Master the Dependent Layout: A Deep Dive into Jetpack Compose SubcomposeLayout

 Solve complex UI dependencies by deferring composition until measurement—a pro-level guide for Android Engineers.

Jetpack Compose SubcomposeLayout

In the standard Jetpack Compose layout pipeline, data flows in one direction: Constraints go down, Sizes come up. Normally, you cannot let Child B know the exact width of Child A before Child B is even composed.

But what if you’re building a UI where a “Secondary Action” should only appear if the “Main Title” leaves enough space? Or a custom layout where a dynamic header’s height dictates the very existence of a footer?

This is where SubcomposeLayout becomes your secret weapon.

The Problem: The “One-Frame Lag” Anti-Pattern

Many developers try to solve dependency issues by using onGloballyPositioned to update a MutableState with a child's size.

Why this fails:

  1. Performance: It triggers a second recomposition immediately after the first, causing potential jank.
  2. Infinite Loops: If the state change affects the size of the thing being measured, you create a recursive layout loop that can crash your app.
  3. Complexity: You have to manage side effects, state nullability, and “empty” initial states while waiting for the first measurement.

SubcomposeLayout solves this by deferring the composition of the dependent child until the measurement of the primary child is finalized—making the logic atomic within a single UI frame.

The “Pro” Implementation: The Smart Adaptive Row

This implementation handles unbounded constraints (ensuring it won’t crash in a horizontal scroll) and uses stable slot keys to optimize recomposition.

// Use stable keys to help the Compose compiler track slots across recompositions
private enum class LayoutSlot { Main, Dependent }

@Composable
fun AdaptiveActionRow(
    mainContent: @Composable () -> Unit,
    dependentAction: @Composable (availableWidth: Int) -> Unit,
    modifier: Modifier = Modifier
) {
    SubcomposeLayout(modifier = modifier) { constraints ->
        // 1. Subcompose and measure the Main Content first
        // We use minWidth = 0 so the child isn't forced to fill the screen
        val mainPlaceables = subcompose(LayoutSlot.Main, mainContent).map {
            it.measure(constraints.copy(minWidth = 0))
        }

        val mainWidth = mainPlaceables.maxOfOrNull { it.width } ?: 0
        
        // 2. Safe Constraint Handling
        // If the parent is in a scrollable Row (maxWidth is Infinity),
        // we fallback to the mainWidth as our boundary to prevent logic errors.
        val maxAvailable = if (constraints.hasBoundedWidth) constraints.maxWidth else mainWidth
        val remainingWidth = (maxAvailable - mainWidth).coerceAtLeast(0)

        // 3. Subcompose the dependent piece with the "Knowledge" of remaining space
        val actionPlaceables = subcompose(LayoutSlot.Dependent) {
            dependentAction(remainingWidth)
        }.map {
            it.measure(
                constraints.copy(
                    minWidth = 0, 
                    maxWidth = remainingWidth // Force the child to respect the gap
                )
            )
        }

        val actionWidth = actionPlaceables.maxOfOrNull { it.width } ?: 0
        val totalWidth = if (constraints.hasBoundedWidth) constraints.maxWidth else mainWidth + actionWidth
        
        val layoutHeight = maxOf(
            mainPlaceables.maxOfOrNull { it.height } ?: 0,
            actionPlaceables.maxOfOrNull { it.height } ?: 0
        )

        // 4. Final Placement Pass
        layout(totalWidth, layoutHeight) {
            mainPlaceables.forEach { it.placeRelative(0, 0) }
            
            // Align the action to the end of the calculated row
            actionPlaceables.forEach { 
                it.placeRelative(totalWidth - actionWidth, 0) 
            }
        }
    }
}

Performance Checklist: When NOT to Subcompose

Because SubcomposeLayout manages separate composition trees for each slot, it is more computationally expensive than a standard Layout.

  • ❌ Don’t use it for Pager Indicators: Indicators depend on state (the current page index), not the measured pixel size of a sibling. A simple Row with animateDpAsState is significantly more performant and follows the Compose "state-down" philosophy.
  • ❌ Don’t use it for standard wrapping: If you need a “Flow” behavior (where items wrap to the next line), use a standard Layout that calculates offsets during the placement phase.
  • ✅ Do use it for Scaffolding: Use it when the presence or UI logic of one child is physically impossible to determine without knowing the exact dimensions of another child first.

Frequently Asked Questions (FAQs)

Why use LayoutSlot enums instead of strings?

While subcompose("Main") works, using an enum or object is a best practice. It prevents typos and ensures the slot keys are stable, which helps the Compose compiler track the subcomposition state more efficiently.

How does this differ from BoxWithConstraints?

BoxWithConstraints is actually built on top of SubcomposeLayout. The difference is scope: BoxWithConstraints gives you the parent's constraints, while SubcomposeLayout gives you the power to measure a sibling first and use those results to decide what to compose next.

Does this handle multi-line wrapping?

The example above is optimized for a single row. If you want the dependentAction to wrap to a second line, you would need to adjust the layoutheight and placeRelative logic to account for a Y-offset.

Join the Conversation!

  • The Infinite Width Challenge: How would you modify this code if you wanted the mainContent to shrink to give the dependentAction at least 50dp of space?
  • Real-World Scenarios: Where is the one place in your current app where a standard Row or ConstraintLayout just isn't cutting it?
  • State vs. Size: Have you ever accidentally used SubcomposeLayout for something that could have been solved with simple animate*AsState?

Let’s discuss in the comments below!

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