compose-component-design

# Compose component design ## Core principle Make reusable components caller-placeable and caller-composable: the component owns its invariant structure while callers retain placement, content, and policy choices that vary by use. ## Procedure 1. State the component's invariant visual structure and identify every varying region, placement concern, and policy choice. 2. Accept and apply a caller modifier at the component root unless a concrete API boundary makes another placement correct. 3. Represent caller-controlled, unconstrained visual regions with slots rather than proliferating primitive content parameters or Boolean shape flags. Keep semantic and design-system constraints as primitive parameters. 4. Keep simple conditional structure inline; extract only a coherent reusable contract. 5. Read the relevant focused reference below before editing public signatures. 6. Finish when callers can position the component, supply variable content, and understand ownership without needing hidden layout or content switches. ## Topic router | Signal | Read | |---|---| | Modifier parameter, root layout placement, modifier ordering, or conditional layout wrappers | [Modifier and layout](references/modifier-layout.md) | | Caller-controlled variable visual regions, optional content, primitive content parameters, or Boolean shape flags | [Slot APIs](references/slot-apis.md) | | Animation belongs to the public component contract | [Compose animations](../compose-animations/SKILL.md) | | State ownership changes while designing the component | [Compose state and effects](../compose-state-and-effects/SKILL.md) | | Semantics or screenshot coverage is needed | [Compose UI testing patterns](../compose-ui-testing-patterns/SKILL.md) | ## RED/GREEN agent scenarios 1. RED exposes `title: String`, `icon: ImageVector?`, and several display flags for a reusable card. GREEN keeps invariant chrome and gives callers the variable regions as named slots. 2. Novel case: a component needs both a root modifier and caller-supplied trailing content. GREEN applies the modifier at the root and supplies a trailing slot without leaking internal layout. 3. Counterexample: a private screen helper has one fixed child and no callers. GREEN keeps it simple instead of inventing slots for hypothetical reuse.