Pagination Strategies

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📖 Concept

Pagination is essential for loading large datasets efficiently on mobile. The Paging 3 library is Google's recommended solution for Android.

Pagination types:

Offset-based: ?page=3&limit=20 → Simple but problematic with live data (inserts cause duplicates/skips)
Cursor-based: ?after=cursor123&limit=20 → Reliable for live data, no duplicates
Keyset-based: ?after_id=456&limit=20 → Similar to cursor but uses a data field as cursor

Paging 3 architecture:

PagingSource (data loading) → Pager → PagingData<T> → LazyColumn/RecyclerView
                                ↑
                         PagingConfig (pageSize, prefetchDistance)

Key components:

PagingSource: Loads pages of data from a single source (network or DB)
RemoteMediator: Loads from network AND saves to DB (offline-first paging)
Pager: Creates the PagingData stream
PagingConfig: Controls page size, prefetch, initial load
cachedIn(viewModelScope): Caches paging data across config changes

💻 Code Example

codeTap to expand ⛶

1// Paging 3 — Network-only PagingSource
2class ArticlePagingSource(
3    private val api: ArticleApi,
4    private val query: String
5) : PagingSource<Int, Article>() {
6    
7    override suspend fun load(params: LoadParams<Int>): LoadResult<Int, Article> {
8        val page = params.key ?: 1
9        return try {
10            val response = api.searchArticles(query, page, params.loadSize)
11            LoadResult.Page(
12                data = response.data.map { it.toDomain() },
13                prevKey = if (page == 1) null else page - 1,
14                nextKey = if (response.pagination.hasNext) page + 1 else null
15            )
16        } catch (e: Exception) {
17            LoadResult.Error(e)
18        }
19    }
20    
21    override fun getRefreshKey(state: PagingState<Int, Article>): Int? {
22        return state.anchorPosition?.let { pos ->
23            state.closestPageToPosition(pos)?.prevKey?.plus(1)
24                ?: state.closestPageToPosition(pos)?.nextKey?.minus(1)
25        }
26    }
27}
28
29// ViewModel with Paging
30@HiltViewModel
31class ArticleViewModel @Inject constructor(
32    private val api: ArticleApi
33) : ViewModel() {
34    
35    private val _query = MutableStateFlow("")
36    
37    val articles: Flow<PagingData<Article>> = _query
38        .debounce(300)
39        .flatMapLatest { query ->
40            Pager(
41                config = PagingConfig(
42                    pageSize = 20,
43                    prefetchDistance = 5,
44                    initialLoadSize = 40
45                ),
46                pagingSourceFactory = { ArticlePagingSource(api, query) }
47            ).flow
48        }
49        .cachedIn(viewModelScope)
50    
51    fun search(query: String) { _query.value = query }
52}
53
54// Compose UI with Paging
55@Composable
56fun ArticleList(viewModel: ArticleViewModel = hiltViewModel()) {
57    val articles = viewModel.articles.collectAsLazyPagingItems()
58    
59    LazyColumn {
60        items(articles.itemCount) { index ->
61            articles[index]?.let { article ->
62                ArticleRow(article)
63            }
64        }
65        
66        // Loading indicator
67        when (articles.loadState.append) {
68            is LoadState.Loading -> item { CircularProgressIndicator() }
69            is LoadState.Error -> item {
70                RetryButton { articles.retry() }
71            }
72            else -> {}
73        }
74    }
75}

🏋️ Practice Exercise

Practice:

Implement a PagingSource for cursor-based pagination
Add a RemoteMediator for offline-first paging with Room
Handle empty states, loading, and error states in paged lists
Implement pull-to-refresh with Paging 3
Compare offset vs cursor pagination — when does offset break?

⚠️ Common Mistakes

Using offset pagination with live data — new items push existing items to next page, causing duplicates or skips
Not using cachedIn(viewModelScope) — paging data is lost on configuration change
Setting pageSize too small — many small network requests are worse than fewer larger ones
Not implementing getRefreshKey — pull-to-refresh doesn't scroll to the right position

💼 Interview Questions

🎤 Mock Interview

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