Implement Smooth Pagination in Jetpack Compose Lists

Pagination is a crucial feature in modern Android apps, enabling seamless loading of large data sets without overwhelming system resources or the user interface. Jetpack Compose, with its declarative UI paradigm, simplifies the process of implementing pagination while offering advanced customization and smooth user experiences.

In this blog post, we'll explore how to implement smooth pagination in Jetpack Compose lists, covering essential concepts, best practices, and advanced techniques. This guide is aimed at intermediate to advanced Android developers familiar with Jetpack Compose and Kotlin.

Understanding Pagination in Jetpack Compose

Pagination divides large data sets into smaller chunks, loaded incrementally as users interact with the app. Jetpack Compose offers tools and patterns that make pagination more efficient, especially in scenarios involving infinite scrolling.

Key Components for Pagination in Jetpack Compose

LazyColumn: The LazyColumn composable is ideal for displaying paginated data. It efficiently handles large lists by recycling and rendering only visible items.
Paging 3 Library: The Paging 3 library integrates seamlessly with Jetpack Compose, managing paginated data sources and offering out-of-the-box support for common use cases.
State and Side-Effects APIs: Manage UI states, such as loading indicators and error messages, using Compose’s state management tools.

Step-by-Step Implementation

1. Set Up Your Project

Start by adding the required dependencies to your build.gradle file:

dependencies {
    implementation "androidx.paging:paging-runtime:3.2.0"
    implementation "androidx.paging:paging-compose:1.0.0-alpha18"
}

2. Create a PagingSource

The PagingSource class fetches data in chunks. Here’s an example implementation:

class ItemPagingSource(private val apiService: ApiService) : PagingSource<Int, Item>() {

    override suspend fun load(params: LoadParams<Int>): LoadResult<Int, Item> {
        return try {
            val currentPage = params.key ?: 1
            val response = apiService.getItems(page = currentPage)
            LoadResult.Page(
                data = response.items,
                prevKey = if (currentPage == 1) null else currentPage - 1,
                nextKey = if (response.items.isEmpty()) null else currentPage + 1
            )
        } catch (e: Exception) {
            LoadResult.Error(e)
        }
    }

    override fun getRefreshKey(state: PagingState<Int, Item>): Int? {
        return state.anchorPosition?.let { position ->
            val closestPage = state.closestPageToPosition(position)
            closestPage?.prevKey?.plus(1) ?: closestPage?.nextKey?.minus(1)
        }
    }
}

3. Set Up the Paging Data Flow

Use the Pager class to generate paginated data:

val pager = Pager(
    config = PagingConfig(
        pageSize = 20,
        enablePlaceholders = false
    ),
    pagingSourceFactory = { ItemPagingSource(apiService) }
)

val pagingData = pager.flow.cachedIn(viewModelScope)

4. Display Data in a LazyColumn

Combine the Paging 3 library with LazyColumn to display items:

@Composable
fun ItemList(pagingData: Flow<PagingData<Item>>) {
    val lazyPagingItems = pagingData.collectAsLazyPagingItems()

    LazyColumn {
        items(lazyPagingItems) { item ->
            item?.let {
                ListItem(item = it)
            }
        }

        lazyPagingItems.apply {
            when {
                loadState.refresh is LoadState.Loading -> {
                    item { CircularProgressIndicator() }
                }
                loadState.append is LoadState.Loading -> {
                    item { CircularProgressIndicator() }
                }
                loadState.append is LoadState.Error -> {
                    val error = loadState.append as LoadState.Error
                    item {
                        Text(
                            text = "Error: ${error.error.message}",
                            modifier = Modifier.clickable { retry() }
                        )
                    }
                }
            }
        }
    }
}

Advanced Techniques for Smooth Pagination

Optimizing for Performance

Prefetch Distance: Configure PagingConfig to fetch pages before the user reaches the end of the list:
```
PagingConfig(
    pageSize = 20,
    prefetchDistance = 5,
    enablePlaceholders = false
)
```
LazyColumn Optimization: Utilize modifiers like Modifier.fillParentMaxSize() and reduce recompositions with proper key usage.

Enhancing User Experience

Loading Indicators: Use contextual loaders like spinners or skeleton screens to keep the UI responsive.
Error Handling: Provide clear messages and actionable retry buttons for seamless recovery from errors.

Empty State: Handle scenarios where no data is available:

if (lazyPagingItems.itemCount == 0 && lazyPagingItems.loadState.refresh !is LoadState.Loading) {
    Text("No items available.", modifier = Modifier.fillMaxSize())
}

Testing Your Pagination Implementation

Ensure your pagination works flawlessly by testing for:

Edge Cases:
- Empty data sets
- Network errors
- Rapid scrolling and data prefetching
Performance Metrics: Use tools like Android Profiler to monitor memory and CPU usage during pagination.
User Experience: Simulate varying network speeds to validate the responsiveness of loaders and error messages.

Best Practices

Separate Concerns: Keep UI logic, data handling, and business logic distinct for maintainability.
Use Dependency Injection: Inject APIs and repositories into your PagingSource using tools like Hilt.
Monitor User Feedback: Continuously refine pagination based on real-world usage and analytics.

Conclusion

Implementing smooth pagination in Jetpack Compose lists requires a blend of effective use of the Paging 3 library, Compose’s declarative patterns, and attention to user experience. By following the steps and best practices outlined in this guide, you can create robust, performant, and user-friendly paginated lists for your Android applications.

Remember, the key to a great paginated experience lies in balancing performance, usability, and adaptability. Dive in, experiment with these techniques, and elevate your app’s user experience!