Keep the tabs adapter over the life of the viewpager (#3255)

Make `tabs` `var` instead of `val` in `MainPagerAdapter` so it can be updated
when tabs change.

Then detach the `tabLayoutMediator`, update the tabs, and call
`notifyItemRangeChanged` in `setupTabs()`.

This fixes a bug (not sure if it's this code, or in ViewPager2) where
assigning a new adapter to the view pager seemed to result in a leak of one
or more fragments. This wasn't user-visible, but it's a leak, and it becomes
user-visible when fragments want to display menus.

This also fixes two other bugs:

1. Be on the left-most tab. Scroll down a bit. Then modify the tabs at
   "Account preferences > tabs", but keep the left-most tab as-is.

   Then go back to MainActivity. Your reading position in the left-most
   tab has been jumped to the top.

2. Be on any non-left-most tab. Then modify the tab list by reordering tabs
   (adding/removing tabs is also OK).

   Then go back to MainActivity. Your tab selection has been overridden,
   and the left-most tab has been selected.

Because the fragments are not destroyed unnecessarily your reading position
is retained. And it remembers the tab you had selected, and as long as that
tab is still present you will be returned to it, even if it's changed
position in the list.

Fixes #3251

app/src/main/java/com/keylesspalace/tusky/MainActivity.kt

@@ -34,6 +34,7 @@ import android.view.View
 import android.widget.ImageView
 import androidx.activity.OnBackPressedCallback
 import androidx.appcompat.app.AlertDialog
+import androidx.appcompat.content.res.AppCompatResources
 import androidx.coordinatorlayout.widget.CoordinatorLayout
 import androidx.core.app.ActivityCompat
 import androidx.core.content.ContextCompat
@@ -170,6 +171,12 @@ class MainActivity : BottomSheetActivity(), ActionButtonActivity, HasAndroidInje
     // We need to know if the emoji pack has been changed
     private var selectedEmojiPack: String? = null
 
+    /** Mediate between binding.viewPager and the chosen tab layout */
+    private var tabLayoutMediator: TabLayoutMediator? = null
+
+    /** Adapter for the different timeline tabs */
+    private lateinit var tabAdapter: MainPagerAdapter
+
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
 
@@ -275,6 +282,13 @@ class MainActivity : BottomSheetActivity(), ActionButtonActivity, HasAndroidInje
 
         fetchAnnouncements()
 
+        // Initialise the tab adapter and set to viewpager. Fragments appear to be leaked if the
+        // adapter changes over the life of the viewPager (the adapter, not its contents), so set
+        // the initial list of tabs to empty, and set the full list later in setupTabs(). See
+        // https://github.com/tuskyapp/Tusky/issues/3251 for details.
+        tabAdapter = MainPagerAdapter(emptyList(), this)
+        binding.viewPager.adapter = tabAdapter
+
         setupTabs(showNotificationTab)
 
         eventHub.events
@@ -655,7 +669,7 @@ class MainActivity : BottomSheetActivity(), ActionButtonActivity, HasAndroidInje
     }
 
     private fun setupTabs(selectNotificationTab: Boolean) {
-        val activeTabLayout = if (preferences.getString("mainNavPosition", "top") == "bottom") {
+        val activeTabLayout = if (preferences.getString(PrefKeys.MAIN_NAV_POSITION, "top") == "bottom") {
             val actionBarSize = getDimension(this, androidx.appcompat.R.attr.actionBarSize)
             val fabMargin = resources.getDimensionPixelSize(R.dimen.fabMargin)
             (binding.composeButton.layoutParams as CoordinatorLayout.LayoutParams).bottomMargin = actionBarSize + fabMargin
@@ -668,29 +682,36 @@ class MainActivity : BottomSheetActivity(), ActionButtonActivity, HasAndroidInje
             binding.tabLayout
         }
 
+        // Save the previous tab so it can be restored later
+        val previousTab = tabAdapter.tabs.getOrNull(binding.viewPager.currentItem)
+
         val tabs = accountManager.activeAccount!!.tabPreferences
 
-        val adapter = MainPagerAdapter(tabs, this)
-        binding.viewPager.adapter = adapter
-        TabLayoutMediator(activeTabLayout, binding.viewPager) { _: TabLayout.Tab?, _: Int -> }.attach()
-        activeTabLayout.removeAllTabs()
-        for (i in tabs.indices) {
-            val tab = activeTabLayout.newTab()
-                .setIcon(tabs[i].icon)
-            if (tabs[i].id == LIST) {
-                tab.contentDescription = tabs[i].arguments[1]
-            } else {
-                tab.setContentDescription(tabs[i].text)
-            }
-            activeTabLayout.addTab(tab)
+        // Detach any existing mediator before changing tab contents and attaching a new mediator
+        tabLayoutMediator?.detach()
 
-            if (tabs[i].id == NOTIFICATIONS) {
-                notificationTabPosition = i
-                if (selectNotificationTab) {
-                    tab.select()
-                }
+        tabAdapter.tabs = tabs
+        tabAdapter.notifyItemRangeChanged(0, tabs.size)
+
+        tabLayoutMediator = TabLayoutMediator(activeTabLayout, binding.viewPager, true) {
+            tab: TabLayout.Tab, position: Int ->
+            tab.icon = AppCompatResources.getDrawable(this@MainActivity, tabs[position].icon)
+            tab.contentDescription = when (tabs[position].id) {
+                LIST -> tabs[position].arguments[position]
+                else -> getString(tabs[position].text)
             }
-        }
+        }.also { it.attach() }
+
+        // Selected tab is either
+        // - Notification tab (if appropriate)
+        // - The previously selected tab (if it hasn't been removed)
+        // - Left-most tab
+        val position = if (selectNotificationTab) {
+            tabs.indexOfFirst { it.id == NOTIFICATIONS }
+        } else {
+            previousTab?.let { tabs.indexOfFirst { it == previousTab } }
+        }.takeIf { it != -1 } ?: 0
+        binding.viewPager.setCurrentItem(position, false)
 
         val pageMargin = resources.getDimensionPixelSize(R.dimen.tab_page_margin)
         binding.viewPager.setPageTransformer(MarginPageTransformer(pageMargin))
@@ -710,18 +731,18 @@ class MainActivity : BottomSheetActivity(), ActionButtonActivity, HasAndroidInje
 
                 binding.mainToolbar.title = tabs[tab.position].title(this@MainActivity)
 
-                refreshComposeButtonState(adapter, tab.position)
+                refreshComposeButtonState(tabAdapter, tab.position)
             }
 
             override fun onTabUnselected(tab: TabLayout.Tab) {}
 
             override fun onTabReselected(tab: TabLayout.Tab) {
-                val fragment = adapter.getFragment(tab.position)
+                val fragment = tabAdapter.getFragment(tab.position)
                 if (fragment is ReselectableFragment) {
                     (fragment as ReselectableFragment).onReselect()
                 }
 
-                refreshComposeButtonState(adapter, tab.position)
+                refreshComposeButtonState(tabAdapter, tab.position)
             }
         }.also {
             activeTabLayout.addOnTabSelectedListener(it)
@@ -730,7 +751,7 @@ class MainActivity : BottomSheetActivity(), ActionButtonActivity, HasAndroidInje
         val activeTabPosition = if (selectNotificationTab) notificationTabPosition else 0
         binding.mainToolbar.title = tabs[activeTabPosition].title(this@MainActivity)
         binding.mainToolbar.setOnClickListener {
-            (adapter.getFragment(activeTabLayout.selectedTabPosition) as? ReselectableFragment)?.onReselect()
+            (tabAdapter.getFragment(activeTabLayout.selectedTabPosition) as? ReselectableFragment)?.onReselect()
         }
 
         updateProfiles()

app/src/main/java/com/keylesspalace/tusky/pager/MainPagerAdapter.kt

@@ -20,7 +20,7 @@ import androidx.fragment.app.FragmentActivity
 import com.keylesspalace.tusky.TabData
 import com.keylesspalace.tusky.util.CustomFragmentStateAdapter
 
-class MainPagerAdapter(val tabs: List<TabData>, activity: FragmentActivity) : CustomFragmentStateAdapter(activity) {
+class MainPagerAdapter(var tabs: List<TabData>, activity: FragmentActivity) : CustomFragmentStateAdapter(activity) {
 
     override fun createFragment(position: Int): Fragment {
         val tab = tabs[position]