[iOS] The scroll position doesn't change when the text is changed from the parent view

[kkebo]

Description

The scroll position doesn't change when the text is changed from the parent view.

ScreenRecording_05-05-2025.03-00-46_1.mp4

How to reproduce the issue

1. Create a new iOS app
2. Add CodeEditorView as a dependency
3. Write the following code:

   import SwiftUI
   import CodeEditorView
   
   struct ContentView: View {
       @State private var text = ""
       @State private var position = CodeEditor.Position()
   
       var body: some View {
           VStack {
               Button("Clear") {
                   text.removeAll()
               }
               .buttonStyle(.borderedProminent)
               .controlSize(.large)
               CodeEditor(text: $text, position: $position, messages: .constant([]))
                   .frame(height: 100)
           }
           .onChange(of: position) { oldValue, newValue in
               print(oldValue, newValue)
           }
       }
   }

4. Run it
5. Tap CodeEditor to put the cursor on it
6. Press the return key repeatedly until it begins to scroll down
7. Tap "Clear" button

Expected behavior

CodeEditor will scroll up to the first line just like when you tap "Select All" and press the delete key.

ScreenRecording_05-05-2025.04-03-31_1.mp4

Actual behavior

The text will be removed, but the scroll position won't be changed and you can't even scroll on it until you input some text. Also, onChange(of: position) won't be called.

Environment

• CodeEditorView 0.15.4 and main (aba6c18)
• iPadOS 18.5 Developer Beta 4

[kkebo]

I thought the issue could be resolved by kkebo@024b6b3, but I found that this commit would cause another issue; the same issue occurs if I press any key after tapping "Clear" button.

[mchakravarty]

Thank for reporting this somewhat tricky problem. Unfortunately, #134 is also problematic. Let me explain why.

First of all, I think, the problem is that updateUIView(:context:) sets context.coordinator.updatingView to avoid update loops. For example, when we update codeView.text, we don't want to trigger setText(:) (from makeUIView(context:) to update the $text binding).

Now, in this case, setting codeView.text leads to an update of the scroll position, which triggers Coordinator.scrollPositionDidChange(:). However, that methods starts on Line 701 with

guard !updatingView else { return }

This effectively suppresses the update of the scroll position, which causes the bug that you discovered.

Now, I'm thinking that because we also have got the test for abs(position.verticalScrollPosition - scrollView.verticalScrollPosition) > 0.0001 in that method, maybe we can remove

guard !updatingView else { return }

without causing any problems.

Would you mind trying whether that would solve your problem, too?

[kkebo]

Unfortunately, #134 is also problematic.

OK, I closed #134 for now.

Would you mind trying whether that would solve your problem, too?

I tried that, but it didn't solve the problem.

I ran the code again printing values out.

diff:

diff --git a/Sources/CodeEditorView/CodeEditor.swift b/Sources/CodeEditorView/CodeEditor.swift
index a141218..3afcc57 100644
--- a/Sources/CodeEditorView/CodeEditor.swift
+++ b/Sources/CodeEditorView/CodeEditor.swift
@@ -645,7 +645,9 @@ extension CodeEditor: UIViewRepresentable {
       if language.languageService !== codeView.language.languageService {
         (codeView.optCodeStorage?.delegate as? CodeStorageDelegate)?.skipNextChangeNotificationToLanguageService = true
       }
+      print("updateUIView:", "before codeView.text = text:", "codeView.vrticalScrollPosition \(codeView.verticalScrollPosition), position.verticalScrollPosition \(position.verticalScrollPosition)")
       codeView.text = text
+      print("updateUIView:", "after codeView.text = text:", "codeView.vrticalScrollPosition \(codeView.verticalScrollPosition), position.verticalScrollPosition \(position.verticalScrollPosition)")
 //      // FIXME: Stupid hack to force redrawing when the language doesn't change. (A language change already forces
 //      // FIXME: redrawing.)
 //      if language == codeView.language {
@@ -665,6 +667,7 @@ extension CodeEditor: UIViewRepresentable {
       }
     }
     if abs(position.verticalScrollPosition - textView.verticalScrollPosition) > 0.0001 {
+      print("updateUIView:", "textView.vrticalScrollPosition \(textView.verticalScrollPosition) = position.verticalScrollPosition \(position.verticalScrollPosition)")
       textView.verticalScrollPosition = position.verticalScrollPosition
     }
     if theme.id != codeView.theme.id { codeView.theme = theme }
@@ -698,7 +701,11 @@ extension CodeEditor: UIViewRepresentable {
     }
 
     func scrollPositionDidChange(_ scrollView: UIScrollView) {
-      guard !updatingView else { return }
+      //guard !updatingView else { return }
+      print("scrollPositionDidChange:", "before:", "position.verticalScrollPosition \(position.verticalScrollPosition), scrollView.verticalScrollPosition \(scrollView.verticalScrollPosition)")
+      defer {
+        print("scrollPositionDidChange:", "after:", "position.verticalScrollPosition \(position.verticalScrollPosition), scrollView.verticalScrollPosition \(scrollView.verticalScrollPosition)")
+      }
 
       if abs(position.verticalScrollPosition - scrollView.verticalScrollPosition) > 0.0001 {
         position.verticalScrollPosition = scrollView.verticalScrollPosition

log:

updateUIView: before codeView.text = text: codeView.vrticalScrollPosition 71.0, position.verticalScrollPosition 71.0
scrollPositionDidChange: before: position.verticalScrollPosition 71.0, scrollView.verticalScrollPosition 0.0
scrollPositionDidChange: after: position.verticalScrollPosition 71.0, scrollView.verticalScrollPosition 0.0
updateUIView: after codeView.text = text: codeView.vrticalScrollPosition 0.0, position.verticalScrollPosition 71.0
updateUIView: textView.vrticalScrollPosition 0.0 = position.verticalScrollPosition 71.0
scrollPositionDidChange: before: position.verticalScrollPosition 71.0, scrollView.verticalScrollPosition 71.0
scrollPositionDidChange: after: position.verticalScrollPosition 71.0, scrollView.verticalScrollPosition 71.0

[kkebo]

I was wondering why the value of position.verticalScrollPosition was not updated:

scrollPositionDidChange: before: position.verticalScrollPosition 71.0, scrollView.verticalScrollPosition 0.0
scrollPositionDidChange: after: position.verticalScrollPosition 71.0, scrollView.verticalScrollPosition 0.0

This causes the value of textView.verticalScrollPosition to be written back to the previous one in the following line:

CodeEditorView/Sources/CodeEditorView/CodeEditor.swift

Line 612 in 5e0e1e8

textView.verticalScrollPosition = position.verticalScrollPosition

updateUIView: textView.vrticalScrollPosition 0.0 = position.verticalScrollPosition 71.0

[mchakravarty]

I was wondering why the value of position.verticalScrollPosition was not updated:

SwiftUI can be a bit weird. I have seen that before, but not with a plain @State variable, but with a @SceneState variable. Updates through bindings sometimes don't happen right away, but later in the SwiftUI view update cycle. Maybe that is what is happening here as well.

But zooming out a bit, the general issue here is that updateUIView(:context:) might update several properties of the code view (e.g., there could be a change to the scroll position and to the text in a single update). In such cases, the order in which we update properties can result in a difference of the final result.

Here updating codeView.text and then the scroll position gets us into trouble, because the update to codeView.text also changes the scroll position. Conversely, if we update codeView.text with a very long text and, in the same update, move the scroll position to the end of that long text, the scroll position update might not work unless the codeView.text has already been updated.

I have had similar problems with the ordering of updating codeView.text and updating the messages.