Support iOS 13 interactive dismissal of compose screen

Tusker/Screens/Compose/ComposeViewController.swift

@@ -34,6 +34,7 @@ class ComposeViewController: UIViewController {
         }
     }
     
+    var hasChanges = false
     var currentDraft: DraftsManager.Draft?
     // Weak so that if a new session is initiated (i.e. XCBManager.currentSession is changed) while the current one is in progress, this one will be released
     weak var xcbSession: XCBSession?
@@ -83,7 +84,7 @@ class ComposeViewController: UIViewController {
         title = "Compose"
         tabBarItem.image = UIImage(systemName: "pencil")
         
-        navigationItem.leftBarButtonItem = UIBarButtonItem(barButtonSystemItem: .cancel, target: self, action: #selector(cancelButtonPressed))
+        navigationItem.leftBarButtonItem = UIBarButtonItem(barButtonSystemItem: .cancel, target: self, action: #selector(showSaveAndClosePrompt))
         navigationItem.rightBarButtonItem = UIBarButtonItem(title: "Post", style: .done, target: self, action: #selector(postButtonPressed))
         postBarButtonItem = navigationItem.rightBarButtonItem
     }
@@ -235,6 +236,14 @@ class ComposeViewController: UIViewController {
         charactersRemainingLabel.text = remaining.description
     }
     
+    func updateHasChanges() {
+        if let currentDraft = currentDraft {
+            hasChanges = statusTextView.text != currentDraft.text
+        } else {
+            hasChanges = !statusTextView.text.isEmpty
+        }
+    }
+    
     func updatePlaceholder() {
         placeholderLabel.isHidden = !statusTextView.text.isEmpty
     }
@@ -304,7 +313,7 @@ class ComposeViewController: UIViewController {
     
     // MARK: - Interaction
     
-    @objc func cancelButtonPressed() {
+    @objc func showSaveAndClosePrompt() {
         guard statusTextView.text.trimmingCharacters(in: .whitespacesAndNewlines) != initialText else {
             close()
             return
@@ -337,6 +346,7 @@ class ComposeViewController: UIViewController {
     
     @objc func contentWarningTextFieldDidChange() {
         updateCharactersRemaining()
+        updateHasChanges()
     }
     
     @objc func visbilityButtonPressed() {
@@ -500,6 +510,7 @@ extension ComposeViewController: UITextViewDelegate {
     func textViewDidChange(_ textView: UITextView) {
         updateCharactersRemaining()
         updatePlaceholder()
+        updateHasChanges()
     }
 }
 
@@ -552,3 +563,24 @@ extension ComposeViewController: DraftsTableViewControllerDelegate {
         }
     }
 }
+
+extension ComposeViewController: UIAdaptivePresentationControllerDelegate {
+    func presentationControllerShouldDismiss(_ presentationController: UIPresentationController) -> Bool {
+        return Preferences.shared.automaticallySaveDrafts || !hasChanges
+    }
+    
+    func presentationControllerDidAttemptToDismiss(_ presentationController: UIPresentationController) {
+        showSaveAndClosePrompt()
+    }
+    
+    // when the compose screen is dismissed interactively, close() isn't called, so we make sure to
+    // complete the X-Callback-URL session and save the draft is automatic saving is enabled
+    // (if automatic saving is off, the draft will get saved/discarded by the user when didAttemptToDismiss is called
+    func presentationControllerDidDismiss(_ presentationController: UIPresentationController) {
+        if Preferences.shared.automaticallySaveDrafts {
+            saveDraft()
+        }
+        xcbSession?.complete(with: .cancel)
+    }
+}
+

Tusker/Screens/Main/MainTabBarViewController.swift

@@ -34,8 +34,10 @@ class MainTabBarViewController: UITabBarController, UITabBarControllerDelegate {
     
     func tabBarController(_ tabBarController: UITabBarController, shouldSelect viewController: UIViewController) -> Bool {
         if viewController is ComposeViewController {
-            let compose = embedInNavigationController(ComposeViewController())
-            tabBarController.present(compose, animated: true)
+            let compose = ComposeViewController()
+            let navigationController = embedInNavigationController(compose)
+            navigationController.presentationController?.delegate = compose
+            present(navigationController, animated: true)
             return false
         }
         return true