From 5c35ee3415e4246eea760435ee3d3039a5781d1a Mon Sep 17 00:00:00 2001
From: Shinokuni <shinokuni@openserver.fr>
Date: Sat, 19 Oct 2019 19:13:34 +0200
Subject: [PATCH] Fix item list scrolling down when updating data with a
 temporary workaround to keep until a new version of androidx paging is
 released with the right fix

---
 .../app/database/RoomFactoryWrapper.java      | 76 +++++++++++++++++++
 .../app/viewmodels/MainViewModel.java         |  3 +-
 2 files changed, 78 insertions(+), 1 deletion(-)
 create mode 100644 app/src/main/java/com/readrops/app/database/RoomFactoryWrapper.java

diff --git a/app/src/main/java/com/readrops/app/database/RoomFactoryWrapper.java b/app/src/main/java/com/readrops/app/database/RoomFactoryWrapper.java
new file mode 100644
index 00000000..4e1175db
--- /dev/null
+++ b/app/src/main/java/com/readrops/app/database/RoomFactoryWrapper.java
@@ -0,0 +1,76 @@
+package com.readrops.app.database;
+
+import androidx.annotation.NonNull;
+import androidx.paging.DataSource;
+import androidx.paging.PositionalDataSource;
+
+/**
+ * Workaround class to avoid item recycler view scrolling down when updating data
+ * This class is to keep until a new version of androidx paging is released with
+ * bug https://issuetracker.google.com/issues/123834703 merged.
+ * @param <T>
+ */
+public class RoomFactoryWrapper<T> extends DataSource.Factory<Integer, T> {
+    final DataSource.Factory<Integer, T> m_wrappedFactory;
+
+    public RoomFactoryWrapper(@NonNull DataSource.Factory<Integer, T> wrappedFactory) {
+        m_wrappedFactory = wrappedFactory;
+    }
+
+    @NonNull
+    @Override
+    public DataSource<Integer, T> create() {
+        return new DataSourceWrapper<>((PositionalDataSource<T>) m_wrappedFactory.create());
+    }
+
+    public static class DataSourceWrapper<T> extends PositionalDataSource<T> {
+        final PositionalDataSource<T> m_wrappedSource;
+
+        DataSourceWrapper(PositionalDataSource<T> wrappedSource) {
+            m_wrappedSource = wrappedSource;
+        }
+
+        @Override
+        public void addInvalidatedCallback(@NonNull InvalidatedCallback onInvalidatedCallback) {
+            m_wrappedSource.addInvalidatedCallback(onInvalidatedCallback);
+        }
+
+        @Override
+        public void removeInvalidatedCallback(
+                @NonNull InvalidatedCallback onInvalidatedCallback) {
+            m_wrappedSource.removeInvalidatedCallback(onInvalidatedCallback);
+        }
+
+        @Override
+        public void invalidate() {
+            m_wrappedSource.invalidate();
+        }
+
+        @Override
+        public boolean isInvalid() {
+            return m_wrappedSource.isInvalid();
+        }
+
+        @Override
+        public void loadInitial(@NonNull LoadInitialParams params,
+                                @NonNull LoadInitialCallback<T> callback) {
+            // Workaround for paging bug: https://issuetracker.google.com/issues/123834703
+            // edit initial load position to start 1/2 load ahead of requested position
+            int newStartPos = params.placeholdersEnabled
+                    ? params.requestedStartPosition
+                    : Math.max(0, params.requestedStartPosition - (params.requestedLoadSize / 2));
+            m_wrappedSource.loadInitial(new LoadInitialParams(
+                    newStartPos,
+                    params.requestedLoadSize,
+                    params.pageSize,
+                    params.placeholdersEnabled
+            ), callback);
+        }
+
+        @Override
+        public void loadRange(@NonNull LoadRangeParams params,
+                              @NonNull LoadRangeCallback<T> callback) {
+            m_wrappedSource.loadRange(params, callback);
+        }
+    }
+}
\ No newline at end of file
diff --git a/app/src/main/java/com/readrops/app/viewmodels/MainViewModel.java b/app/src/main/java/com/readrops/app/viewmodels/MainViewModel.java
index e809040e..3daf2e4c 100644
--- a/app/src/main/java/com/readrops/app/viewmodels/MainViewModel.java
+++ b/app/src/main/java/com/readrops/app/viewmodels/MainViewModel.java
@@ -12,6 +12,7 @@ import androidx.paging.PagedList;
 import com.readrops.app.activities.MainActivity;
 import com.readrops.app.database.Database;
 import com.readrops.app.database.ItemsListQueryBuilder;
+import com.readrops.app.database.RoomFactoryWrapper;
 import com.readrops.app.database.entities.account.Account;
 import com.readrops.app.database.entities.Feed;
 import com.readrops.app.database.entities.Folder;
@@ -68,7 +69,7 @@ public class MainViewModel extends AndroidViewModel {
         if (lastFetch != null)
             itemsWithFeed.removeSource(lastFetch);
 
-        lastFetch = new LivePagedListBuilder<>(db.itemDao().selectAll(queryBuilder.getQuery()),
+        lastFetch = new LivePagedListBuilder<>(new RoomFactoryWrapper<>(db.itemDao().selectAll(queryBuilder.getQuery())),
                 new PagedList.Config.Builder()
                         .setPageSize(100)
                         .setPrefetchDistance(150)