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Jetpack Compose constraint and single-pass layout model

Document status: reviewed. Canonical source.

Summary

Jetpack Compose lays out a UI tree by passing constraints to children, measuring each child, deciding parent size and then placing children. Standard layout prohibits measuring a child more than once; intrinsic measurement and subcomposition are separate mechanisms.

Evidence

  • The layout basics define a single pass in which parents initiate measurement, constraints descend, and resolved sizes and placement instructions return up the tree. Measurement and placement are distinct sub-phases. https://developer.android.com/develop/ui/compose/layouts/basics#the-layout-model (retrieved 2026-08-29).
  • Custom layout requires measuring children, deciding size and placing children. Compose rejects ordinary multi-pass child measurement. https://developer.android.com/develop/ui/compose/layouts/custom (retrieved 2026-08-29).
  • Modifier order changes the constraint and layout nodes wrapped around a composable. Equivalent visible output does not imply equivalent authored modifier structure. https://developer.android.com/develop/ui/compose/layouts/constraints-modifiers (retrieved 2026-08-29).

NUIF relevance

Borrow bounded min/max constraints, intrinsic queries, row/column alignment and the measurement/placement separation for a lowering profile.

Adapt NUIF stack/flex semantics into a generated, profile-owned Kotlin DSL subset with stable identity comments or modifiers. Compile and screenshot tests require a pinned Android Gradle Plugin, Compose version, SDK and font set.

Reject arbitrary Kotlin/Compose import as a lossless document operation. Composable execution, state, modifier order, subcomposition and platform resources require runtime evaluation and cannot be recovered from resolved geometry alone.