app: de-duplicate paging sources

Signed-off-by: Harsh Shandilya <me@msfjarvis.dev>

app/src/main/java/dev/msfjarvis/lobsters/data/remote/HottestPostsPagingSource.kt

@@ -1,33 +0,0 @@
-package dev.msfjarvis.lobsters.data.remote
-
-import androidx.paging.PagingSource
-import androidx.paging.PagingState
-import dev.msfjarvis.lobsters.data.repo.LobstersRepository
-import dev.msfjarvis.lobsters.model.LobstersPost
-
-class HottestPostsPagingSource(
-  private val lobstersRepository: LobstersRepository,
-) : PagingSource<Int, LobstersPost>() {
-
-  override suspend fun load(params: LoadParams<Int>): LoadResult<Int, LobstersPost> {
-    return try {
-      val page = params.key ?: 1
-      // Update cache before fetching a list.
-      // This is done to make sure that we can update the isSaved status of incoming posts.
-      lobstersRepository.updateCache()
-      val posts = lobstersRepository.fetchHottestPosts(page)
-
-      LoadResult.Page(
-        data = posts,
-        prevKey = if (page == 1) null else page - 1,
-        nextKey = page.plus(1)
-      )
-    } catch (e: Exception) {
-      LoadResult.Error(e)
-    }
-  }
-
-  override fun getRefreshKey(state: PagingState<Int, LobstersPost>): Int {
-    return state.pages.size + 1
-  }
-}

app/src/main/java/dev/msfjarvis/lobsters/data/remote/LobstersPagingSource.kt

@@ -2,20 +2,16 @@ package dev.msfjarvis.lobsters.data.remote
 
 import androidx.paging.PagingSource
 import androidx.paging.PagingState
-import dev.msfjarvis.lobsters.data.repo.LobstersRepository
 import dev.msfjarvis.lobsters.model.LobstersPost
 
-class NewestPostsPagingSource(
-  private val lobstersRepository: LobstersRepository,
+class LobstersPagingSource(
+  private val getMorePosts: suspend (Int) -> List<LobstersPost>,
 ) : PagingSource<Int, LobstersPost>() {
 
   override suspend fun load(params: LoadParams<Int>): LoadResult<Int, LobstersPost> {
     return try {
       val page = params.key ?: 1
-      // Update cache before fetching a list.
-      // This is done to make sure that we can update the isSaved status of incoming posts.
-      lobstersRepository.updateCache()
-      val posts = lobstersRepository.fetchNewestPosts(page)
+      val posts = getMorePosts(page)
 
       LoadResult.Page(
         data = posts,

app/src/main/java/dev/msfjarvis/lobsters/ui/viewmodel/LobstersViewModel.kt

@@ -8,9 +8,9 @@ import androidx.paging.cachedIn
 import dagger.hilt.android.lifecycle.HiltViewModel
 import dev.msfjarvis.lobsters.data.local.SavedPost
 import dev.msfjarvis.lobsters.data.preferences.ClawPreferences
-import dev.msfjarvis.lobsters.data.remote.HottestPostsPagingSource
-import dev.msfjarvis.lobsters.data.remote.NewestPostsPagingSource
+import dev.msfjarvis.lobsters.data.remote.LobstersPagingSource
 import dev.msfjarvis.lobsters.data.repo.LobstersRepository
+import dev.msfjarvis.lobsters.model.LobstersPost
 import dev.msfjarvis.lobsters.model.LobstersPostDetails
 import javax.inject.Inject
 import kotlinx.coroutines.flow.Flow
@@ -31,18 +31,18 @@ constructor(
   val savedPosts = _savedPosts.asStateFlow()
   val hottestPosts =
     Pager(PagingConfig(25)) {
-        HottestPostsPagingSource(lobstersRepository).also { hottestPostsPagingSource = it }
+        LobstersPagingSource(getPostFetcher(hottest = true)).also { hottestPostsPagingSource = it }
       }
       .flow
       .cachedIn(viewModelScope)
   val newestPosts =
     Pager(PagingConfig(25)) {
-        NewestPostsPagingSource(lobstersRepository).also { newestPostsPagingSource = it }
+        LobstersPagingSource(getPostFetcher(newest = true)).also { hottestPostsPagingSource = it }
       }
       .flow
       .cachedIn(viewModelScope)
-  private var hottestPostsPagingSource: HottestPostsPagingSource? = null
-  private var newestPostsPagingSource: NewestPostsPagingSource? = null
+  private var hottestPostsPagingSource: LobstersPagingSource? = null
+  private var newestPostsPagingSource: LobstersPagingSource? = null
 
   init {
     lobstersRepository
@@ -86,6 +86,26 @@ constructor(
     return lobstersRepository.isPostSaved(postId)
   }
 
+  /**
+   * Returns a lambda that can be used to fetch the next round of hottest or newest posts. Having a
+   * single paging source makes it significantly easier to make changes to the logic both here and
+   * in [LobstersPagingSource].
+   */
+  private fun getPostFetcher(
+    newest: Boolean = false,
+    hottest: Boolean = false,
+  ): suspend (page: Int) -> List<LobstersPost> {
+    check(newest != hottest) { "Must pick one of newest or hottest" }
+    return { page ->
+      lobstersRepository.updateCache()
+      if (newest) {
+        lobstersRepository.fetchNewestPosts(page)
+      } else {
+        lobstersRepository.fetchHottestPosts(page)
+      }
+    }
+  }
+
   private fun savePost(post: SavedPost) {
     viewModelScope.launch {
       lobstersRepository.addPost(post)