@Preview_mdpi
@Composable
fun ColumnSpacerSample() {
    Column(modifier = Modifier.size(320.dp, 240.dp)) {
        Image(
            painter = painterResource(R.drawable.leaf1),
            contentDescription = null,
            modifier = Modifier.height(100.dp),
            contentScale = ContentScale.None
        )
        Spacer(modifier = Modifier.height(30.dp))
        Image(
            painter = painterResource(R.drawable.leaf2),
            contentDescription = null,
            modifier = Modifier.height(100.dp),
            contentScale = ContentScale.None
        )
    }
}

@Composable
@NonRestartableComposable
fun Spacer(modifier: Modifier) { ... }

@Composable
@NonRestartableComposable
fun Spacer(modifier: Modifier) {
    Layout({}, measurePolicy = SpacerMeasurePolicy, modifier = modifier)
}

private object SpacerMeasurePolicy : MeasurePolicy {

    override fun MeasureScope.measure(
        measurables: List<Measurable>,
        constraints: Constraints
    ): MeasureResult {
        return with(constraints) { 
            val width = if (hasFixedWidth) maxWidth else 0	   // widthの算出
            val height = if (hasFixedHeight) maxHeight else 0  // heightの算出
            layout(width, height) {}                           // サイズを通知
        }
    }
}

@Composable
fun Canvas(modifier: Modifier, onDraw: DrawScope.() -> Unit) =
    Spacer(modifier.drawBehind(onDraw))

@Preview_mdpi
@Composable
fun CanvasSample() {
    Box(modifier = Modifier.size(320.dp, 240.dp)) {
        Canvas(
            modifier = Modifier
                .size(120.dp)	// 120x120へ固定化
                .background(Color.LightGray)
        ) {
            drawRect(			// 矩形：DrawScopeに定義された描画関数
                Color.Blue,
                topLeft = Offset(10.dp.toPx(), 10.dp.toPx()),
                size = Size(100.dp.toPx(), 100.dp.toPx())
            )
            drawCircle(			// 円：DrawScopeに定義された描画関数
                Color.Red,
                radius = 50.dp.toPx()
            )
        }
    }
}