compose-component-design

name	compose-component-design
description	Use when designing or reviewing reusable Jetpack Compose component APIs with modifier parameters, root layout placement, caller-provided variable content, primitive content parameters, optional content, or boolean shape flags.

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
Caller-controlled variable visual regions, optional content, primitive content parameters, or Boolean shape flags	Slot APIs
Animation belongs to the public component contract	Compose animations
State ownership changes while designing the component	Compose state and effects
Semantics or screenshot coverage is needed	Compose UI testing patterns

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.