Understanding Jetpack Compose Layout Model and Custom Layout Techniques

Jetpack Compose uses a three‑stage layout model—Composition, Layout, and Draw—where the Composition phase builds a UI tree from @Composable functions, the Layout phase measures each component once to determine size and position (similar to Android's onMeasure/onLayout but with a single measurement), and the Draw phase renders the components onto a Canvas.

The @Composable inline fun Layout(content: @Composable () -> Unit, modifier: Modifier = Modifier, measurePolicy: MeasurePolicy) function is the entry point for custom layouts; developers provide a MeasurePolicy that defines how child measurables are measured and placed.

Typical layout steps inside MeasurePolicy are:

Measure all child measurables.

Determine the size of the parent layout.

Place each child at its final coordinates.

Example code for a custom column layout:

@Composable
fun CustomColumnLayout(
    modifier: Modifier,
    content: @Composable () -> Unit
) {
    Layout(
        content = content,
        modifier = modifier,
        measurePolicy = object : MeasurePolicy {
            override fun MeasureScope.measure(
                measurables: List
,
                constraints: Constraints
            ): MeasureResult {
                // measurement and placement logic
                ...
            }
        }
    )
}

The Modifier system is a chain of elements that can modify size, padding, background, etc. Each call such as fillMaxSize() , padding(5.dp) , or background(Color.White) returns a new Modifier by invoking then() , which either returns the existing instance (when the argument is Modifier ) or creates a CombinedModifier that links the previous and new elements.

Key interfaces:

Modifier – base interface with foldIn , foldOut , any , all , and then methods.

Element – a sub‑interface of Modifier that implements the folding operations.

CombinedModifier – holds an outer and inner Modifier to represent the chain.

During composition, the materializerOf(modifier) step converts the chain into a concrete layout node structure. It traverses the chain with foldIn and foldOut , turning ComposedModifier instances into real Modifier objects that are later wrapped into LayoutNodeWrapper objects such as ModifiedLayoutNode .

When a LayoutModifier like PaddingModifier is encountered, its measure implementation adjusts the incoming constraints, measures the child, and then adds the padding back to the final size before placing the child:

override fun MeasureScope.measure(
    measurable: Measurable,
    constraints: Constraints
): MeasureResult {
    val horizontal = start.roundToPx() + end.roundToPx()
    val vertical = top.roundToPx() + bottom.roundToPx()
    val placeable = measurable.measure(constraints.offset(-horizontal, -vertical))
    val width = constraints.constrainWidth(placeable.width + horizontal)
    val height = constraints.constrainHeight(placeable.height + vertical)
    return layout(width, height) {
        if (rtlAware) {
            placeable.placeRelative(start.roundToPx(), top.roundToPx())
        } else {
            placeable.place(start.roundToPx(), top.roundToPx())
        }
    }
}

Besides using a custom MeasurePolicy , developers can create a custom layout directly on a Modifier via the layout extension, which receives a Measurable and Constraints and returns a MeasureResult :

fun Modifier.customLayout() = layout { measurable, constraints ->
    val placeable = measurable.measure(constraints)
    // optional custom logic here
    layout(constraints.maxWidth, constraints.maxHeight) {
        placeable.placeRelative(0, 0)
    }
}

Both approaches are analogous to Android's custom ViewGroup (using Layout ) and custom View (using Modifier.layout ) respectively, offering flexible ways to meet complex UI requirements.

The article concludes that mastering the Layout phase and the Modifier chain enables developers to build precise, performant custom components in Jetpack Compose.