fix: markdown parser doesn't recognize softline breaks

example/pubspec.yaml

@@ -56,7 +56,7 @@ dependency_overrides:
     git:
       url: https://github.com/AppFlowy-IO/AppFlowy-plugins.git
       path: "packages/appflowy_editor_plugins"
-      ref: "b228456"
+      ref: "2d3d4fb"
 
 dev_dependencies:
   flutter_test:

lib/src/plugins/markdown/decoder/document_markdown_decoder.dart

@@ -24,15 +24,13 @@ class DocumentMarkdownDecoder extends Converter<String, Document> {
       ],
       encodeHtml: false,
     ).parse(input);
-    final document = Document.blank();
 
+    final document = Document.blank();
     final nodes = mdNodes
-        .map((mdNode) => _parseNode(mdNode))
-        .toList()
+        .map((e) => _parseNode(e))
         .whereNotNull()
-        .toList()
-        .flattened;
-
+        .flattened
+        .toList(growable: false); // avoid lazy evaluation
     if (nodes.isNotEmpty) {
       document.insert([0], nodes);
     }

lib/src/plugins/markdown/decoder/parser/markdown_paragraph_parser.dart

@@ -28,11 +28,51 @@
       }
     }
 
-    final deltaDecoder = DeltaMarkdownDecoder();
-    return [
-      paragraphNode(
-        delta: deltaDecoder.convertNodes(element.children),
-      ),
-    ];
+    if (ec == null || ec.isEmpty) {
+      // return empty paragraph node if there is no children
+      return [
+        paragraphNode(),
+      ];
+    }
+
+    // Split the paragraph node by <br> tag
+    final splitContent = _splitByBrTag(ec);
+
+    // Transform each split content into a paragraph node
+    return splitContent.map((content) {
+      final deltaDecoder = DeltaMarkdownDecoder();
+      final delta = deltaDecoder.convertNodes(content);
+      return paragraphNode(delta: delta);
+    }).toList();
   }
 }
+
+// split the <p> children by <br> tag, mostly it's used for handling the soft line break
+// for example:
+// ```html
+// <p>
+// Hello<br>World
+// </p>
+// ```
+// will be split into:
+// ```document
+// Hello
+//
+// World
+// ```
+List<List<md.Node>> _splitByBrTag(List<md.Node> nodes) {
+  return nodes
+      .fold<List<List<md.Node>>>(
+        [[]],
+        (acc, node) {
+          if (node is md.Element && node.tag == 'br') {
+            acc.add([]);
+          } else {
+            acc.last.add(node);
+          }
+          return acc;
+        },
+      )
+      .where((group) => group.isNotEmpty)
+      .toList();
+}

test/plugins/markdown/document_markdown_test.dart

@@ -12,6 +12,14 @@ void main() {
       expect(document.toJson(), data);
     });
 
+    test('soft line break with two spaces', () {
+      const markdown = 'first line  \nsecond line';
+      final document = markdownToDocument(markdown);
+      expect(document.root.children.length, 2);
+      expect(document.root.children[0].delta?.toPlainText(), 'first line');
+      expect(document.root.children[1].delta?.toPlainText(), 'second line');
+    });
+
     test('documentToMarkdown()', () {
       final document = markdownToDocument(markdownDocument);
       final markdown = documentToMarkdown(document);