compose-ui

---
name: compose-ui
description: Best practices for building UI with Jetpack Compose, focusing on state hoisting, detailed performance optimizations, and theming. Use this when writing or refactoring Composable functions.
---

# Jetpack Compose Best Practices

## Instructions

Follow these guidelines to create performant, reusable, and testable Composables.

### 1. State Hoisting (Unidirectional Data Flow)
Make Composables **stateless** whenever possible by moving state to the caller.

*   **Pattern**: Function signature should usually look like:
    ```kotlin
    @Composable
    fun MyComponent(
        value: String,              // State flows down
        onValueChange: (String) -> Unit, // Events flow up
        modifier: Modifier = Modifier // Standard modifier parameter
    )
    ```
*   **Benefit**: Decouples the UI from simple state storage, making it easier to preview and test.
*   **ViewModel Integration**: The screen-level Composable retrieves state from the ViewModel (`viewModel.uiState.collectAsStateWithLifecycle()`) and passes it down.

### 2. Modifiers
*   **Default Parameter**: Always provide a `modifier: Modifier = Modifier` as the first optional parameter.
*   **Application**: Apply this `modifier` to the *root* layout element of your Composable.
*   **Ordering matters**: `padding().clickable()` is different from `clickable().padding()`. Generally apply layout-affecting modifiers (like padding) *after* click listeners if you want the padding to be clickable.

### 3. Performance Optimization
*   **`remember`**: Use `remember { ... }` to cache expensive calculations across recompositions.
*   **`derivedStateOf`**: Use `derivedStateOf { ... }` when a state changes frequently (like scroll position) but the UI only needs to react to a threshold or summary (e.g., show "Jump to Top" button). This prevents unnecessary recompositions.
    ```kotlin
    val showButton by remember {
        derivedStateOf { listState.firstVisibleItemIndex > 0 }
    }
    ```
*   **Lambda Stability**: Prefer method references (e.g., `viewModel::onEvent`) or remembered lambdas to prevent unstable types from triggering recomposition of children.

### 4. Theming and Resources
*   Use `MaterialTheme.colorScheme` and `MaterialTheme.typography` instead of hardcoded colors or text styles.
*   Organize simple UI components into specific files (e.g., `DesignSystem.kt` or `Components.kt`) if they are shared across features.

### 5. Previews
*   Create a private preview function for every public Composable.
*   Use `@Preview(showBackground = true)` and include Light/Dark mode previews if applicable.
*   Pass dummy data (static) to the stateless Composable for the preview.