fix: support export markdown with inline math equation (#1164)

lib/src/plugins/markdown/encoder/delta_markdown_encoder.dart

@@ -15,8 +15,13 @@ class DeltaMarkdownEncoder extends Converter<Delta, String> {
       if (op is TextInsert) {
         final attributes = op.attributes;
         if (attributes != null) {
+          final formula = attributes[BuiltInAttributeKey.formula] ?? '';
           buffer.write(_prefixSyntax(attributes));
-          buffer.write(op.text);
+          if (formula.isNotEmpty) {
+            buffer.write(formula);
+          } else {
+            buffer.write(op.text);
+          }
           buffer.write(_suffixSyntax(attributes));
         } else {
           buffer.write(op.text);
@@ -52,6 +57,10 @@ class DeltaMarkdownEncoder extends Converter<Delta, String> {
       syntax += '[';
     }
 
+    if (attributes[BuiltInAttributeKey.formula] != null) {
+      syntax += r'$';
+    }
+
     return syntax;
   }
 
@@ -83,6 +92,10 @@ class DeltaMarkdownEncoder extends Converter<Delta, String> {
       syntax += '_';
     }
 
+    if (attributes[BuiltInAttributeKey.formula] != null) {
+      syntax += r'$';
+    }
+
     return syntax;
   }
 }

test/plugins/markdown/encoder/delta_markdown_encoder_test.dart

@@ -137,5 +137,22 @@ void main() async {
         '***<u>`Welcome`</u>*** ***~~to~~*** ***[AppFlowy](https://appflowy.io)***',
       );
     });
+
+    test('formula', () {
+      final delta = Delta(
+        operations: [
+          TextInsert('This is a formula:'),
+          TextInsert(
+            '\$',
+            attributes: {
+              BuiltInAttributeKey.formula: 'E = MC ^ 2',
+            },
+          ),
+          TextInsert('.'),
+        ],
+      );
+      final result = DeltaMarkdownEncoder().convert(delta);
+      expect(result, 'This is a formula:\$E = MC ^ 2\$.');
+    });
   });
 }