Migrating from XML Themes to Jetpack Compose

With Jetpack Compose redefining how we build Android UIs, developers face the exciting challenge of transitioning from XML-based layouts and themes to the fully declarative Compose UI paradigm. While the shift offers enhanced flexibility and expressiveness, migrating XML themes and styles to Jetpack Compose requires a nuanced understanding of Compose’s theming model.

In this blog post, we’ll explore:

  • Why migrating to Jetpack Compose is worth the effort.

  • A deep dive into the Compose theming model.

  • Best practices for translating XML themes to Compose.

  • Strategies for handling common challenges during the migration process.

This guide is tailored for intermediate to advanced Android developers familiar with XML-based themes and eager to adopt Compose for modern app development.

Why Migrate to Jetpack Compose?

Simplified Development Workflow

Compose eliminates the verbosity of XML, enabling developers to define UI elements and their styles programmatically in Kotlin. This integration between logic and UI reduces the cognitive load when maintaining themes across an app.

Dynamic and Reactive UI

Jetpack Compose’s declarative nature ensures that UI components react dynamically to state changes. This makes theme updates more intuitive compared to traditional XML-based approaches.

Future-Proofing Your App

Google’s focus on Compose signals a shift in Android’s development trajectory. Embracing Compose themes now ensures your app stays aligned with modern practices and is easier to maintain as Compose evolves.

Understanding Jetpack Compose Theming

Key Concepts in Compose Theming

Compose introduces a unified and flexible theming model that revolves around the MaterialTheme object. Here are its core components:

  1. Colors: Defined using a ColorScheme (or ColorPalette in earlier Compose versions), representing primary, secondary, background, and other colors.

  2. Typography: Managed through a Typography object, which specifies text styles like headline, body, and caption.

  3. Shapes: Controlled by the Shapes class, defining corner radii for buttons, cards, etc.

Material 3 Support

Compose fully supports Material Design 3 (M3), allowing developers to take advantage of dynamic color theming and improved accessibility. If your app targets Material Design 3, the migration process should align with M3’s guidelines.

Mapping XML Themes to Compose

Let’s translate XML-based themes and styles into Jetpack Compose components step-by-step.

Colors

In XML:

<color name="colorPrimary">#6200EE</color>
<color name="colorPrimaryVariant">#3700B3</color>
<color name="colorOnPrimary">#FFFFFF</color>

In Compose:

val LightColorScheme = ColorScheme(
    primary = Color(0xFF6200EE),
    primaryContainer = Color(0xFF3700B3),
    onPrimary = Color(0xFFFFFFFF),
    background = Color(0xFFFFFFFF),
    onBackground = Color(0xFF000000),
    // Add more colors as needed
)

val DarkColorScheme = ColorScheme(
    primary = Color(0xFFBB86FC),
    primaryContainer = Color(0xFF3700B3),
    onPrimary = Color(0xFF000000),
    background = Color(0xFF121212),
    onBackground = Color(0xFFFFFFFF),
)

Typography

In XML:

<style name="TextAppearance.Headline1">
    <item name="android:fontFamily">sans-serif</item>
    <item name="android:textSize">96sp</item>
    <item name="android:fontWeight">bold</item>
</style>

In Compose:

val AppTypography = Typography(
    displayLarge = TextStyle(
        fontFamily = FontFamily.SansSerif,
        fontWeight = FontWeight.Bold,
        fontSize = 96.sp
    ),
    bodyLarge = TextStyle(
        fontFamily = FontFamily.Serif,
        fontWeight = FontWeight.Normal,
        fontSize = 16.sp
    )
    // Define other text styles
)

Shapes

In XML:

<dimen name="corner_radius_small">4dp</dimen>
<dimen name="corner_radius_medium">8dp</dimen>
<dimen name="corner_radius_large">16dp</dimen>

In Compose:

val AppShapes = Shapes(
    small = RoundedCornerShape(4.dp),
    medium = RoundedCornerShape(8.dp),
    large = RoundedCornerShape(16.dp)
)

Assembling the Theme

In XML:

<style name="AppTheme" parent="Theme.MaterialComponents.DayNight">
    <item name="colorPrimary">@color/colorPrimary</item>
    <item name="colorPrimaryVariant">@color/colorPrimaryVariant</item>
    <item name="colorOnPrimary">@color/colorOnPrimary</item>
    <item name="shapeAppearanceCornerSmall">@dimen/corner_radius_small</item>
    <item name="android:fontFamily">sans-serif</item>
</style>

In Compose:

@Composable
fun AppTheme(
    darkTheme: Boolean = isSystemInDarkTheme(),
    content: @Composable () -> Unit
) {
    val colors = if (darkTheme) DarkColorScheme else LightColorScheme

    MaterialTheme(
        colorScheme = colors,
        typography = AppTypography,
        shapes = AppShapes,
        content = content
    )
}

Best Practices for Migration

Incremental Migration

Migrating an entire app at once can be overwhelming. Start by integrating Jetpack Compose in smaller, isolated features or screens while maintaining existing XML themes for the rest of the app.

Use interop APIs

Leverage ComposeView and AndroidView to integrate Compose and XML layouts seamlessly during migration. This allows you to adopt Compose incrementally without rewriting entire screens.

Testing Themes

Ensure your themes work as expected across light and dark modes. Compose provides preview annotations like @Preview and @Preview(uiMode = Configuration.UI_MODE_NIGHT_YES) to validate themes visually.

Embrace Material Design 3

If your app uses Material Design, adopt Material 3 components and dynamic color features to align with modern design standards and improve user experience.

Common Challenges and Solutions

Challenge: Lack of Parity with XML Attributes

Some attributes available in XML, like certain elevation settings or background tints, may not have direct equivalents in Compose.

Solution: Compose provides greater flexibility through modifiers. For example:

Box(modifier = Modifier.background(Color.Gray).elevation(4.dp))

Challenge: Legacy Code Dependencies

Apps with heavy reliance on XML themes may have dependencies on resources or styles that are difficult to replicate in Compose.

Solution: Gradually refactor dependencies into modular components that can be reused in both Compose and XML.

Conclusion

Migrating from XML themes to Jetpack Compose is a transformative step toward modernizing your Android app development workflow. By understanding the Compose theming model, translating XML themes effectively, and addressing migration challenges, you can harness the full potential of Jetpack Compose to deliver dynamic, future-proof UIs.

Start small, experiment with Compose’s theming capabilities, and embrace the declarative paradigm to elevate your development experience and app quality. Happy coding!