Compose Layout System Internals

Compose Layout System Internals

1. Layout Pipeline

Compose Layout은 세 단계로 구성된다.
Compose layout has three major stages.

Measure → Layout (position) → Draw

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2. MeasurePolicy

커스텀 레이아웃은 measurePolicy로 동작한다.

@Composable
fun MyLayout(content: @Composable () -> Unit) {
    Layout(
        content = content,
        measurePolicy = { measurables, constraints ->
            val placeables = measurables.map { it.measure(constraints) }
            val width = placeables.maxOf { it.width }
            val height = placeables.sumOf { it.height }

            layout(width, height) {
                var y = 0
                placeables.forEach { p ->
                    p.place(0, y)
                    y += p.height
                }
            }
        }
    )
}

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3. Constraints & Density

Constraints:

• minWidth, maxWidth
• minHeight, maxHeight

Density:

• dp/sp를 px로 변환
• LocalDensity로 제공

CompositionLocal은 Layout behavior를 간접적으로 제어할 수 있다.
예: LocalSpacing, LocalLayoutDirection, LocalDensity.

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4. Intrinsic Measurements

intrinsicSize는 컨텐츠 기반 크기 계산에 사용된다.
Intrinsic sizes allow measuring based on content.

Modifier.width(IntrinsicSize.Min) 등.

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5. LayoutNode와 SlotTable

SlotTable이 UI 구조를 표현하면, LayoutNode는 실제 레이아웃 트리를 표현한다.

SlotTable (logical)
LayoutNode tree (physical)

CompositionLocal 값은 SlotTable → LayoutNode로 전달되어 measure/draw에 영향을 준다.