Add a menu option to load the newest notifications (#3708)

- Create a flow with new items (arbitrary ints) when a reload from the top should happen
- Combine this flow with notificationFilter, so changes to either of them trigger a reload
- Provide a menu item in NotificationsFragment to initiate the reload
- Handle the action in the view model

app/src/main/java/com/keylesspalace/tusky/components/notifications/NotificationsFragment.kt

@@ -454,6 +454,10 @@ class NotificationsFragment :
                 onRefresh()
                 true
             }
+            R.id.load_newest -> {
+                viewModel.accept(InfallibleUiAction.LoadNewest)
+                true
+            }
             else -> false
         }
     }

app/src/main/java/com/keylesspalace/tusky/components/notifications/NotificationsViewModel.kt

@@ -59,6 +59,7 @@ import kotlinx.coroutines.flow.distinctUntilChanged
 import kotlinx.coroutines.flow.filter
 import kotlinx.coroutines.flow.filterIsInstance
 import kotlinx.coroutines.flow.flatMapLatest
+import kotlinx.coroutines.flow.getAndUpdate
 import kotlinx.coroutines.flow.map
 import kotlinx.coroutines.flow.onEach
 import kotlinx.coroutines.flow.onStart
@@ -123,6 +124,12 @@ sealed class InfallibleUiAction : UiAction() {
      * can do.
      */
     data class SaveVisibleId(val visibleId: String) : InfallibleUiAction()
+
+    /** Ignore the saved reading position, load the page with the newest items */
+    // Resets the account's `lastNotificationId`, which can't fail, which is why this is
+    // infallible. Reloading the data may fail, but that's handled by the paging system /
+    // adapter refresh logic.
+    object LoadNewest : InfallibleUiAction()
 }
 
 /** Actions the user can trigger on an individual notification. These may fail. */
@@ -300,6 +307,9 @@ class NotificationsViewModel @Inject constructor(
     /** Flow of user actions received from the UI */
     private val uiAction = MutableSharedFlow<UiAction>()
 
+    /** Flow that can be used to trigger a full reload */
+    private val reload = MutableStateFlow(0)
+
     /** Flow of successful action results */
     // Note: This is a SharedFlow instead of a StateFlow because success state does not need to be
     // retained. A message is shown once to a user and then dismissed. Re-collecting the flow
@@ -342,6 +352,18 @@ class NotificationsViewModel @Inject constructor(
                 )
             }
 
+        // Reset the last notification ID to "0" to fetch the newest notifications, and
+        // increment `reload` to trigger creation of a new PagingSource.
+        viewModelScope.launch {
+            uiAction
+                .filterIsInstance<InfallibleUiAction.LoadNewest>()
+                .collectLatest {
+                    account.lastNotificationId = "0"
+                    accountManager.saveAccount(account)
+                    reload.getAndUpdate { it + 1 }
+                }
+        }
+
         // Save the visible notification ID
         viewModelScope.launch {
             uiAction
@@ -465,11 +487,12 @@ class NotificationsViewModel @Inject constructor(
             }
         }
 
-        pagingData = notificationFilter
+        // Re-fetch notifications if either of `notificationFilter` or `reload` flows have
+        // new items.
+        pagingData = combine(notificationFilter, reload) { action, _ -> action }
             .flatMapLatest { action ->
                 getNotifications(filters = action.filter, initialKey = getInitialKey())
-            }
-            .cachedIn(viewModelScope)
+            }.cachedIn(viewModelScope)
 
         uiState = combine(notificationFilter, getUiPrefs()) { filter, prefs ->
             UiState(

app/src/main/res/menu/fragment_notifications.xml

@@ -22,4 +22,9 @@
         android:id="@+id/action_refresh"
         android:title="@string/action_refresh"
         app:showAsAction="never" />
+
+    <item
+        android:id="@+id/load_newest"
+        android:title="@string/load_newest_notifications"
+        app:showAsAction="never" />
 </menu>

app/src/main/res/values/strings.xml

@@ -810,4 +810,5 @@
         you follow.\n\nTo explore accounts you can either discover them in one of the other timelines.
         For example the local timeline of your instance [iconics gmd_group]. Or you can search them
         by name [iconics gmd_search]; for example search for Tusky to find our Mastodon account.</string>
+    <string name="load_newest_notifications">Load newest notifications</string>
 </resources>