osx: better scroll view management

osx/kordophone2/ConversationView.swift

@@ -14,7 +14,7 @@ struct ConversationView: View
     @Binding var entryModel: MessageEntryView.ViewModel
     
     var body: some View {
-        VStack {
+        VStack(spacing: 0.0) {
             TranscriptView(model: $transcriptModel)
             MessageEntryView(viewModel: $entryModel)
         }

osx/kordophone2/Transcript/TranscriptDisplayItems.swift

@@ -10,7 +10,7 @@ import SwiftUI
 
 extension TranscriptView.ViewModel
 {
-    internal func rebuildDisplayItems(animated: Bool = false) {
+    internal func rebuildDisplayItems(animated: Bool = false, completion: () -> Void = {}) {
         var displayItems: [DisplayItem] = []
         var lastDate: Date = .distantPast
         var lastSender: Display.Sender? = nil
@@ -53,6 +53,7 @@ extension TranscriptView.ViewModel
         let animation: Animation? = animated ? .default : nil
         withAnimation(animation) {
             self.displayItems = displayItems
+            completion()
         }
     }
 }

osx/kordophone2/Transcript/TranscriptView.swift

@@ -13,24 +13,56 @@ struct TranscriptView: View
     
     @Environment(\.xpcClient) private var xpcClient
 
+    init(model: Binding<ViewModel>) {
+        self._model = model
+    }
+
     var body: some View {
-        ScrollView {
-            LazyVStack(spacing: 6.0) {
-                ForEach($model.displayItems.reversed()) { item in
-                    displayItemView(item.wrappedValue)
-                        .id(item.id)
-                        .scaleEffect(CGSize(width: 1.0, height: -1.0))
-                        .transition(
-                            .push(from: .top)
-                            .combined(with: .opacity)
-                        )
+        ScrollViewReader { proxy in
+            ScrollView {
+                // For resetting scroll position to the "bottom"
+                EmptyView()
+                    .id(ViewID.bottomAnchor)
+
+                LazyVStack(spacing: 6.0) {
+                    ForEach($model.displayItems.reversed()) { item in
+                        displayItemView(item.wrappedValue)
+                            .id(item.id)
+                            .scaleEffect(CGSize(width: 1.0, height: -1.0))
+                            .transition(
+                                .push(from: .top)
+                                .combined(with: .opacity)
+                            )
+                    }
+                }
+                .padding()
+            }
+
+            // Flip vertically so newest messages are at the bottom.
+            .scaleEffect(CGSize(width: 1.0, height: -1.0))
+
+            // Watch for xpc events
+            .task { await watchForMessageListChanges() }
+
+            // On conversation change, reload displayed messages and mark as read.
+            .onChange(of: model.displayedConversation) { oldValue, newValue in
+                Task {
+                    guard oldValue != newValue else { return }
+
+                    // Reload NOW
+                    await model.reloadMessages(animated: false) {
+                        // Once that's done, scroll to the "bottom" (actually top)
+                        proxy.scrollTo(ViewID.bottomAnchor, anchor: .top)
+                    }
+                }
+
+                Task.detached {
+                    // Mark as read on server, and trigger a sync.
+                    await model.markAsRead()
+                    await model.triggerSync()
                 }
             }
-            .padding()
         }
-        .scaleEffect(CGSize(width: 1.0, height: -1.0))
-        .id(model.displayedConversation?.id)
-        .task { await watchForMessageListChanges() }
     }
     
     private func watchForMessageListChanges() async {
@@ -82,6 +114,13 @@ struct TranscriptView: View
 
     // MARK: - Types
 
+    enum ViewID: String
+    {
+        case bottomAnchor
+    }
+
+    // MARK: - View Model
+
     @Observable
     class ViewModel
     {
@@ -94,7 +133,6 @@ struct TranscriptView: View
 
         init(messages: [Display.Message] = []) {
             self.messages = messages
-            observeDisplayedConversation()
             rebuildDisplayItems()
         }
         
@@ -106,7 +144,7 @@ struct TranscriptView: View
             needsReload = .yes(animated)
             Task { @MainActor [weak self] in
                 guard let self else { return }
-                await reloadMessages()
+                await reloadIfNeeded()
             }
         }
         
@@ -115,22 +153,6 @@ struct TranscriptView: View
             setNeedsReload(animated: false)
         }
         
-        private func observeDisplayedConversation() {
-            withObservationTracking {
-                _ = displayedConversation
-            } onChange: {
-                Task { @MainActor [weak self] in
-                    guard let self else { return }
-                    
-                    await markAsRead()
-                    await triggerSync()
-                    
-                    setNeedsReload(animated: false)
-                    observeDisplayedConversation()
-                }
-            }
-        }
-        
         func markAsRead() async {
             guard let displayedConversation else { return }
             
@@ -151,10 +173,14 @@ struct TranscriptView: View
             }
         }
 
-        private func reloadMessages() async {
+        func reloadIfNeeded(completion: () -> Void = {}) async {
             guard case .yes(let animated) = needsReload else { return }
             needsReload = .no
 
+            await reloadMessages(animated: animated, completion: completion)
+        }
+
+        func reloadMessages(animated: Bool, completion: () -> Void) async {
             guard let displayedConversation else { return }
             
             do {
@@ -167,8 +193,10 @@ struct TranscriptView: View
                 // Only animate for incoming messages.
                 let shouldAnimate = (newIds.count == 1)
 
-                self.messages = clientMessages
-                self.rebuildDisplayItems(animated: animated && shouldAnimate)
+                await MainActor.run {
+                    self.messages = clientMessages
+                    self.rebuildDisplayItems(animated: animated && shouldAnimate, completion: completion)
+                }
             } catch {
                 print("Message fetch error: \(error)")
             }