compose-slot-api-pattern

Compose: slot API pattern

Core principle

A reusable Compose component describes layout structure. Callers provide variable visual content through slots.

API review procedure

1. Confirm the component is reusable. For a true single-use composable, do not add slot ceremony.
2. Mark which regions vary by caller: headline, supporting text, leading visual, trailing visual, actions, body.
3. Replace caller-controlled primitive content and shape flags with slots.
4. Add receiver scopes only when the slot is emitted inside a layout whose scope APIs callers should use.
5. Make absent optional regions nullable (null), so the component can omit their containers and spacing.
6. Put repeated default content or tokens in XxxDefaults.
7. Pair this with the modifier rules in compose-modifier-and-layout-style.

1. Replace primitive content with @Composable slots

Where the component asks for caller-controlled content, prefer a @Composable () -> Unit slot. Where the slot is structurally required, leave it non-nullable with no default. Where it's optional, make it nullable with a null default.

// ❌ BAD — primitive parameters; trailing area is the only slot; everything else is locked
@Composable
fun SettingsRow(
    title: String,
    onClick: () -> Unit,
    modifier: Modifier = Modifier,
    subtitle: String? = null,
    leadingIcon: ImageVector? = null,
    trailing: (@Composable () -> Unit)? = null,
) { … }

// ✅ GOOD — every visual region is a slot; the row describes structure, not content
@Composable
fun SettingsRow(
    headlineContent: @Composable () -> Unit,
    onClick: () -> Unit,
    modifier: Modifier = Modifier,
    supportingContent: (@Composable () -> Unit)? = null,
    leadingContent: (@Composable () -> Unit)? = null,
    trailingContent: (@Composable () -> Unit)? = null,
) { … }

Call sites stay short when the typical content is a one-liner:

SettingsRow(
    headlineContent = { Text("Account") },
    leadingContent = { Icon(Icons.Default.Person, contentDescription = null) },
    trailingContent = { SettingsRowDefaults.Chevron() },
    onClick = { … },
)

The unusual cases no longer require new component parameters:

SettingsRow(
    headlineContent = {
        Row(verticalAlignment = Alignment.CenterVertically) {
            Text("Inbox")
            Spacer(Modifier.width(8.dp))
            Badge { Text("3") }
        }
    },
    onClick = { … },
)

Slot naming

• Use xxxContent for free-form @Composable () -> Unit slots (headlineContent, supportingContent, trailingContent) — matches Material 3.
• Use a singular noun (title, icon, actions) when the slot is semantically constrained and the component name disambiguates (Scaffold(topBar = { … }, bottomBar = { … }, floatingActionButton = { … })).
• Don't use content and other xxxContent slots together — pick one convention per component.

2. Scope receivers when the slot emits into a layout

If the slot's content will sit inside a Row/Column/Box whose layout features (Modifier.weight, BoxScope.matchParentSize, alignment) should be available to the caller, declare the slot as a receiver lambda: @Composable RowScope.() -> Unit.

// ❌ BAD — actions render inside a Row, but callers can't use RowScope.weight()
@Composable
fun MyTopBar(
    title: @Composable () -> Unit,
    actions: @Composable () -> Unit = {},   // ← caller has no Row scope
)

// ✅ GOOD — caller gets RowScope; .weight() and alignment-by works inside
@Composable
fun MyTopBar(
    title: @Composable () -> Unit,
    actions: @Composable RowScope.() -> Unit = {},
)

This is what makes TopAppBar(actions = { IconButton(…); IconButton(…) }) work — the caller is implicitly inside a RowScope.

Don't bolt a scope receiver onto every slot reflexively. The receiver should match the actual parent layout the slot emits into. If the slot is rendered inside a Box, use BoxScope. If it's inside a Column, use ColumnScope. If the parent is not a standard layout (or none of its scope APIs are useful in slot content), no receiver.

3. Optional slots — nullable with null default

For slots that may be absent, prefer (@Composable () -> Unit)? = null over @Composable () -> Unit = {}:

// ❌ BAD — empty default; "no leading content" is the empty lambda
leadingContent: @Composable () -> Unit = {}

// ✅ GOOD — null means "no slot"; the component can omit space/padding when absent
leadingContent: (@Composable () -> Unit)? = null

With a nullable slot, the component can branch on leadingContent != null and skip the slot's container, spacing, and padding entirely. With an empty default, the layout often still allocates space for absent content.

4. Defaults live in XxxDefaults

When you find yourself documenting "the trailing slot should usually be a chevron" or "pass MaterialTheme.colorScheme.surface for the default background", co-locate the helpers in a XxxDefaults object next to the component:

object SettingsRowDefaults {
    @Composable
    fun Chevron() = Icon(
        imageVector = Icons.AutoMirrored.Filled.KeyboardArrowRight,
        contentDescription = null,
    )

    @Composable
    fun TrailingValue(text: String) = Text(
        text = text,
        style = MaterialTheme.typography.bodyMedium,
        color = MaterialTheme.colorScheme.onSurfaceVariant,
    )
}

Call sites stay declarative for the common cases and the slot is still fully open for one-offs:

SettingsRow(
    headlineContent = { Text("Notifications") },
    trailingContent = { SettingsRowDefaults.Chevron() },
    onClick = { … },
)

This matches Material 3's ButtonDefaults, TopAppBarDefaults, etc. — defaults that are themselves composable belong here, not as new component parameters with MaterialTheme.x.y defaults expanded inline.

Quick reference

Symptom	Diagnosis	Fix
title: String, subtitle: String?, leadingIcon: ImageVector? on a reusable component	Primitive content params (§1)	Convert to xxxContent: (@Composable () -> Unit)? slots
Multiple boolean flags (showChevron, showSwitch) selecting trailing shapes	Enumerating shapes (§1)	One trailingContent: (@Composable () -> Unit)? slot
A mode: Mode.Sealed parameter listing variants	Same as flag soup (§1)	Slot it
actions: @Composable () -> Unit = {} inside a Row body	Missing scope receiver (§2)	actions: @Composable RowScope.() -> Unit = {}
slot: @Composable () -> Unit = {} for an optional area	Empty-lambda default (§3)	slot: (@Composable () -> Unit)? = null and branch on it
Component param defaultColor: Color = MaterialTheme.colorScheme.surface	Defaults inlined (§4)	Move to XxxDefaults.color and reference it
Common trailing content repeats at every call site	Missing default helper (§4)	Add XxxDefaults.Chevron() etc.

When NOT to apply

• Single-use components. A composable used in exactly one place, with no plan to reuse, doesn't benefit from slot flexibility — and the slot indirection makes the code harder to read for the one reader. Primitive params + inline content is fine. (As soon as a second call site appears, slot it.)
• Design-system primitives where every caller must look identical. A Heading2(text: String) exists because you want every H2 to look the same; making it headlineContent: @Composable () -> Unit invites callers to break the rule. Keep it primitive. (Conversely: if Heading2 ever needs a badge inline, slot it.)
• Semantic parameters the component intentionally owns. If the component owns typography, iconography, accessibility wording, or product consistency, a primitive parameter may be the constraint you want.
• Constrained-type parameters that genuinely are constrained. A Switch(checked: Boolean, onCheckedChange: ...) doesn't need its checked indicator to be a slot. Booleans-with-callbacks are not "content."
• Performance-critical fast paths (rare in app code; common in framework primitives). A slot is an allocated lambda. In the deepest LazyList item layer, sometimes primitives win. If you're not writing the framework, this doesn't apply.

Red flags during review

Thought	Reality
"Title is always a String — making it a slot is over-engineering"	"Always today" is the trap. Material's ListItem.headlineContent exists because tomorrow someone wants a Text + Badge. The slot is 8 characters of extra wrapping at every call site ({ Text(…) }); the refactor to add a slot later edits every existing call site.
"Lambdas are heavier than strings"	At the scale of typical Compose UI, this isn't measurable — and the framework's own components (Button, ListItem, TopAppBar, Scaffold) all slot. If your component is in the hottest of hot paths, see "When NOT to apply."
"I'll add a slot later if someone asks"	The slot turns one parameter into two parameters (the slot itself + maybe an internal flag) and edits every call site. The shape change isn't a "later" change.
"I'll model the variants with a sealed Trailing type instead"	Sealed enumeration is bounded; slots are unbounded. A sealed type works until the day someone needs a variant you didn't anticipate — at which point you're back to editing the component. The slot avoids the cycle.
"The leading area is always an icon, the trailing area varies — I'll slot only the trailing"	This is the partial-slot trap. The "always-an-icon" assumption breaks the first time a row needs an avatar or a flag emoji or a coloured shape. Slot leading too.
"There's only one call site today"	If there's only one call site, you're probably not designing a reusable component yet. See "When NOT to apply" — primitives are fine for a true single-use. The moment you copy-paste it, slot it.

Related

• compose-modifier-and-layout-style — the modifier-parameter rule (§1–§3 there) travels with slot APIs. A reusable component takes a modifier parameter and slots its content; the caller owns both placement and what to place.