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):

const val LazyListTag = "LazyListTag"