Constraints

BoxConstraints is what a parent tells a child before asking it how big it wants to be. It travels down the tree during measurement; sizes travel back up.

use Atelier\Layout\Constraint\BoxConstraints;
use Atelier\Layout\Geometry\Size;

BoxConstraints::tight(200, 48)->constrain(new Size(320, 40));   // 200 x 48
BoxConstraints::unconstrained()->constrain(new Size(320, 40));  // 320 x 40

A tight constraint admits exactly one size and the child has no say. An unconstrained one lets the child report its intrinsic size. constrain() is the operation that turns a wish into an allowed size.

Where They Come From

You rarely build constraints by hand. Containers derive them: a grid slot constrains its child to the slot, a stack constrains on the cross axis and lets the main axis flow, a fixed frame reports the same size whatever it is asked.

Constraints matter when you implement LayoutNodeInterface yourself. measure() receives them and must return an IntrinsicSize that respects them; solve() receives the final Rect and places content in it.

public function measure(LayoutContext $context, BoxConstraints $constraints): IntrinsicSize;
public function solve(LayoutContext $context, Rect $rect): PlacedNode;

The split is deliberate. Measurement may run several times as a parent explores sizes; solving runs once, on the answer. Keep measure() free of side effects.

Flexible Children

FlexibleLayoutNodeInterface adds flex(): float. A node that reports a non-zero flex asks for a share of the leftover main-axis space instead of its measured size. Frame::stretch() and Spacer both implement it.

Flex is resolved after fixed children are measured, which is why a stretch child never squeezes a fixed one: it only ever receives what is left.

Grid track sizes are the other place where fixed and flexible meet, and Distribution decides what happens to the leftover space when no child is flexible at all.