Measurable

fun measure(constraints: Constraints): Placeable

Measures the layout with constraints, returning a Placeable layout that has its new size. A Measurable can only be measured once inside a layout pass.

val Measurable.constraintLayoutId: Any?

val Measurable.constraintLayoutTag: Any?

val Measurable.layoutId: Any?

Retrieves the tag associated to a composable with the Modifier.layoutId modifier. For a parent data value to be returned by this property when not using the Modifier.layoutId modifier, the parent data value should implement the LayoutIdParentData interface.

Example usage:

import androidx.compose.foundation.layout.Box
import androidx.compose.ui.Modifier
import androidx.compose.ui.layout.Layout
import androidx.compose.ui.layout.layout
import androidx.compose.ui.layout.layoutId
import androidx.compose.ui.unit.Constraints

Layout({
    // Here the Containers are only needed to apply the modifiers. You could use the
    // modifier on header and footer directly if they are composables accepting modifiers.
    Box(Modifier.layoutId("header")) { header() }
    Box(Modifier.layoutId("footer")) { footer() }
}) { measurables, constraints ->
    val placeables =
        measurables.map { measurable ->
            when (measurable.layoutId) {
                // You should use appropriate constraints. Here we measure fake constraints.
                "header" -> measurable.measure(Constraints.fixed(100, 100))
                "footer" -> measurable.measure(constraints)
                else -> error("Unexpected tag")
            }
        }
    // Size should be derived from children measured sizes on placeables,
    // but this is simplified for the purposes of the example.
    layout(100, 100) { placeables.forEach { it.placeRelative(0, 0) } }
}