Fix text editing issues on characters added right after mention link

Changelog-Fixed: Fix text editing issues on characters added right after mention link
Closes: https://github.com/damus-io/damus/issues/1375
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>
Tested-by: William Casarin <jb55@jb55.com>
Signed-off-by: William Casarin <jb55@jb55.com>

damus/Views/PostView.swift

@@ -204,6 +204,8 @@ struct PostView: View {
             TextViewWrapper(attributedText: $post, postTextViewCanScroll: $postTextViewCanScroll, cursorIndex: newCursorIndex, getFocusWordForMention: { word, range in
                 focusWordAttributes = (word, range)
                 self.newCursorIndex = nil
+            }, updateCursorPosition: { newCursorIndex in
+                self.newCursorIndex = newCursorIndex
             })
                 .environmentObject(tagModel)
                 .focused($focus)

damus/Views/TextViewWrapper.swift

@@ -14,6 +14,7 @@ struct TextViewWrapper: UIViewRepresentable {
     
     let cursorIndex: Int?
     var getFocusWordForMention: ((String?, NSRange?) -> Void)? = nil
+    let updateCursorPosition: ((Int) -> Void)
     
     func makeUIView(context: Context) -> UITextView {
         let textView = UITextView()
@@ -34,11 +35,11 @@ struct TextViewWrapper: UIViewRepresentable {
 
     func updateUIView(_ uiView: UITextView, context: Context) {
         uiView.isScrollEnabled = postTextViewCanScroll
-        let range = uiView.selectedRange
         uiView.attributedText = attributedText
 
         TextViewWrapper.setTextProperties(uiView)
         setCursorPosition(textView: uiView)
+        let range = uiView.selectedRange
 
         uiView.selectedRange = NSRange(location: range.location + tagModel.diff, length: range.length)
         tagModel.diff = 0
@@ -52,16 +53,18 @@ struct TextViewWrapper: UIViewRepresentable {
     }
 
     func makeCoordinator() -> Coordinator {
-        Coordinator(attributedText: $attributedText, getFocusWordForMention: getFocusWordForMention)
+        Coordinator(attributedText: $attributedText, getFocusWordForMention: getFocusWordForMention, updateCursorPosition: updateCursorPosition)
     }
 
     class Coordinator: NSObject, UITextViewDelegate {
         @Binding var attributedText: NSMutableAttributedString
         var getFocusWordForMention: ((String?, NSRange?) -> Void)? = nil
+        let updateCursorPosition: ((Int) -> Void)
 
-        init(attributedText: Binding<NSMutableAttributedString>, getFocusWordForMention: ((String?, NSRange?) -> Void)?) {
+        init(attributedText: Binding<NSMutableAttributedString>, getFocusWordForMention: ((String?, NSRange?) -> Void)?, updateCursorPosition: @escaping ((Int) -> Void)) {
             _attributedText = attributedText
             self.getFocusWordForMention = getFocusWordForMention
+            self.updateCursorPosition = updateCursorPosition
         }
 
         func textViewDidChange(_ textView: UITextView) {
@@ -96,6 +99,65 @@ struct TextViewWrapper: UIViewRepresentable {
             }
             return NSRange(location: startOffset, length: length)
         }
+
+        // This `UITextViewDelegate` method is automatically called by the editor when edits occur, to check whether a change should occur
+        // We will use this method to manually handle edits concerning mention ("@") links, to avoid manual text edits to attributed mention links
+        func textView(_ textView: UITextView, shouldChangeTextIn range: NSRange, replacementText text: String) -> Bool {
+            guard let attributedString = textView.attributedText else {
+                return true     // If we cannot get an attributed string, just fail gracefully and allow changes
+            }
+            var mutable = NSMutableAttributedString(attributedString: attributedString)
+            
+            let entireRange = NSRange(location: 0, length: attributedString.length)
+            var shouldAllowChange = true
+            var performEditActionManually = false
+
+            attributedString.enumerateAttribute(.link, in: entireRange, options: []) { (value, linkRange, stop) in
+                guard value != nil else {
+                    return  // This range is not a link. Skip checking.
+                }
+                
+                if range.contains(linkRange.upperBound) && range.contains(linkRange.lowerBound) {
+                    // Edit range engulfs all of this link's range.
+                    // This link will naturally disappear, so no work needs to be done in this range.
+                    return
+                }
+                else if linkRange.intersection(range) != nil {
+                    // If user tries to change an existing link directly, remove the link attribute
+                    mutable.removeAttribute(.link, range: linkRange)
+                    // Perform action manually to flush above changes to the view, and to prevent the character being added from having an attributed link property
+                    performEditActionManually = true
+                    return
+                }
+                else if range.location == linkRange.location + linkRange.length && range.length == 0 {
+                    // If we are inserting a character at the right edge of a link, UITextInput tends to include the new character inside the link.
+                    // Therefore, we need to manually append that character outside of the link
+                    performEditActionManually = true
+                    return
+                }
+            }
+            
+            if performEditActionManually {
+                shouldAllowChange = false
+                addUnattributedText(text, to: &mutable, inRange: range)
+                attributedText = mutable
+                
+                // Move caret to the end of the newly changed text.
+                updateCursorPosition(range.location + text.count)
+            }
+
+            return shouldAllowChange
+        }
+
+        func addUnattributedText(_ text: String, to attributedString: inout NSMutableAttributedString, inRange range: NSRange) {
+            if range.length == 0 {
+                attributedString.insert(NSAttributedString(string: text, attributes: nil), at: range.location)
+            }
+            else {
+                attributedString.replaceCharacters(in: range, with: text)
+            }
+        }
+        
     }
 }