[G1-Sync] Manual knowledge update

2026-05-09 21:08:02 +09:00
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+---
+id: android-paging-3-patterns
+title: Android Paging 3 — 효율적 페이지네이션
+category: Coding
+status: draft
+source_trust_level: B
+verification_status: conceptual
+created_at: 2026-05-09
+updated_at: 2026-05-09
+tags: [android, paging, list, vibe-coding]
+tech_stack: { language: "Kotlin / Jetpack Paging 3", applicable_to: ["Android"] }
+applied_in: []
+aliases: [PagingSource, RemoteMediator, PagingData, LazyColumn]
+---
+
+# Android Paging 3
+
+> 큰 list 를 chunk 로 fetch + 캐시 + 무한 스크롤. **PagingSource (단일 source)** 또는 **RemoteMediator + Room (network + DB)** 두 패턴. Compose / RecyclerView 모두 지원.
+
+## 📖 핵심 개념
+- PagingSource: load(params) → PagingSourceLoadResult.
+- Pager: configuration (pageSize, prefetch).
+- PagingData: ViewModel 에서 Compose / Adapter 로 흐름.
+
+## 💻 코드 패턴
+
+### 단일 PagingSource (network only)
+```kotlin
+class UserPagingSource(private val api: UserApi) : PagingSource<Int, User>() {
+    override fun getRefreshKey(state: PagingState<Int, User>): Int? {
+        return state.anchorPosition?.let { state.closestPageToPosition(it)?.prevKey?.plus(1) }
+    }
+
+    override suspend fun load(params: LoadParams<Int>): LoadResult<Int, User> = try {
+        val page = params.key ?: 1
+        val res = api.fetchUsers(page = page, size = params.loadSize)
+        LoadResult.Page(
+            data = res.items,
+            prevKey = if (page == 1) null else page - 1,
+            nextKey = if (res.items.isEmpty()) null else page + 1,
+        )
+    } catch (e: IOException) {
+        LoadResult.Error(e)
+    }
+}
+
+class UserViewModel(api: UserApi) : ViewModel() {
+    val users: Flow<PagingData<User>> = Pager(
+        config = PagingConfig(pageSize = 20, prefetchDistance = 3),
+        pagingSourceFactory = { UserPagingSource(api) }
+    ).flow.cachedIn(viewModelScope)
+}
+```
+
+### Compose 사용
+```kotlin
+@Composable
+fun UserList(viewModel: UserViewModel) {
+    val users = viewModel.users.collectAsLazyPagingItems()
+
+    LazyColumn {
+        items(users.itemCount, key = users.itemKey { it.id }) { index ->
+            users[index]?.let { UserRow(it) }
+        }
+
+        when (val s = users.loadState.append) {
+            is LoadState.Loading -> item { Spinner() }
+            is LoadState.Error -> item { ErrorRow(s.error) { users.retry() } }
+            else -> Unit
+        }
+    }
+}
+```
+
+### RemoteMediator (network + DB cache)
+```kotlin
+@OptIn(ExperimentalPagingApi::class)
+class UserRemoteMediator(
+    private val api: UserApi, private val db: AppDb
+) : RemoteMediator<Int, UserEntity>() {
+
+    override suspend fun load(loadType: LoadType, state: PagingState<Int, UserEntity>): MediatorResult {
+        try {
+            val page = when (loadType) {
+                LoadType.REFRESH -> 1
+                LoadType.PREPEND -> return MediatorResult.Success(endOfPaginationReached = true)
+                LoadType.APPEND -> {
+                    val last = state.lastItemOrNull() ?: return MediatorResult.Success(true)
+                    last.page + 1
+                }
+            }
+            val res = api.fetchUsers(page = page, size = state.config.pageSize)
+            db.withTransaction {
+                if (loadType == LoadType.REFRESH) db.userDao().clear()
+                db.userDao().upsertAll(res.items.map { it.toEntity(page) })
+            }
+            return MediatorResult.Success(endOfPaginationReached = res.items.isEmpty())
+        } catch (e: Exception) {
+            return MediatorResult.Error(e)
+        }
+    }
+}
+
+// ViewModel
+val users = Pager(
+    config = PagingConfig(pageSize = 20),
+    remoteMediator = UserRemoteMediator(api, db),
+    pagingSourceFactory = { db.userDao().pagingSource() }
+).flow.cachedIn(viewModelScope)
+```
+
+## 🤔 의사결정 기준
+| 상황 | 패턴 |
+|---|---|
+| Network only, 캐시 불필요 | PagingSource |
+| Network + DB 캐시 / 오프라인 | RemoteMediator + Room |
+| 검색 (key 가 string) | PagingSource<String, ...> |
+| Cursor pagination | key = cursor |
+| 작은 list (50개 미만) | Paging 불필요 — 그냥 Flow<List> |
+
+## ❌ 안티패턴
+- **cachedIn 없이**: configuration change 마다 새 fetch. cachedIn(viewModelScope).
+- **getRefreshKey 잘못**: refresh 시 첫 page 로 다시 → 사용자 위치 잃음.
+- **key 로 unstable id (timestamp)**: 같은 row 가 다른 page 에 나타남.
+- **error 상태 무시**: 사용자 멈춤 모름. retry button.
+- **endOfPaginationReached 잘못 판정**: 무한 fetch 또는 일찍 멈춤.
+
+## 🤖 LLM 활용 힌트
+- 신규 = Paging 3 + Compose collectAsLazyPagingItems.
+- offline 필요 = RemoteMediator.
+
+## 🔗 관련 문서
+- [[Android_Room_Patterns]]
+- [[React_Virtualization_Lists]]