Custom layouts in Compose

Needed a custom layout that wasn't available in Compose, so I tried to create one using Layout composable. It's more involved than I expected.

The Layout composable gives you full control over measuring and positioning children, but you need to understand how Compose's measurement system works.

@Composable
fun CustomLayout(
    modifier: Modifier = Modifier,
    content: @Composable () -> Unit
) {
    Layout(
        modifier = modifier,
        content = content
    ) { measurables, constraints ->
        // Measure children
        val placeables = measurables.map { it.measure(constraints) }
        
        // Calculate size
        val width = placeables.maxOf { it.width }
        val height = placeables.sumOf { it.height }
        
        // Position and return layout
        layout(width, height) {
            var yPosition = 0
            placeables.forEach { placeable ->
                placeable.placeRelative(x = 0, y = yPosition)
                yPosition += placeable.height
            }
        }
    }
}

But here's what confused me - constraints work differently than I expected. When you measure a child, you pass constraints to it, and those constraints affect how the child measures itself. If you want a child to take all available space, you need to pass Constraints.fixed() or Constraints.loose().

Also, if you're measuring children multiple times (which you shouldn't do, but sometimes need to), make sure you understand the implications. Compose's measurement system assumes single-pass measurement for performance.

One gotcha - if you're using Modifier.layout() for simple custom positioning, that's easier than creating a full custom layout. But for complex layouts, you'll need the Layout composable.

And remember - custom layouts can hurt performance if not done carefully. Make sure you're not doing unnecessary work in the measurement or placement phases.

Thanks for reading! Check out more posts on the blog if you'd like.