jetpack-compose

Jetpack Compose

Jetpack Compose is Android's modern toolkit for building native UIs. It simplifies and accelerates UI development on Android with less code, powerful tools, and intuitive Kotlin APIs.

When to Use

• New Android application development (Grid/​List content, complex layouts).
• Migrating existing View-based apps incrementally (Interoperability).
• Sharing UI logic with Kotlin Multiplatform (Compose Multiplatform).

Quick Start

// build.gradle.kts needs composed enabled

// MainActivity.kt
import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.material3.*
import androidx.compose.runtime.*
import androidx.lifecycle.viewmodel.compose.viewModel

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            MaterialTheme {
                MyApp()
            }
        }
    }
}

@Composable
fun MyApp(viewModel: CounterViewModel = viewModel()) {
    // Collecting state from ViewModel
    val count by viewModel.uiState.collectAsState()

    Scaffold(
        floatingActionButton = {
            FloatingActionButton(onClick = { viewModel.increment() }) {
                Text("+")
            }
        }
    ) { padding ->
        Text(
            text = "Count: $count",
            modifier = Modifier.padding(padding)
        )
    }
}

Core Concepts

Composable Functions

Functions annotated with @Composable are the building blocks. They describe specialized UI widgets or layouts. They can call other Composables.

Recomposition

When the state of a Composable changes, the framework re-executes the function to update the UI. Smart logic ensures only necessary parts are redrawn.

Modifiers

The Modifier object allows you to decorate or augment a composable (layout, appearance, interactions, etc.). They are chainable and order-sensitive.

Common Patterns

State Hoisting

State should be moved up to the caller to make components stateless and reusable.

• Stateful: Owns state (remember { mutableStateOf(...) }).
• Stateless: Receives state as parameters and emits events via lambdas.

ViewModel & StateFlow

Use ViewModel to hold business logic and expose screen state via StateFlow or Compose State. Collect it in the UI using collectAsStateWithLifecycle().

Navigation Compose

Define a NavHost with composable destinations. Pass arguments and navigate using a type-safe approach (library dependent) or string routes (default).

Best Practices

Do:

• Use Material 3 (androidx.compose.material3) for the latest design specs.
• Use remember and derivedStateOf to optimize performance.
• Use LazyColumn / LazyRow for lists (equivalent to RecyclerView).
• Use Preview annotations to visualize UI without running the app.

Don't:

• Don't perform expensive operations in the composition phase (use LaunchedEffect or ViewModel).
• Don't create state inside a loop.
• Don't ignore the Modifier parameter in reusable components (always allow caller to pass one).

Troubleshooting

References

• Jetpack Compose Documentation
• Compose Material 3
• Accompanist Libraries