Save and retrieve top bar state in SavedInstanceState bundle

Save information about the selected feed id and top bar filter ui state in the
SavedInstanceState Bundle and recover it when the activity is destroyed due to
system-initiated destruction (low-memory or "Don't keep activities"	dev setting)
to keep the experience consistent.

WordPress/src/main/java/org/wordpress/android/ui/reader/ReaderFragment.kt

@@ -210,7 +210,12 @@ class ReaderFragment : Fragment(R.layout.reader_fragment_layout), MenuProvider,
 
         observeJetpackOverlayEvent(savedInstanceState)
 
-        viewModel.start()
+        viewModel.start(savedInstanceState)
+    }
+
+    override fun onSaveInstanceState(outState: Bundle) {
+        super.onSaveInstanceState(outState)
+        viewModel.onSaveInstanceState(outState)
     }
 
     private fun updateUiState(uiState: ReaderViewModel.ReaderUiState) {

WordPress/src/main/java/org/wordpress/android/ui/reader/viewmodels/ReaderViewModel.kt

@@ -1,14 +1,18 @@
 package org.wordpress.android.ui.reader.viewmodels
 
+import android.os.Bundle
+import android.os.Parcelable
 import androidx.annotation.DrawableRes
 import androidx.annotation.StringRes
+import androidx.core.os.BundleCompat
 import androidx.lifecycle.LiveData
 import androidx.lifecycle.MutableLiveData
 import kotlinx.coroutines.CoroutineDispatcher
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Job
 import kotlinx.coroutines.delay
 import kotlinx.coroutines.withContext
+import kotlinx.parcelize.Parcelize
 import org.greenrobot.eventbus.EventBus
 import org.greenrobot.eventbus.Subscribe
 import org.greenrobot.eventbus.ThreadMode.MAIN
@@ -114,24 +118,31 @@ class ReaderViewModel @Inject constructor(
         EventBus.getDefault().register(this)
     }
 
-    fun start() {
+    fun start(savedInstanceState: Bundle?) {
         if (tagsRequireUpdate()) _updateTags.value = Event(Unit)
         if (initialized) return
-        loadTabs()
+        loadTabs(savedInstanceState)
         if (jetpackBrandingUtils.shouldShowJetpackPoweredBottomSheet()) showJetpackPoweredBottomSheet()
     }
 
+    fun onSaveInstanceState(out: Bundle) {
+        _topBarUiState.value?.let {
+            out.putString(KEY_TOP_BAR_UI_STATE_SELECTED_ITEM_ID, it.selectedItem.id)
+            out.putParcelable(KEY_TOP_BAR_UI_STATE_FILTER_UI_STATE, it.filterUiState)
+        }
+    }
+
     private fun showJetpackPoweredBottomSheet() {
 //        _showJetpackPoweredBottomSheet.value = Event(true)
     }
 
-    private fun loadTabs() {
+    private fun loadTabs(savedInstanceState: Bundle? = null) {
         launch {
             val currentContentUiState = _uiState.value as? ContentUiState
             val tagList = loadReaderTabsUseCase.loadTabs()
             if (tagList.isNotEmpty() && readerTagsList != tagList) {
                 updateReaderTagsList(tagList)
-                updateTopBarUiState()
+                updateTopBarUiState(savedInstanceState)
                 _uiState.value = ContentUiState(
                     tabUiStates = tagList.map { TabUiState(label = UiStringText(it.label)) },
                     selectedReaderTag = selectedReaderTag(),
@@ -352,20 +363,34 @@ class ReaderViewModel @Inject constructor(
             readerTagsList[readerTopBarMenuHelper.getReaderTagIndexFromMenuItem(it.selectedItem)]
         }
 
-    private suspend fun updateTopBarUiState() {
+    private suspend fun updateTopBarUiState(savedInstanceState: Bundle? = null) {
         withContext(bgDispatcher) {
             val menuItems = readerTopBarMenuHelper.createMenu(readerTagsList)
 
             // if menu is exactly the same as before, don't update
             if (_topBarUiState.value?.menuItems == menuItems) return@withContext
 
-            // if there's already a selected item, use it, otherwise use the first item
+
+            // if there's already a selected item, use it, otherwise use the first item, also try to use the saved state
+            val savedStateSelectedId = savedInstanceState?.getString(KEY_TOP_BAR_UI_STATE_SELECTED_ITEM_ID)
             val selectedItem = _topBarUiState.value?.selectedItem
-                ?: menuItems.first { it is MenuElementData.Item.Single } as MenuElementData.Item.Single
+                ?: menuItems.filterSingleItems()
+                    .let { singleItems ->
+                        singleItems.firstOrNull { it.id == savedStateSelectedId } ?: singleItems.first()
+                    }
 
-            // if there's a selected item and filter state, also use the filter state
+            // if there's a selected item and filter state, also use the filter state, also try to use the saved state
             val filterUiState = _topBarUiState.value?.filterUiState
                 ?.takeIf { _topBarUiState.value?.selectedItem != null }
+                ?: savedInstanceState
+                    ?.let {
+                        BundleCompat.getParcelable(
+                            it,
+                            KEY_TOP_BAR_UI_STATE_FILTER_UI_STATE,
+                            TopBarUiState.FilterUiState::class.java
+                        )
+                    }
+                    ?.takeIf { selectedItem.id == savedStateSelectedId }
 
             _topBarUiState.postValue(
                 TopBarUiState(
@@ -525,13 +550,14 @@ class ReaderViewModel @Inject constructor(
         val onDropdownMenuClick: () -> Unit,
         val isSearchActionVisible: Boolean = false,
     ) {
+        @Parcelize
         data class FilterUiState(
             val blogsFilterCount: Int,
             val tagsFilterCount: Int,
             val selectedItem: ReaderFilterSelectedItem? = null,
             val showBlogsFilter: Boolean = blogsFilterCount > 0,
             val showTagsFilter: Boolean = tagsFilterCount > 0,
-        )
+        ) : Parcelable
     }
 
     sealed class ReaderUiState(
@@ -573,6 +599,9 @@ class ReaderViewModel @Inject constructor(
         private const val QUICK_START_DISCOVER_TAB_STEP_DELAY = 2000L
         private const val QUICK_START_PROMPT_DURATION = 5000
         private const val FILTER_UPDATE_DELAY = 50L
+
+        private const val KEY_TOP_BAR_UI_STATE_SELECTED_ITEM_ID = "topBarUiState_selectedItem_id"
+        private const val KEY_TOP_BAR_UI_STATE_FILTER_UI_STATE = "topBarUiState_filterUiState"
     }
 }