Fix broken view pager tabs implementation

- Fragments were being recreated from scratch (losing their state) every
time some configuration change occurred (e.g. screen rotation).
- Use `FragmentStatePagerAdapter` instead, as it is built to work with
them and manage their states.

app/src/main/java/org/schabi/newpipe/fragments/MainFragment.java

@@ -1,5 +1,6 @@
 package org.schabi.newpipe.fragments;
 
+import android.content.Context;
 import android.os.Bundle;
 import android.util.Log;
 import android.view.LayoutInflater;
@@ -15,7 +16,7 @@ import androidx.appcompat.app.ActionBar;
 import androidx.appcompat.app.AppCompatActivity;
 import androidx.fragment.app.Fragment;
 import androidx.fragment.app.FragmentManager;
-import androidx.fragment.app.FragmentPagerAdapter;
+import androidx.fragment.app.FragmentStatePagerAdapter;
 import androidx.viewpager.widget.ViewPager;
 
 import com.google.android.material.tabs.TabLayout;
@@ -52,32 +53,19 @@ public class MainFragment extends BaseFragment implements TabLayout.OnTabSelecte
         super.onCreate(savedInstanceState);
         setHasOptionsMenu(true);
 
-        destroyOldFragments();
-
         tabsManager = TabsManager.getManager(activity);
         tabsManager.setSavedTabsListener(() -> {
             if (DEBUG) {
                 Log.d(TAG, "TabsManager.SavedTabsChangeListener: onTabsChanged called, isResumed = " + isResumed());
             }
             if (isResumed()) {
-                updateTabs();
+                setupTabs();
             } else {
                 hasTabsChanged = true;
             }
         });
     }
 
-    private void destroyOldFragments() {
-        for (Fragment fragment : getChildFragmentManager().getFragments()) {
-            if (fragment != null) {
-                getChildFragmentManager()
-                        .beginTransaction()
-                        .remove(fragment)
-                        .commitNowAllowingStateLoss();
-            }
-        }
-    }
-
     @Override
     public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
         return inflater.inflate(R.layout.fragment_main, container, false);
@@ -90,23 +78,17 @@ public class MainFragment extends BaseFragment implements TabLayout.OnTabSelecte
         tabLayout = rootView.findViewById(R.id.main_tab_layout);
         viewPager = rootView.findViewById(R.id.pager);
 
-        /*  Nested fragment, use child fragment here to maintain backstack in view pager. */
-        pagerAdapter = new SelectedTabsPagerAdapter(getChildFragmentManager());
-        viewPager.setAdapter(pagerAdapter);
-
         tabLayout.setupWithViewPager(viewPager);
         tabLayout.addOnTabSelectedListener(this);
-        updateTabs();
+
+        setupTabs();
     }
 
     @Override
     public void onResume() {
         super.onResume();
 
-        if (hasTabsChanged) {
-            hasTabsChanged = false;
-            updateTabs();
-        }
+        if (hasTabsChanged) setupTabs();
     }
 
     @Override
@@ -153,14 +135,21 @@ public class MainFragment extends BaseFragment implements TabLayout.OnTabSelecte
     // Tabs
     //////////////////////////////////////////////////////////////////////////*/
 
-    public void updateTabs() {
+    public void setupTabs() {
         tabsList.clear();
         tabsList.addAll(tabsManager.getTabs());
-        pagerAdapter.notifyDataSetChanged();
 
-        viewPager.setOffscreenPageLimit(pagerAdapter.getCount());
+        if (pagerAdapter == null || !pagerAdapter.sameTabs(tabsList)) {
+            pagerAdapter = new SelectedTabsPagerAdapter(requireContext(), getChildFragmentManager(), tabsList);
+        }
+        // Clear previous tabs/fragments and set new adapter
+        viewPager.setAdapter(pagerAdapter);
+        viewPager.setOffscreenPageLimit(tabsList.size());
+
         updateTabsIconAndDescription();
         updateCurrentTitle();
+
+        hasTabsChanged = false;
     }
 
     private void updateTabsIconAndDescription() {
@@ -194,26 +183,30 @@ public class MainFragment extends BaseFragment implements TabLayout.OnTabSelecte
         updateCurrentTitle();
     }
 
-    private class SelectedTabsPagerAdapter extends FragmentPagerAdapter {
+    private static class SelectedTabsPagerAdapter extends FragmentStatePagerAdapter {
+        private final Context context;
+        private final List<Tab> internalTabsList;
 
-        private SelectedTabsPagerAdapter(FragmentManager fragmentManager) {
-            super(fragmentManager);
+        private SelectedTabsPagerAdapter(Context context, FragmentManager fragmentManager, List<Tab> tabsList) {
+            super(fragmentManager, BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT);
+            this.context = context;
+            this.internalTabsList = new ArrayList<>(tabsList);
         }
 
         @Override
         public Fragment getItem(int position) {
-            final Tab tab = tabsList.get(position);
+            final Tab tab = internalTabsList.get(position);
 
             Throwable throwable = null;
             Fragment fragment = null;
             try {
-                fragment = tab.getFragment(requireContext());
+                fragment = tab.getFragment(context);
             } catch (ExtractionException e) {
                 throwable = e;
             }
 
             if (throwable != null) {
-                ErrorActivity.reportError(activity, throwable, activity.getClass(), null,
+                ErrorActivity.reportError(context, throwable, null, null,
                         ErrorActivity.ErrorInfo.make(UserAction.UI_ERROR, "none", "", R.string.app_ui_crash));
                 return new BlankFragment();
             }
@@ -234,15 +227,11 @@ public class MainFragment extends BaseFragment implements TabLayout.OnTabSelecte
 
         @Override
         public int getCount() {
-            return tabsList.size();
+            return internalTabsList.size();
         }
 
-        @Override
-        public void destroyItem(ViewGroup container, int position, Object object) {
-            getChildFragmentManager()
-                    .beginTransaction()
-                    .remove((Fragment) object)
-                    .commitNowAllowingStateLoss();
+        public boolean sameTabs(List<Tab> tabsToCompare) {
+            return internalTabsList.equals(tabsToCompare);
         }
     }
 }

app/src/main/java/org/schabi/newpipe/settings/tabs/Tab.java

@@ -28,6 +28,8 @@ import org.schabi.newpipe.util.KioskTranslator;
 import org.schabi.newpipe.util.ServiceHelper;
 import org.schabi.newpipe.util.ThemeHelper;
 
+import java.util.Objects;
+
 public abstract class Tab {
     Tab() {
     }
@@ -47,6 +49,8 @@ public abstract class Tab {
 
     @Override
     public boolean equals(Object obj) {
+        if (obj == this) return true;
+
         return obj instanceof Tab && obj.getClass().equals(this.getClass())
                 && ((Tab) obj).getTabId() == this.getTabId();
     }
@@ -340,6 +344,13 @@ public abstract class Tab {
             kioskId = jsonObject.getString(JSON_KIOSK_ID_KEY, "<no-id>");
         }
 
+        @Override
+        public boolean equals(Object obj) {
+            return super.equals(obj) &&
+                    kioskServiceId == ((KioskTab) obj).kioskServiceId
+                    && Objects.equals(kioskId, ((KioskTab) obj).kioskId);
+        }
+
         public int getKioskServiceId() {
             return kioskServiceId;
         }
@@ -409,6 +420,14 @@ public abstract class Tab {
             channelName = jsonObject.getString(JSON_CHANNEL_NAME_KEY, "<no-name>");
         }
 
+        @Override
+        public boolean equals(Object obj) {
+            return super.equals(obj) &&
+                    channelServiceId == ((ChannelTab) obj).channelServiceId
+                    && Objects.equals(channelUrl, ((ChannelTab) obj).channelUrl)
+                    && Objects.equals(channelName, ((ChannelTab) obj).channelName);
+        }
+
         public int getChannelServiceId() {
             return channelServiceId;
         }