Handle unsupported attributes in Markdown conversion + horizontal lines

packages/fleather/lib/src/widgets/theme.dart

@@ -29,16 +29,18 @@ class FleatherTheme extends InheritedWidget {
   }
 
   /// The data from the closest [FleatherTheme] instance that encloses the given
-  /// context.
+  /// [context].
   ///
   /// Returns `null` if there is no [FleatherTheme] in the given build context
   /// and [nullOk] is set to `true`. If [nullOk] is set to `false` (default)
   /// then this method asserts.
   static FleatherThemeData? of(BuildContext context, {bool nullOk = false}) {
     final widget = context.dependOnInheritedWidgetOfExactType<FleatherTheme>();
     if (widget == null && nullOk) return null;
-    assert(widget != null,
-        '$FleatherTheme.of() called with a context that does not contain a FleatherTheme.');
+    assert(
+      widget != null,
+      '${FleatherTheme.of} called with a context that does not contain a $FleatherTheme.',
+    );
     return widget!.data;
   }
 }

packages/parchment/lib/codecs.dart

@@ -1,14 +1,18 @@
 /// Provides codecs to convert Parchment documents to other formats.
 library parchment.codecs;
 
-import 'src/codecs/markdown.dart';
 import 'src/codecs/html.dart';
+import 'src/codecs/markdown.dart';
 
-export 'src/codecs/markdown.dart';
 export 'src/codecs/html.dart';
+export 'src/codecs/markdown.dart';
 
 /// Markdown codec for Parchment documents.
 const parchmentMarkdown = ParchmentMarkdownCodec();
 
+/// Not strict markdown codec for Parchment documents.
+const parchmentMarkdownNotStrict =
+    ParchmentMarkdownCodec(strictEncoding: false);
+
 /// HTML codec for Parchment documents.
 const parchmentHtml = ParchmentHtmlCodec();

packages/parchment/lib/src/codecs/markdown.dart

@@ -1,23 +1,28 @@
 import 'dart:convert';
 
-import 'package:parchment_delta/parchment_delta.dart';
-
-import '../document.dart';
-import '../document/attributes.dart';
-import '../document/block.dart';
-import '../document/leaf.dart';
-import '../document/line.dart';
+import '../../parchment.dart';
 
 class ParchmentMarkdownCodec extends Codec<ParchmentDocument, String> {
-  const ParchmentMarkdownCodec();
+  const ParchmentMarkdownCodec({this.strictEncoding = true});
+
+  /// Whether to strictly stick to the Markdown syntax during the encoding.
+  ///
+  /// If this option is enabled, during the encoding, if attributes that are
+  /// not natively supported by the Markdown syntax exist, an exception will be
+  /// thrown. Otherwise, they will be converted in the best way possible
+  /// (for example with HTML tags, plain text or placeholders).
+  ///
+  /// Currently supported attributes:
+  ///   - Underline with `<u>...</u>`
+  final bool strictEncoding;
 
   @override
   Converter<String, ParchmentDocument> get decoder =>
       _ParchmentMarkdownDecoder();
 
   @override
   Converter<ParchmentDocument, String> get encoder =>
-      _ParchmentMarkdownEncoder();
+      _ParchmentMarkdownEncoder(strict: strictEncoding);
 }
 
 class _ParchmentMarkdownDecoder extends Converter<String, ParchmentDocument> {
@@ -354,6 +359,10 @@ class _ParchmentMarkdownDecoder extends Converter<String, ParchmentDocument> {
 }
 
 class _ParchmentMarkdownEncoder extends Converter<ParchmentDocument, String> {
+  const _ParchmentMarkdownEncoder({required this.strict});
+
+  final bool strict;
+
   static final simpleBlocks = <ParchmentAttribute, String>{
     ParchmentAttribute.bq: '> ',
     ParchmentAttribute.ul: '* ',
@@ -403,7 +412,16 @@ class _ParchmentMarkdownEncoder extends Converter<ParchmentDocument, String> {
     ParchmentAttribute? currentBlockAttribute;
 
     void handleLine(LineNode node) {
-      if (node.hasBlockEmbed) return;
+      if (node.hasBlockEmbed) {
+        if ((node.children.single as EmbedNode).value ==
+            BlockEmbed.horizontalRule) {
+          _writeHorizontalLineTag(buffer);
+        } else {
+          buffer.write('[object]');
+        }
+
+        return;
+      }
 
       for (final attr in node.style.lineAttributes) {
         if (attr.key == ParchmentAttribute.block.key) {
@@ -508,8 +526,12 @@ class _ParchmentMarkdownEncoder extends Converter<ParchmentDocument, String> {
       // no-op already handled in handleBlock
     } else if (attribute?.key == ParchmentAttribute.indent.key) {
       // no-op already handled in handleBlock
-    } else {
+    } else if (!strict && attribute?.key == ParchmentAttribute.underline.key) {
+      _writeUnderlineTag(buffer, close: close);
+    } else if (strict) {
       throw ArgumentError('Cannot handle $attribute');
+    } else {
+      _writeObjectTag(buffer);
     }
   }
 
@@ -521,6 +543,14 @@ class _ParchmentMarkdownEncoder extends Converter<ParchmentDocument, String> {
     buffer.write('_');
   }
 
+  void _writeUnderlineTag(StringBuffer buffer, {bool close = false}) {
+    if (close) {
+      buffer.write('</u>');
+    } else {
+      buffer.write('<u>');
+    }
+  }
+
   void _writeInlineCodeTag(StringBuffer buffer) {
     buffer.write('`');
   }
@@ -529,8 +559,11 @@ class _ParchmentMarkdownEncoder extends Converter<ParchmentDocument, String> {
     buffer.write('~~');
   }
 
-  void _writeLinkTag(StringBuffer buffer, ParchmentAttribute<String> link,
-      {bool close = false}) {
+  void _writeLinkTag(
+    StringBuffer buffer,
+    ParchmentAttribute<String> link, {
+    bool close = false,
+  }) {
     if (close) {
       buffer.write('](${link.value})');
     } else {
@@ -560,4 +593,12 @@ class _ParchmentMarkdownEncoder extends Converter<ParchmentDocument, String> {
       }
     }
   }
+
+  void _writeHorizontalLineTag(StringBuffer buffer) {
+    buffer.write('---');
+  }
+
+  void _writeObjectTag(StringBuffer buffer) {
+    buffer.write('[object]');
+  }
 }

packages/parchment/test/codecs/markdown_test.dart

@@ -144,6 +144,20 @@ void main() {
       runFor('~~strike through~~\n\n', true);
     });
 
+    test('underline', () {
+      final delta = Delta()
+        ..insert('an ')
+        ..insert('underlined', ParchmentAttribute.underline.toJson())
+        ..insert(' text')
+        ..insert('\n');
+
+      final result =
+          parchmentMarkdownNotStrict.encode(ParchmentDocument.fromDelta(delta));
+      final expected = 'an <u>underlined</u> text\n\n';
+
+      expect(result, expected);
+    });
+
     test('intersecting inline styles', () {
       final markdown = 'This **house _is a_ circus**\n\n';
       final document = parchmentMarkdown.decode(markdown);