ConstrainedScope

The scope of a container whose child stands between the constraints that container offers it and its own measurement: a fill, a padding, an offset, an aspect ratio or a default minimum size each narrows what reaches the child, states what the child plus its own space occupies, and places the child inside that.

Row, Column and Box offer this to their content, so a child of any of them declares these alongside what that container's own scope offers, and a custom Layout offers this alone.

layout is a member of this scope, and every modifier built on it, such as fillMaxWidth, padding or aspectRatio, takes this scope as its context, so that one declared on a child of a standard Swing layout manager, which honors none of them, fails at compile time rather than at runtime. A modifier of your own declares the same context and builds on layout or on the modifiers built on it:

context(scope: ConstrainedScope)
fun SwingModifier.halfWidth(): SwingModifier =
    layout { measurable, constraints ->
        val maxWidth = if (constraints.hasBoundedWidth) constraints.maxWidth / 2 else constraints.maxWidth
        val placeable = measurable.measure(constraints.copy(minWidth = 0, maxWidth = maxWidth))
        layout(placeable.width, placeable.height) { placeable.place(0, 0) }
    }

Inheritors