Layout

A composable that measures and places its content by a MeasurePolicy of your own: a container whose placement rules you write, with no LayoutManager to go with them.

Row, Column and Box are each one of these; a policy you write is asked the same questions they are. Children are written plainly, and the policy is handed one Measurable per child in declaration order:

Layout(
    content = {
        Label(text = "Status")
        Button(text = "Close", onClick = ::close)
    },
    measurePolicy = { measurables, constraints ->
        // A stack imposes no minimum of its own: each child takes the height left after earlier ones.
        var remainingHeight = constraints.maxHeight
        val placeables = measurables.map { measurable ->
            val placeable = measurable.measure(
                Constraints(maxWidth = constraints.maxWidth, maxHeight = remainingHeight)
            )
            remainingHeight = (remainingHeight - placeable.height).coerceAtLeast(0)
            placeable
        }
        // The extent is named from the children and held inside the offer. Under an unbounded offer,
        // the same body answers the unbounded question `preferredLayoutSize` asks.
        val width = constraints.constrainWidth(placeables.maxOfOrNull { it.width } ?: 0)
        val height = constraints.constrainHeight(constraints.maxHeight - remainingHeight)
        layout(width, height) {
            var y = 0
            for (placeable in placeables) {
                placeable.placeRelative(0, y)
                y += placeable.height
            }
        }
    },
)

A child's own layout modifiers - a padding, an offset, an aspect ratio, a default minimum size - stand between the constraints the policy offers and what the child is measured under, which is why the content receiver is ConstrainedScope. Placements of your own go in a scope of your own extending it, whose builders append a value the policy reads back through Measurable.parentData.

A policy that reads a snapshot State is observed: a read made while measuring or answering an intrinsic query measures the container and its ancestors again when the state changes, and a read made only while placing places the children again, invalidating no ancestor. A read made while the container paints its own decoration repaints it. A child's paint is observed by the child, so a plain component reading state while it paints is not observed; draw such content in a Canvas. A policy that writes, while measuring, a state it also reads is measured again after the write, and settles once a pass writes the value the state already holds. A pass at the extent the container already settled on places that result again without running the policy, until the container is invalidated.

Parameters

A Layout with no children that can participate in layout, drawing, and input through modifier.

Parameters