Merge pull request #1081 from claraf3/pagingWithNetworkSample-cleanUp

Clean up query order logic in PagingWithNetwork sample

PagingWithNetworkSample/app/src/main/java/com/android/example/paging/pagingwithnetwork/reddit/db/RedditPostDao.kt

@@ -28,12 +28,10 @@ interface RedditPostDao {
     @Insert(onConflict = OnConflictStrategy.REPLACE)
     suspend fun insertAll(posts: List<RedditPost>)
 
-    @Query("SELECT * FROM posts WHERE subreddit = :subreddit ORDER BY indexInResponse ASC")
+    // Order by _id which is an immutable key to help maintain consistent item ordering
+    @Query("SELECT * FROM posts WHERE subreddit = :subreddit ORDER BY _id ASC")
     fun postsBySubreddit(subreddit: String): PagingSource<Int, RedditPost>
 
     @Query("DELETE FROM posts WHERE subreddit = :subreddit")
     suspend fun deleteBySubreddit(subreddit: String)
-
-    @Query("SELECT MAX(indexInResponse) + 1 FROM posts WHERE subreddit = :subreddit")
-    suspend fun getNextIndexInSubreddit(subreddit: String): Int
 }

PagingWithNetworkSample/app/src/test-common/java/com/android/example/paging/pagingwithnetwork/repository/PostFactory.kt

@@ -32,9 +32,9 @@ class PostFactory {
                 created = System.currentTimeMillis(),
                 thumbnail = null,
                 subreddit = subredditName,
-                url = null
+                url = null,
+                _id = id
         )
-        post.indexInResponse = -1
         return post
     }
 }

PagingWithNetworkSample/lib/src/main/java/com/android/example/paging/pagingwithnetwork/reddit/vo/RedditPost.kt

@@ -25,7 +25,11 @@ import com.google.gson.annotations.SerializedName
 @Entity(tableName = "posts",
         indices = [Index(value = ["subreddit"], unique = false)])
 data class RedditPost(
-        @PrimaryKey
+        // A unique Int key starting at 0 which autoIncrements by 1 with every insert.
+        // We need this immutable key to maintain consistent item ordering so that the UI
+        // does not scroll out of order when new posts are appended and inserted into this db.
+        @PrimaryKey(autoGenerate = true)
+        val _id: Int?,
         @SerializedName("name")
         val name: String,
         @SerializedName("title")
@@ -43,6 +47,4 @@ data class RedditPost(
         val created: Long,
         val thumbnail: String?,
         val url: String?) {
-    // to be consistent w/ changing backend order, we need to keep a data like this
-    var indexInResponse: Int = -1
 }