Working with Custom Layouts in Jetpack Compose: Flexibility Beyond the Box

Part of the series "Android Development Series by Mircea Ioan Soit"

Jetpack Compose offers a variety of pre-built layouts like Column, Row, Box, and LazyColumn, which are useful in most cases. However, when it comes to advanced and highly custom UI designs, you may need to create your own layout. Custom layouts give you total control over how your UI elements are measured and positioned, ensuring your app meets unique design requirements.

In this article, we’ll walk through the process of creating a custom layout in Jetpack Compose, giving you the tools to go beyond the limitations of the default layouts.

1. Understanding the Need for Custom Layouts

Jetpack Compose abstracts the complex layout mechanics through its built-in components, but in some cases, you may encounter designs that require:

• Custom alignment of components,
• Non-linear positioning (e.g., circular layouts),
• Overlapping elements with specific z-index control,
• Dynamic layouts that respond to external data in unique ways.

When Compose’s built-in layouts don’t fit your needs, creating a custom layout is the way forward.

2. The Basics of Custom Layouts in Compose

In Jetpack Compose, you can create a custom layout using the Layout composable. This gives you control over both measurement and placement of child composables.

a) Creating a Simple Custom Layout

Here’s how to create a basic custom layout that arranges its children in a diagonal fashion:

@Composable
fun DiagonalLayout(
    modifier: Modifier = Modifier,
    content: @Composable () -> Unit
) {
    Layout(
        modifier = modifier,
        content = content
    ) { measurables, constraints ->
        // Measure each child
        val placeables = measurables.map { measurable ->
            measurable.measure(constraints)
        }

        // Set the layout size
        layout(constraints.maxWidth, constraints.maxHeight) {
            // Position each child in a diagonal line
            var yPosition = 0
            placeables.forEach { placeable ->
                placeable.placeRelative(x = yPosition, y = yPosition)
                yPosition += placeable.height
            }
        }
    }
}        

This layout positions its children diagonally, with each child’s x and y position increasing as it gets placed.

b) Custom Measurement Logic

In this custom layout, measurables.measure(constraints) measures each child based on the constraints passed from the parent. You can apply any custom measurement logic here to ensure the children are sized and positioned according to your needs.

3. Building More Complex Layouts

Custom layouts can be much more advanced. Let’s take a look at an example of a radial or circular layout, where children are placed around a central point, like a clock.

a) Radial Layout Example

@Composable
fun RadialLayout(
    modifier: Modifier = Modifier,
    content: @Composable () -> Unit
) {
    Layout(
        modifier = modifier,
        content = content
    ) { measurables, constraints ->
        val placeables = measurables.map { measurable ->
            measurable.measure(constraints)
        }

        val radius = constraints.maxWidth / 2

        layout(constraints.maxWidth, constraints.maxHeight) {
            val centerX = constraints.maxWidth / 2
            val centerY = constraints.maxHeight / 2
            val angleStep = 360 / placeables.size

            placeables.forEachIndexed { index, placeable ->
                val angle = Math.toRadians((angleStep * index).toDouble())
                val x = centerX + (radius * Math.cos(angle)).toInt() - placeable.width / 2
                val y = centerY + (radius * Math.sin(angle)).toInt() - placeable.height / 2

                placeable.placeRelative(x, y)
            }
        }
    }
}        

This custom layout arranges its children in a circular manner around a central point. You can modify the radius or angleStep to adjust the appearance.

4. Handling Dynamic Content in Custom Layouts

One of the strengths of Compose is its declarative nature. Even in custom layouts, you can take advantage of this feature by making your layout responsive to dynamic data. For example, you could adjust the number of children in a radial layout based on user input or external data.

@Composable
fun DynamicRadialLayout(
    itemCount: Int,
    modifier: Modifier = Modifier,
    content: @Composable (Int) -> Unit
) {
    Layout(
        modifier = modifier,
        content = {
            for (i in 0 until itemCount) {
                content(i)
            }
        }
    ) { measurables, constraints ->
        val placeables = measurables.map { measurable ->
            measurable.measure(constraints)
        }

        val radius = constraints.maxWidth / 2

        layout(constraints.maxWidth, constraints.maxHeight) {
            val centerX = constraints.maxWidth / 2
            val centerY = constraints.maxHeight / 2
            val angleStep = 360 / placeables.size

            placeables.forEachIndexed { index, placeable ->
                val angle = Math.toRadians((angleStep * index).toDouble())
                val x = centerX + (radius * Math.cos(angle)).toInt() - placeable.width / 2
                val y = centerY + (radius * Math.sin(angle)).toInt() - placeable.height / 2

                placeable.placeRelative(x, y)
            }
        }
    }
}        

In this layout, the number of items to be placed can be determined at runtime, making it highly adaptable.

5. Advanced Placement with Modifier.offset

If you need to make minor adjustments to where composables are placed within a layout, Compose’s Modifier.offset can come in handy. This allows you to shift elements by a specific number of pixels (or DPs) on the x or y axis.

@Composable
fun OffsetBox(modifier: Modifier = Modifier) {
    Box(
        modifier = modifier
            .size(100.dp)
            .background(Color.Red)
            .offset(x = 20.dp, y = 30.dp)
    )
}        

This composable shifts the red box 20dp to the right and 30dp down from its default position. Combining offset with custom layouts can give you even finer control over UI placement.

6. Testing Custom Layouts

When creating custom layouts, it's essential to test them thoroughly to ensure they behave as expected across various screen sizes and configurations. Compose’s @Preview feature can be used to quickly iterate and verify the appearance of your custom layouts.

@Preview(showBackground = true)
@Composable
fun PreviewDiagonalLayout() {
    DiagonalLayout {
        Text("Item 1")
        Text("Item 2")
        Text("Item 3")
    }
}        

This preview allows you to see how the DiagonalLayout positions its children in real-time without running the app on a device.

7. Best Practices for Custom Layouts

• Measure and place efficiently: Always keep performance in mind. Avoid unnecessary recompositions by using proper state management techniques.
• Adhere to constraints: Make sure your layout respects the constraints passed by the parent, especially for width, height, and padding.
• Test across devices: Ensure your custom layouts scale properly and behave as expected across different screen sizes and orientations.

8. Conclusion: Expanding Your Layout Flexibility

Custom layouts in Jetpack Compose provide the flexibility to design unique and complex UIs that go beyond standard design patterns. With Compose’s powerful measurement and placement APIs, you can handle advanced use cases like radial layouts, dynamic content, and non-linear positioning with ease. Mastering custom layouts will unlock a new level of creativity in your Android app designs.