From 93dc5c6881e84046b34e509ac36f45327a62383b Mon Sep 17 00:00:00 2001
From: Amir Panahandeh <amirpanahandeh@yahoo.com>
Date: Sun, 10 Dec 2023 12:35:42 +0330
Subject: [PATCH] Preserve line style when entering new line

Fixes #158
---
 packages/parchment/lib/src/heuristics.dart    |  2 +-
 .../lib/src/heuristics/insert_rules.dart      | 24 ++++++++++---------
 .../test/heuristics/insert_rules_test.dart    | 12 ++++++----
 3 files changed, 22 insertions(+), 16 deletions(-)

diff --git a/packages/parchment/lib/src/heuristics.dart b/packages/parchment/lib/src/heuristics.dart
index 1a9f8a20..6eb71c11 100644
--- a/packages/parchment/lib/src/heuristics.dart
+++ b/packages/parchment/lib/src/heuristics.dart
@@ -33,7 +33,7 @@ class ParchmentHeuristics {
       PreserveBlockStyleOnInsertRule(),
       // Lines
       PreserveLineStyleOnSplitRule(),
-      ResetLineFormatOnNewLineRule(),
+      PreserveLineFormatOnNewLineRule(),
       // Inlines
       PreserveInlineStylesRule(),
       // Catch-all
diff --git a/packages/parchment/lib/src/heuristics/insert_rules.dart b/packages/parchment/lib/src/heuristics/insert_rules.dart
index ac42396b..7c0b5f82 100644
--- a/packages/parchment/lib/src/heuristics/insert_rules.dart
+++ b/packages/parchment/lib/src/heuristics/insert_rules.dart
@@ -63,6 +63,7 @@ class CatchAllInsertRule extends InsertRule {
   }
 }
 
+//TODO: Investigate if PreserveLineStyleOnSplitRule and PreserveLineFormatOnNewLineRule can be combined into a single heuristic
 /// Preserves line format when user splits the line into two.
 ///
 /// This rule ignores scenarios when the line is split on its edge, meaning
@@ -112,13 +113,13 @@ class PreserveLineStyleOnSplitRule extends InsertRule {
   }
 }
 
-/// Resets format for a newly inserted line when insert occurred at the end
-/// of a line (right before a newline).
+/// Preserves line format when insert occurred
+/// at the end of a line (right before a newline).
 ///
-/// This handles scenarios when a new line is added when at the end of a
+/// This also handles scenarios when a new line is added when at the end of a
 /// heading line. The newly added line should be a regular paragraph.
-class ResetLineFormatOnNewLineRule extends InsertRule {
-  const ResetLineFormatOnNewLineRule();
+class PreserveLineFormatOnNewLineRule extends InsertRule {
+  const PreserveLineFormatOnNewLineRule();
 
   @override
   Delta? apply(Delta document, int index, Object data) {
@@ -135,17 +136,18 @@ class ResetLineFormatOnNewLineRule extends InsertRule {
 
     final targetText = target.data as String;
 
+    Map<String, dynamic>? resetStyle;
     if (targetText.startsWith('\n')) {
       if (target.attributes != null &&
           target.attributes!.containsKey(ParchmentAttribute.heading.key)) {
         // Reset heading style
-        final resetStyle = ParchmentAttribute.heading.unset.toJson();
-        return Delta()
-          ..retain(index)
-          ..insert('\n', target.attributes)
-          ..retain(1, resetStyle)
-          ..trim();
+        resetStyle = ParchmentAttribute.heading.unset.toJson();
       }
+      return Delta()
+        ..retain(index)
+        ..insert('\n', target.attributes)
+        ..retain(1, resetStyle)
+        ..trim();
     }
     return null;
   }
diff --git a/packages/parchment/test/heuristics/insert_rules_test.dart b/packages/parchment/test/heuristics/insert_rules_test.dart
index c7bcbc85..e976896c 100644
--- a/packages/parchment/test/heuristics/insert_rules_test.dart
+++ b/packages/parchment/test/heuristics/insert_rules_test.dart
@@ -59,8 +59,8 @@ void main() {
     });
   });
 
-  group('$ResetLineFormatOnNewLineRule', () {
-    final rule = const ResetLineFormatOnNewLineRule();
+  group('$PreserveLineFormatOnNewLineRule', () {
+    final rule = const PreserveLineFormatOnNewLineRule();
 
     test('applies when line-break is inserted at the end of line', () {
       final doc = Delta()
@@ -75,10 +75,14 @@ void main() {
       expect(actual, expected);
     });
 
-    test("doesn't apply without style reset if not needed", () {
+    test('applies without style reset if not needed', () {
       final doc = Delta()..insert('Hello world\n');
       final actual = rule.apply(doc, 11, '\n');
-      expect(actual, isNull);
+      expect(actual, isNotNull);
+      final expected = Delta()
+        ..retain(11)
+        ..insert('\n');
+      expect(actual, expected);
     });
 
     test('applies at the beginning of a document', () {