Compose in Large Codebases: A Practical Guide

Jetpack Compose, the modern UI toolkit for Android, is revolutionizing the way we build user interfaces. Its declarative nature, coupled with Kotlin’s conciseness, allows developers to create complex UIs with less code and improved readability. However, integrating Compose into large, existing codebases can present unique challenges. This article provides a comprehensive guide on how to effectively adopt Compose in large codebases.

Understanding the Landscape of Large Codebases

Large codebases often come with legacy code, established architectural patterns, and diverse team structures. Introducing a new UI toolkit like Compose requires careful planning and a strategic approach.

Challenges of Adopting Compose in Large Codebases

• Incremental Adoption: Migrating an entire codebase to Compose at once is usually not feasible.
• Compatibility: Ensuring Compose interoperates smoothly with existing UI elements (Views) is critical.
• Learning Curve: The team needs to adapt to the declarative UI paradigm of Compose.
• Performance: Optimizing Compose to maintain or improve existing app performance is essential.
• Architectural Alignment: Fitting Compose into existing architectural patterns (MVVM, MVP, etc.) is necessary.

Strategies for Adopting Compose in Large Codebases

1. Incremental Migration

The most practical approach is to adopt Compose incrementally. Start by integrating Compose into new features or smaller, isolated parts of the app. This allows the team to learn and adapt to Compose gradually without disrupting the entire application.

// Example: Using Compose in a Fragment

class MyFragment : Fragment() {
    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View {
        return ComposeView(requireContext()).apply {
            setContent {
                MyComposableContent()
            }
        }
    }
}

@Composable
fun MyComposableContent() {
    Text("Hello from Compose!")
}

2. Interoperability with Existing Views

Jetpack Compose provides excellent interoperability with the existing View system. You can use ComposeView to host Compose content within a View hierarchy, and AndroidView to embed traditional Views inside a Compose UI.

Embedding Compose in a View (ComposeView)

As shown above, ComposeView can be used in Fragments or Activities to host Compose content.

Embedding Views in Compose (AndroidView)

import androidx.compose.ui.viewinterop.AndroidView
import android.widget.TextView

@Composable
fun ViewInCompose() {
    AndroidView(
        factory = { context ->
            TextView(context).apply {
                text = "Hello from TextView!"
                textSize = 20f
            }
        },
        update = { textView ->
            // Update properties of the TextView here
            textView.text = "Updated Text from Compose!"
        }
    )
}

This allows you to reuse existing custom Views within your Compose UI, bridging the gap during the migration process.

3. Modularization

Modularizing your codebase can greatly simplify Compose adoption. Breaking down the app into smaller, independent modules allows you to migrate individual modules to Compose without affecting others. This also promotes better code organization and faster build times.

Each module can be migrated to Compose independently:

• Feature Modules: Contains specific features of the application.
• UI Component Library: Houses reusable Compose components that can be used across the app.

4. Component Library Approach

Create a component library using Compose that mirrors existing View-based components. This approach allows developers to gradually replace View-based components with their Compose equivalents while maintaining a consistent look and feel.

// Legacy View-based Button
class LegacyButton(context: Context) : Button(context) {
    // Implementation details
}

// Compose-based Button
@Composable
fun ComposeButton(text: String, onClick: () -> Unit) {
    Button(onClick = onClick) {
        Text(text)
    }
}

Over time, the Compose-based components can completely replace the legacy ones.

5. Theming and Design Systems

Jetpack Compose simplifies theming with its built-in theming capabilities. Create a consistent design system by defining colors, typography, and shapes that are used throughout your Compose UI.

import androidx.compose.material.MaterialTheme
import androidx.compose.material.Shapes
import androidx.compose.material.Typography
import androidx.compose.material.darkColors
import androidx.compose.runtime.Composable
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.text.TextStyle
import androidx.compose.ui.text.font.FontFamily
import androidx.compose.ui.text.font.FontWeight
import androidx.compose.ui.unit.sp

private val DarkColorPalette = darkColors(
    primary = Color(0xFFBB86FC),
    primaryVariant = Color(0xFF3700B3),
    secondary = Color(0xFF03DAC5)
)

val MyTypography = Typography(
    body1 = TextStyle(
        fontFamily = FontFamily.Default,
        fontWeight = FontWeight.Normal,
        fontSize = 16.sp
    )
)

val MyShapes = Shapes()

@Composable
fun MyAppTheme(content: @Composable () -> Unit) {
    MaterialTheme(
        colors = DarkColorPalette,
        typography = MyTypography,
        shapes = MyShapes,
        content = content
    )
}

Then use your custom theme in your composables:

@Composable
fun MyScreen() {
    MyAppTheme {
        Surface(color = MaterialTheme.colors.background) {
            Text("Hello, World!", style = MaterialTheme.typography.body1)
        }
    }
}

6. ViewModel and State Management

Adopt a consistent state management solution, such as ViewModel with LiveData or StateFlow, to manage UI state in Compose. This aligns with best practices for modern Android development and ensures data consistency.

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel

class MyViewModel : ViewModel() {
    private val _uiState = MutableLiveData("Initial State")
    val uiState: LiveData = _uiState

    fun updateState(newState: String) {
        _uiState.value = newState
    }
}

// In your Composable
@Composable
fun MyComposable(viewModel: MyViewModel) {
    val uiState = viewModel.uiState.observeAsState()

    Text(uiState.value ?: "Loading...")

    Button(onClick = { viewModel.updateState("New State") }) {
        Text("Update State")
    }
}

7. Testing Strategy

Implement a robust testing strategy for your Compose UI. Jetpack Compose provides excellent testing APIs for unit and UI testing. Leverage these APIs to ensure the correctness and reliability of your Compose code.

import androidx.compose.ui.test.junit4.createComposeRule
import androidx.compose.ui.test.onNodeWithText
import androidx.compose.ui.test.performClick
import org.junit.Rule
import org.junit.Test

class MyComposableTest {
    @get:Rule
    val composeTestRule = createComposeRule()

    @Test
    fun testButtonUpdatesText() {
        composeTestRule.setContent {
            MyComposable(MyViewModel()) // Assuming a mock ViewModel
        }

        composeTestRule.onNodeWithText("Update State").performClick()
        composeTestRule.onNodeWithText("New State").assertExists()
    }
}

Best Practices for Compose Adoption

• Start Small: Begin with simple UI components or new features.
• Stay Updated: Keep your Compose dependencies up to date to leverage the latest features and bug fixes.
• Code Reviews: Conduct thorough code reviews to ensure code quality and consistency.
• Documentation: Maintain clear documentation for your Compose components and patterns.
• Performance Monitoring: Continuously monitor performance to identify and address any bottlenecks.

Conclusion

Adopting Jetpack Compose in large codebases is a journey that requires a strategic, incremental approach. By leveraging interoperability with existing Views, modularizing the codebase, creating component libraries, and adhering to best practices for state management and testing, developers can successfully integrate Compose while maintaining code quality and app performance. With careful planning and execution, Compose can transform the UI layer of even the most complex Android applications.