From 7f7863b7be12dbdf8bfa038f6282bc1b0f3d6ab6 Mon Sep 17 00:00:00 2001
From: ByteHamster <info@bytehamster.com>
Date: Sun, 11 Oct 2020 19:26:15 +0200
Subject: [PATCH] Generate unique view IDs

---
 .../antennapod/activity/MainActivity.java     | 24 +++++++++++++++++++
 .../fragment/ItemPagerFragment.java           | 14 ++++++++++-
 2 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/app/src/main/java/de/danoeh/antennapod/activity/MainActivity.java b/app/src/main/java/de/danoeh/antennapod/activity/MainActivity.java
index eaa423708..fffe5be60 100644
--- a/app/src/main/java/de/danoeh/antennapod/activity/MainActivity.java
+++ b/app/src/main/java/de/danoeh/antennapod/activity/MainActivity.java
@@ -19,6 +19,7 @@ import androidx.annotation.Nullable;
 import androidx.appcompat.app.ActionBarDrawerToggle;
 import androidx.appcompat.app.AlertDialog;
 import androidx.appcompat.widget.Toolbar;
+import androidx.core.view.ViewCompat;
 import androidx.drawerlayout.widget.DrawerLayout;
 import androidx.fragment.app.Fragment;
 import androidx.fragment.app.FragmentManager;
@@ -67,6 +68,7 @@ public class MainActivity extends CastEnabledActivity {
     public static final String EXTRA_FEED_ID = "fragment_feed_id";
     public static final String EXTRA_OPEN_PLAYER = "open_player";
     public static final String EXTRA_REFRESH_ON_START = "refresh_on_start";
+    public static final String KEY_GENERATED_VIEW_ID = "generated_view_id";
 
     private DrawerLayout drawerLayout;
     private View navDrawer;
@@ -88,6 +90,9 @@ public class MainActivity extends CastEnabledActivity {
     public void onCreate(Bundle savedInstanceState) {
         lastTheme = UserPreferences.getNoTitleTheme();
         setTheme(lastTheme);
+        if (savedInstanceState != null) {
+            ensureGeneratedViewIdGreaterThan(savedInstanceState.getInt(KEY_GENERATED_VIEW_ID, 0));
+        }
         super.onCreate(savedInstanceState);
         StorageUtils.checkStorageAvailability(this);
         setContentView(R.layout.main);
@@ -132,6 +137,25 @@ public class MainActivity extends CastEnabledActivity {
         sheetBehavior.setBottomSheetCallback(bottomSheetCallback);
     }
 
+    /**
+     * ViewCompat.generateViewId stores the current ID in a static variable.
+     * When the process is killed, the variable gets reset.
+     * This makes sure that we do not get ID collisions
+     * and therefore errors when trying to restore state from another view.
+     */
+    @SuppressWarnings("StatementWithEmptyBody")
+    private void ensureGeneratedViewIdGreaterThan(int minimum) {
+        while (ViewCompat.generateViewId() <= minimum) {
+            // Generate new IDs
+        }
+    }
+
+    @Override
+    protected void onSaveInstanceState(@NonNull Bundle outState) {
+        super.onSaveInstanceState(outState);
+        outState.putInt(KEY_GENERATED_VIEW_ID, ViewCompat.generateViewId());
+    }
+
     private BottomSheetBehavior.BottomSheetCallback bottomSheetCallback =
             new BottomSheetBehavior.BottomSheetCallback() {
         @Override
diff --git a/app/src/main/java/de/danoeh/antennapod/fragment/ItemPagerFragment.java b/app/src/main/java/de/danoeh/antennapod/fragment/ItemPagerFragment.java
index c198ce258..b6e4190e8 100644
--- a/app/src/main/java/de/danoeh/antennapod/fragment/ItemPagerFragment.java
+++ b/app/src/main/java/de/danoeh/antennapod/fragment/ItemPagerFragment.java
@@ -38,6 +38,8 @@ import io.reactivex.schedulers.Schedulers;
 public class ItemPagerFragment extends Fragment {
     private static final String ARG_FEEDITEMS = "feeditems";
     private static final String ARG_FEEDITEM_POS = "feeditem_pos";
+    private static final String KEY_PAGER_ID = "pager_id";
+    private ViewPager2 pager;
 
     /**
      * Creates a new instance of an ItemPagerFragment.
@@ -77,12 +79,16 @@ public class ItemPagerFragment extends Fragment {
         feedItems = getArguments().getLongArray(ARG_FEEDITEMS);
         int feedItemPos = getArguments().getInt(ARG_FEEDITEM_POS);
 
-        ViewPager2 pager = layout.findViewById(R.id.pager);
+        pager = layout.findViewById(R.id.pager);
         // FragmentStatePagerAdapter documentation:
         // > When using FragmentStatePagerAdapter the host ViewPager must have a valid ID set.
         // When opening multiple ItemPagerFragments by clicking "item" -> "visit podcast" -> "item" -> etc,
         // the ID is no longer unique and FragmentStatePagerAdapter does not display any pages.
         int newId = ViewCompat.generateViewId();
+        if (savedInstanceState != null && savedInstanceState.getInt(KEY_PAGER_ID, 0) != 0) {
+            // Restore state by using the same ID as before. ID collisions are prevented in MainActivity.
+            newId = savedInstanceState.getInt(KEY_PAGER_ID, 0);
+        }
         pager.setId(newId);
         pager.setAdapter(new ItemPagerAdapter(this));
         pager.setCurrentItem(feedItemPos, false);
@@ -99,6 +105,12 @@ public class ItemPagerFragment extends Fragment {
         return layout;
     }
 
+    @Override
+    public void onSaveInstanceState(@NonNull Bundle outState) {
+        super.onSaveInstanceState(outState);
+        outState.putInt(KEY_PAGER_ID, pager.getId());
+    }
+
     @Override
     public void onDestroyView() {
         super.onDestroyView();