Building Custom Layouts in Jetpack Compose: A Step-by-Step Guide

Creating Custom Layouts in Jetpack Compose

Jetpack Compose, the modern UI toolkit for Android, provides a variety of built-in layout types to help you design your app’s user interface. However, sometimes these pre-existing layouts may not meet your project’s specific requirements. In such cases, it’s essential to know how to create custom layouts to fit your needs.

Why Create Custom Layouts?

While Jetpack Compose offers many developer tools for building faster Android apps, including various layout options, these existing layouts may not always satisfy project design requirements. By creating custom layouts, you can tailor your app’s UI to meet your exact needs, resulting in a more polished and user-friendly experience.

Overview of Layouts in Jetpack Compose

Some common layouts in Jetpack Compose include:

• Box: a layout that places its views on top of another
• Column: a layout that places its views in a vertical sequence
• Row: a layout that places its views in a horizontal sequence
• ConstraintLayout: a layout that places its views relative to others

Recently, LazyVerticalGrid and LazyHorizontalGrid, which were grid layouts under testing, were fully released. Along with this update came an exciting new layout called LazyLayout. This layout only composes and lays out currently needed items, making it efficient for scrollable layouts.

Steps to Build a Custom Layout in Jetpack Compose

To illustrate the process of building a custom layout, let’s create a simple example called ReverseFlowRow. This layout places its views next to each other, moving on to the next line when the current line is filled up. However, it starts arranging its views from the end position to the start position of the screen.

The Theory Behind Jetpack Compose Layouts

To display views on the screen, Jetpack Compose composes the UI tree of nodes (which represent views), lays out each view in the UI tree, and draws each of them to the screen. For the purposes of this article, we are only interested in the laying out of views, since we can handle creating a custom layout during this process.

The process of laying out the views within a layout happens in three steps:

1. Measuring all views in the layout (i.e., the children)
2. Deciding what size the layout should be
3. Placing the children within the boundaries of the layout

Using the Layout Composable

In Jetpack Compose, laying out views can be achieved using the Layout composable. The most important part of this code is MeasurePolicy, which defines the measurement of child views, the size of the layout, and the placing of the child views in the layout.

Measuring All Views in the Custom Layout

We measure each child with constraints by calling measure(constraints) on each of them. This returns a Placeable, which corresponds to a child layout that can be positioned by its parent layout.

Adding Size Constraints to the Custom Layout

Next, we define the size of the layout by calling the layout() method and specifying at least its width and height.

Placing Views Within the Layout

Finally, we place the measured children, also called Placeable elements, in the layout by calling the placeRelative() method.

Final Jetpack Compose Project Code

Our complete layout will look like this:

kotlin
@Composable
fun ReverseFlowRow(
modifier: Modifier = Modifier,
mainAxisSpacing: Dp = 0.dp,
crossAxisSpacing: Dp = 0.dp,
content: @Composable () -> Unit
) {
Layout(
modifier = modifier,
content = content
) { measurables, constraints ->
val placeables = measurables.map { it.measure(constraints) }
val width = constraints.maxWidth
val height = constraints.maxHeight
layout(width, height) {
var x = width
var y = 0
placeables.forEach { placeable ->
if (x > width - placeable.width) {
x = width
y += placeable.height + crossAxisSpacing.roundToPx()
}
placeable.placeRelative(x - placeable.width, y)
x -= placeable.width + mainAxisSpacing.roundToPx()
}
}
}
}


Testing Our Custom Layout

To preview our layout, we can wrap it within a composable function that is annotated with @Preview. This enables us to run a sample of the layout without the extra configuration required for an Android application.

By following these steps and understanding the concepts behind Jetpack Compose layouts, you can create your own custom layouts to fit your app’s specific needs, resulting in a more polished and user-friendly experience.