyListTag)) { items(100) { Item("item-$it") } } }
    rule.onNodeWithTag(LazyListTag).performScrollToIndex(7)
    rule.onNodeWithTag("item-7").assertIsDisplayed()
}

Pattern: counting items — onChildren() vs state.layoutInfo

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// WRONG
@Test
fun has100Items() {
    rule.setContent { LazyColumn(Modifier.testTag(LazyListTag)) { items(100) { … } } }
    rule.onNodeWithTag(LazyListTag).onChildren().assertCountEquals(100)
}
// WRONG because: only the items currently composed appear as children. The viewport may
// contain three. The total item count lives on LazyListState, not the semantics tree.

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// RIGHT
@Test
fun has100Items() {
    lateinit var state: LazyListState
    rule.setContent {
        state = rememberLazyListState()
        LazyColumn(Modifier.testTag(LazyListTag), state = state) { items(100) { Item(it) } }
    }
    rule.runOnIdle {
        assertThat(state.layoutInfo.totalItemsCount).isEqualTo(100)
        assertThat(state.layoutInfo.visibleItemsInfo).isNotEmpty()
    }
}

Pattern: scroll by tag for disambiguation

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// WRONG
@Test
fun scrollList() {
    rule.setContent {
        Column {
            LazyColumn { items(20) { … } }                          // list A
            LazyColumn(Modifier.testTag(LazyListTag)) { items(20) { … } } // list B
        }
    }
    rule.onNode(hasScrollAction()).performScrollToIndex(5)
}
// WRONG because: hasScrollAction matches both lists, so onNode throws "found 2 nodes". Use the
// container's tag to disambiguate.

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// RIGHT
rule.onNodeWithTag(LazyListTag).performScrollToIndex(5)

Pattern: find by tag, never by item text

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// WRONG
rule.onNodeWithText("Snack #7").assertIsDisplayed()
// WRONG because: copy edits, plurals, and i18n break every test. Skydoves directive #1.

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// RIGHT — tag each item by its stable key in production
items(snacks, key = { it.id }) { snack ->
    Row(Modifier.testTag(snack.id.toString())) { Text(snack.name) }
}
// then, in the test:
rule.onNodeWithTag(snack.id.toString()).assertIsDisplayed()

Mandatory rules

• MUST tag the container with Modifier.testTag(...) declared as a const val in production source (skydoves directive #1). MUST NOT find lazy items by their visible text.
• MUST verify visibility with assertIsDisplayed / assertIsNotDisplayed. MUST NOT use assertExists / assertDoesNotExist to prove that an item is on-screen.
• MUST use the semantic scroll action for index- or key-based scrolls: performScrollToIndex(...), performScrollToKey(...). The semantic action is what LazyListState.scrollToItem exposes via Modifier.semantics.
• MUST read state.layoutInfo inside rule.runOnIdle { … }, never directly from the test thread.
• MUST set rule.mainClock.autoAdvance = false before any test that asserts on intermediate frames of an item-placement animation. See skydoves directive #3 and ../../synchronization/testing-animations-deterministically/SKILL.md.
• MUST NOT call rule.onNodeWithTag(LazyListTag).onChildren().assertCountEquals(totalItems) to verify the total count — only composed items appear as children.
• PREFERRED: scroll by key (performScrollToKey) when testing reorderable / paginated content; the test survives data churn.
• PREFERRED: parameterize Orientation.Vertical / Orientation.Horizontal with a base class (BaseLazyListTestWithOrientation-style) instead of duplicating tests.

Verification

• The container has Modifier.testTag(LazyListTag); each item has Modifier.testTag(<stable key>).
• Every visibility assertion uses assertIsDisplayed / assertIsNotDisplayed. No assertExists is used as a stand-in for "on-screen".
• Scrolling uses performScrollToIndex / performScrollToKey (preferred) or performTouchInput { swipeUp() } (fallback when no semantics).
• Per-item index / offset assertions read state.layoutInfo.visibleItemsInfo inside runOnIdle.
• Tests for item-placement animations set rule.mainClock.autoAdvance = false (typically in @Before) and step frames manually.
• No onNodeWithText("…") is used to find a list item.

References

• Compose testing cheat sheet — finders & lazy lists: https://developer.android.com/develop/ui/compose/testing-cheatsheet
• Lists and grids: https://developer.android.com/develop/ui/compose/lists
• Canonical scroll-to-key test: compose/ui/ui-test/src/androidDeviceTest/kotlin/androidx/compose/ui/test/actions/ScrollToKeyTest.kt:50-95
• Canonical layoutInfo probe: compose/foundation/foundation/integration-tests/lazy-tests/src/androidTest/kotlin/androidx/compose/foundation/lazy/list/LazyListRequestScrollTest.kt:155-184
• Per-item test tags + scroll: compose/foundation/foundation/integration-tests/lazy-tests/src/androidTest/kotlin/androidx/compose/foundation/lazy/list/LazyListTest.kt:141, 285-296
• Item-placement animation harness: compose/foundation/foundation/integration-tests/lazy-tests/src/androidTest/kotlin/androidx/compose/foundation/lazy/list/LazyListItemPlacementAnimationTest.kt:120-148, 1724-1738
• Vertical/horizontal base class: compose/foundation/foundation/integration-tests/lazy-tests/src/androidTest/kotlin/androidx/compose/foundation/lazy/list/BaseLazyListTestWithOrientation.kt:55-114
• hasScrollToKeyAction matcher: compose/ui/ui-test/src/commonMain/kotlin/androidx/compose/ui/test/Filters.kt