compose-slot-api-pattern

Replace primitive content parameters like title: String and leadingIcon: ImageVector with @Composable () -> Unit slots such as headlineContent and leadingContent. The component then owns layout structure while callers supply whatever composables each region needs.

Use a scope receiver when the slot emits into a matching layout container and callers need its modifiers, such as Modifier.weight inside a Row. Declare actions: @Composable RowScope.() -> Unit so callers get the same capabilities as Material's TopAppBar actions slot.

Prefer (@Composable () -> Unit)? = null over an empty lambda default. A null slot lets the component skip the container, spacing, and padding entirely, while an empty lambda still allocates layout space for content that renders nothing.

Skip slots for single-use components, design-system primitives that must look identical everywhere, and semantic parameters the component intentionally owns like Switch's checked state. Slot indirection adds reading cost without benefit when there is only one call site.

A sealed variant type is bounded and requires editing the component whenever an unanticipated variant appears. Slots are unbounded, so new visual shapes work without touching the component, which is why Material 3 uses slots for ListItem and TopAppBar.