Combine Flow with Paging in Jetpack Compose for Efficient Loading

In modern Android app development, Jetpack Compose has revolutionized UI design by enabling developers to create declarative and responsive interfaces. When building dynamic apps that consume large datasets—like paginated lists from APIs or databases—combining Kotlin Flow, Paging 3, and Jetpack Compose offers an efficient, scalable, and seamless solution. This blog post dives deep into leveraging these tools to load, display, and handle large datasets effectively in a Compose-based application.

Why Combine Flow, Paging, and Compose?

Kotlin Flow is a reactive stream API that helps manage asynchronous data streams. Paging 3 is designed for loading large data sets incrementally, minimizing resource consumption. Integrating these tools with Jetpack Compose ensures that:

Performance: Data is loaded and displayed incrementally, reducing memory usage and network overhead.
Scalability: Compose’s declarative UI updates seamlessly with changes in the dataset.
Responsiveness: Flow ensures reactive data updates, while Paging handles pagination efficiently.

Setting Up Dependencies

Before diving into the implementation, ensure you have the required dependencies in your build.gradle file:

implementation "androidx.paging:paging-compose:<latest_version>"
implementation "androidx.paging:paging-runtime:<latest_version>"
implementation "androidx.compose.ui:ui:<latest_version>"
implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:<latest_version>"

Replace <latest_version> with the current versions compatible with your project.

Creating a Paging Source

A PagingSource is the core component that defines how data is loaded incrementally. Here’s an example of a PagingSource implementation for loading items from a REST API:

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 { anchorPosition ->
            state.closestPageToPosition(anchorPosition)?.prevKey?.plus(1)
                ?: state.closestPageToPosition(anchorPosition)?.nextKey?.minus(1)
        }
    }
}

Integrating PagingSource with a Pager

Use Pager to configure and create a Flow of paged data:

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

With this Flow, data is loaded incrementally and provided as pages to the consumer.

Consuming the Paging Data in Compose

Jetpack Compose provides the LazyColumn composable to efficiently display large datasets. Combine it with collectAsLazyPagingItems for seamless integration:

@Composable
fun ItemListScreen(viewModel: ItemListViewModel) {
    val items = viewModel.itemsFlow.collectAsLazyPagingItems()

    LazyColumn {
        items(items) { item ->
            item?.let {
                ItemRow(item)
            }
        }

        items.apply {
            when {
                loadState.append is LoadState.Loading -> {
                    item { LoadingIndicator() }
                }
                loadState.append is LoadState.Error -> {
                    val error = loadState.append as LoadState.Error
                    item { RetryButton { retry() } }
                }
            }
        }
    }
}

@Composable
fun ItemRow(item: Item) {
    Text(text = item.name, style = MaterialTheme.typography.body1)
}

@Composable
fun LoadingIndicator() {
    CircularProgressIndicator(modifier = Modifier.fillMaxWidth())
}

@Composable
fun RetryButton(onRetry: () -> Unit) {
    Button(onClick = onRetry) {
        Text("Retry")
    }
}

This code showcases how paginated data streams from the Pager Flow are consumed and displayed using Compose’s LazyColumn.

Advanced Use Case: Handling UI States

Compose simplifies managing UI states, such as loading, error, and empty states. Extend the above example to handle these scenarios:

@Composable
fun ItemListScreen(viewModel: ItemListViewModel) {
    val items = viewModel.itemsFlow.collectAsLazyPagingItems()

    when (items.loadState.refresh) {
        is LoadState.Loading -> LoadingIndicator()
        is LoadState.Error -> ErrorView(
            errorMessage = (items.loadState.refresh as LoadState.Error).error.message,
            onRetry = { items.retry() }
        )
        else -> {
            if (items.itemCount == 0) {
                EmptyStateView()
            } else {
                LazyColumn {
                    items(items) { item ->
                        item?.let { ItemRow(it) }
                    }
                }
            }
        }
    }
}

@Composable
fun ErrorView(errorMessage: String?, onRetry: () -> Unit) {
    Column(
        modifier = Modifier.fillMaxSize(),
        verticalArrangement = Arrangement.Center,
        horizontalAlignment = Alignment.CenterHorizontally
    ) {
        Text(text = errorMessage ?: "Unknown error", color = Color.Red)
        Spacer(modifier = Modifier.height(8.dp))
        RetryButton(onRetry)
    }
}

@Composable
fun EmptyStateView() {
    Text(text = "No items available", modifier = Modifier.fillMaxWidth(), textAlign = TextAlign.Center)
}

This approach ensures that the UI remains responsive and informative under different states.

Performance Best Practices

To ensure the best user experience, follow these performance tips:

Use Paging’s Placeholders: When applicable, enable placeholders to improve the perceived loading speed.
Optimize List Items: Use lightweight composables for list items to reduce recomposition overhead.
Avoid Unnecessary Recompositions: Use remember and rememberLazyListState to retain UI state across recompositions.
Handle Errors Gracefully: Provide clear feedback and retry options for errors in data loading.
Test for Edge Cases: Ensure your app handles edge cases like empty pages, network errors, and configuration changes.

Conclusion

Combining Kotlin Flow, Paging 3, and Jetpack Compose provides an efficient and modern way to handle large datasets in Android apps. With the powerful integration between Paging and Compose, developers can build scalable and responsive UIs that handle pagination effortlessly.

By following the best practices and advanced techniques discussed in this post, you can elevate the performance and user experience of your Compose applications while efficiently managing paginated data.

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