Persist more state when switching accounts

Closes #486

Tusker/Scenes/MainSceneDelegate.swift

@@ -225,7 +225,6 @@ class MainSceneDelegate: UIResponder, UIWindowSceneDelegate, TuskerSceneDelegate
             window!.windowScene!.title = account.instanceURL.host!
         }
         
-        let newRoot = createAppUI()
         if let container = window?.rootViewController as? AccountSwitchingContainerViewController {
             let direction: AccountSwitchingContainerViewController.AnimationDirection
             if animated,
@@ -235,9 +234,9 @@ class MainSceneDelegate: UIResponder, UIWindowSceneDelegate, TuskerSceneDelegate
             } else {
                 direction = .none
             }
-            container.setRoot(newRoot, for: account, animating: direction)
+            container.setRoot(createAppUI, for: account, animating: direction)
         } else {
-            window!.rootViewController = AccountSwitchingContainerViewController(root: newRoot, for: account)
+            window!.rootViewController = AccountSwitchingContainerViewController(root: createAppUI(), for: account)
         }
     }
     

Tusker/Screens/Main/AccountSwitchingContainerViewController.swift

@@ -23,7 +23,7 @@ class AccountSwitchingContainerViewController: UIViewController {
     private(set) var currentAccountID: String
     private(set) var root: AccountSwitchableViewController
     
-    private var userActivities: [String: NSUserActivity] = [:]
+    private var viewControllers: [String: (AccountSwitchableViewController?, NSUserActivity)] = [:]
     
     init(root: AccountSwitchableViewController, for account: UserAccountInfo) {
         self.currentAccountID = account.id
@@ -42,27 +42,43 @@ class AccountSwitchingContainerViewController: UIViewController {
         embedChild(root)
     }
     
-    func setRoot(_ newRoot: AccountSwitchableViewController, for account: UserAccountInfo, animating direction: AnimationDirection) {
+    override func didReceiveMemoryWarning() {
+        super.didReceiveMemoryWarning()
+        viewControllers = viewControllers.mapValues { (_, activity) in
+            (nil, activity)
+        }
+    }
+    
+    func setRoot(_ newRootProvider: () -> AccountSwitchableViewController, for account: UserAccountInfo, animating direction: AnimationDirection) {
         let oldRoot = self.root
         if direction == .none {
             oldRoot.removeViewAndController()
         }
         if let activity = oldRoot.stateRestorationActivity() {
             stateRestorationLogger.debug("AccountSwitchingContainer: saving \(activity.activityType, privacy: .public) for \(self.currentAccountID, privacy: .public)")
-            userActivities[currentAccountID] = activity
+            viewControllers[currentAccountID] = (oldRoot, activity)
+        }
+        
+        let newRoot: AccountSwitchableViewController
+        if let (existingRoot, activity) = viewControllers.removeValue(forKey: account.id) {
+            if let existingRoot {
+                newRoot = existingRoot
+                stateRestorationLogger.debug("AccountSwitchingContainer: reusing existing VC for \(account.id, privacy: .public)")
+            } else {
+                newRoot = newRootProvider()
+                stateRestorationLogger.debug("AccountSwitchingContainer: restoring \(activity.activityType, privacy: .public) for \(account.id, privacy: .public)")
+                let context = StateRestorationUserActivityHandlingContext(root: newRoot)
+                _ = activity.handleResume(manager: UserActivityManager(scene: view.window!.windowScene!, context: context))
+                context.finalize(activity: activity)
+            }
+        } else {
+            newRoot = newRootProvider()
         }
         
         self.currentAccountID = account.id
         self.root = newRoot
         embedChild(newRoot)
         
-        if let activity = userActivities.removeValue(forKey: account.id) {
-            stateRestorationLogger.debug("AccountSwitchingContainer: restoring \(activity.activityType, privacy: .public) for \(account.id, privacy: .public)")
-            let context = StateRestorationUserActivityHandlingContext(root: newRoot)
-            _ = activity.handleResume(manager: UserActivityManager(scene: view.window!.windowScene!, context: context))
-            context.finalize(activity: activity)
-        }
-        
         if direction != .none {
             if UIAccessibility.prefersCrossFadeTransitions {
                 newRoot.view.alpha = 0
@@ -92,6 +108,7 @@ class AccountSwitchingContainerViewController: UIViewController {
                 #endif
 
                 // only one edge is affected in each direction, i have no idea why
+                let origAdditionalSafeAreaInsets = oldRoot.additionalSafeAreaInsets
                 if direction == .upwards {
                     oldRoot.additionalSafeAreaInsets.bottom = view.safeAreaInsets.bottom
                 } else {
@@ -102,6 +119,7 @@ class AccountSwitchingContainerViewController: UIViewController {
                     oldRoot.view.transform = CGAffineTransform(translationX: 0, y: -newInitialOffset).scaledBy(x: scale, y: scale)
                     newRoot.view.transform = .identity
                 } completion: { (_) in
+                    oldRoot.additionalSafeAreaInsets = origAdditionalSafeAreaInsets
                     oldRoot.removeViewAndController()
                     newRoot.view.layer.masksToBounds = false
                 }

Tusker/Screens/Utilities/UIViewController+Children.swift

@@ -68,7 +68,7 @@ extension UIView {
         
         if layout {
             subview.frame = bounds
-            
+            subview.translatesAutoresizingMaskIntoConstraints = false
             NSLayoutConstraint.activate([
                 subview.leadingAnchor.constraint(equalTo: leadingAnchor),
                 subview.trailingAnchor.constraint(equalTo: trailingAnchor),