From 5e91e20c512b5c7a1eb2d92d9e1e3b9aa2a52d6a Mon Sep 17 00:00:00 2001
From: Harsh Shandilya <me@msfjarvis.dev>
Date: Tue, 18 Jul 2023 22:45:38 +0530
Subject: [PATCH] feat(android): add specialized PagingSource for search
 endpoint

---
 .../claw/android/paging/SearchPagingSource.kt | 67 +++++++++++++++++++
 1 file changed, 67 insertions(+)
 create mode 100644 android/src/main/kotlin/dev/msfjarvis/claw/android/paging/SearchPagingSource.kt

diff --git a/android/src/main/kotlin/dev/msfjarvis/claw/android/paging/SearchPagingSource.kt b/android/src/main/kotlin/dev/msfjarvis/claw/android/paging/SearchPagingSource.kt
new file mode 100644
index 00000000..11146005
--- /dev/null
+++ b/android/src/main/kotlin/dev/msfjarvis/claw/android/paging/SearchPagingSource.kt
@@ -0,0 +1,67 @@
+/*
+ * Copyright © 2021-2023 Harsh Shandilya.
+ * Use of this source code is governed by an MIT-style
+ * license that can be found in the LICENSE file or at
+ * https://opensource.org/licenses/MIT.
+ */
+package dev.msfjarvis.claw.android.paging
+
+import androidx.paging.PagingSource
+import androidx.paging.PagingState
+import com.slack.eithernet.ApiResult
+import dagger.assisted.Assisted
+import dagger.assisted.AssistedFactory
+import dagger.assisted.AssistedInject
+import dev.msfjarvis.claw.android.paging.LobstersPagingSource.Companion.PAGE_SIZE
+import dev.msfjarvis.claw.android.paging.LobstersPagingSource.Companion.STARTING_PAGE_INDEX
+import dev.msfjarvis.claw.core.injection.IODispatcher
+import dev.msfjarvis.claw.model.LobstersPost
+import java.io.IOException
+import kotlinx.coroutines.CoroutineDispatcher
+import kotlinx.coroutines.withContext
+
+/**
+ * Duplicate of [LobstersPagingSource] with an additional optimization to not continue loading
+ * further pages if nothing is found in the current one. This is required to prevent the app from
+ * hammering the search endpoint with up to 70 consecutive requests for the search results of a
+ * blank query.
+ */
+class SearchPagingSource
+@AssistedInject
+constructor(
+  @Assisted private val remoteFetcher: RemoteFetcher<LobstersPost>,
+  @IODispatcher private val ioDispatcher: CoroutineDispatcher,
+) : PagingSource<Int, LobstersPost>() {
+  override fun getRefreshKey(state: PagingState<Int, LobstersPost>): Int? {
+    return state.anchorPosition?.let { anchorPosition ->
+      (anchorPosition / PAGE_SIZE).coerceAtLeast(STARTING_PAGE_INDEX)
+    }
+  }
+
+  override suspend fun load(params: LoadParams<Int>): LoadResult<Int, LobstersPost> {
+    val page = params.key ?: STARTING_PAGE_INDEX
+    return when (val result = withContext(ioDispatcher) { remoteFetcher.getItemsAtPage(page) }) {
+      is ApiResult.Success -> {
+        // Optimization: prevent fetching more pages if we found no items, this means
+        // there is no active search query.
+        val nextKey = if (result.value.isEmpty()) null else page + 1
+        LoadResult.Page(
+          itemsBefore = (page - 1) * PAGE_SIZE,
+          data = result.value,
+          prevKey = if (page == STARTING_PAGE_INDEX) null else page - 1,
+          nextKey = nextKey,
+        )
+      }
+      is ApiResult.Failure.NetworkFailure -> LoadResult.Error(result.error)
+      is ApiResult.Failure.UnknownFailure -> LoadResult.Error(result.error)
+      is ApiResult.Failure.HttpFailure,
+      is ApiResult.Failure.ApiFailure ->
+        LoadResult.Error(IOException("API returned an invalid response"))
+    }
+  }
+
+  @AssistedFactory
+  interface Factory {
+    fun create(remoteFetcher: RemoteFetcher<LobstersPost>): SearchPagingSource
+  }
+}