Remember notifications tab after app restart (#1048)

Mastodon/Scene/Notification/NotificationViewModel.swift

@@ -25,9 +25,26 @@ final class NotificationViewModel {
     // output
     let scopes = NotificationTimelineViewModel.Scope.allCases
     @Published var viewControllers: [UIViewController] = []
-    @Published var currentPageIndex = 0
-
+    @Published var currentPageIndex = 0 {
+        didSet {
+            lastPageIndex = currentPageIndex
+        }
+    }
     
+    private var lastPageIndex: Int {
+        get {
+            UserDefaults.shared.getLastSelectedNotificationsTab(
+                accessToken: authContext.mastodonAuthenticationBox.userAuthorization.accessToken
+            )
+        }
+        set {
+            UserDefaults.shared.setLastSelectedNotificationsTab(
+                accessToken: authContext.mastodonAuthenticationBox.userAuthorization.accessToken,
+                value: newValue
+            )
+        }
+    }
+
     init(context: AppContext, authContext: AuthContext) {
         self.context = context
         self.authContext = authContext
@@ -58,7 +75,14 @@ extension NotificationViewModel: PageboyViewControllerDataSource {
     }
     
     func defaultPage(for pageboyViewController: PageboyViewController) -> PageboyViewController.Page? {
-        return .first
+        guard
+            let pageCount = pageboyViewController.pageCount,
+            pageCount > 1,
+            (0...(pageCount - 1)).contains(lastPageIndex)
+        else {
+            return .first /// this should never happen, but in case we somehow manage to acquire invalid data in `lastPageIndex` let's make sure not to crash the app.
+        }
+        return .at(index: lastPageIndex)
     }
     
 }

MastodonSDK/Sources/MastodonCommon/Preference/Preference+Notification.swift

@@ -20,7 +20,8 @@ extension UserDefaults {
     }
     
     private static let notificationCountKeyPrefix = "notification_count"
-    
+    private static let notificationsLastTabIndexKeyPrefix = "last_notification_tab_index"
+
     public func getNotificationCountWithAccessToken(accessToken: String) -> Int {
         let prefix = UserDefaults.notificationCountKeyPrefix
         let key = UserDefaults.deriveKey(from: accessToken, prefix: prefix)
@@ -38,6 +39,17 @@ extension UserDefaults {
         setNotificationCountWithAccessToken(accessToken: accessToken, value: count + 1)
     }
     
+    @objc public func getLastSelectedNotificationsTab(accessToken: String) -> Int {
+        let prefix = UserDefaults.notificationsLastTabIndexKeyPrefix
+        let key = UserDefaults.deriveKey(from: accessToken, prefix: prefix)
+        return integer(forKey: key)
+    }
+    
+    @objc public func setLastSelectedNotificationsTab(accessToken: String, value: Int) {
+        let prefix = UserDefaults.notificationsLastTabIndexKeyPrefix
+        let key = UserDefaults.deriveKey(from: accessToken, prefix: prefix)
+        setValue(value, forKey: key)
+    }
 }
 
 extension UserDefaults {