[flutter_markdown] Allow for custom block element (#5815)

Fixes problem with adding custom block syntax.

Issue: flutter/flutter#135848

Author: dawidope

packages/flutter_markdown/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 0.6.22
+
+* Introduces a new `MarkdownElementBuilder.isBlockElement()` method to specify if custom element
+  is a block.
+
 ## 0.6.21+1
 
 * Adds `onSelectionChanged` to the constructors of `Markdown` and `MarkdownBody`.

packages/flutter_markdown/lib/src/builder.dart

@@ -10,7 +10,7 @@ import '_functions_io.dart' if (dart.library.js_interop) '_functions_web.dart';
 import 'style_sheet.dart';
 import 'widget.dart';
 
-const List<String> _kBlockTags = <String>[
+final List<String> _kBlockTags = <String>[
   'p',
   'h1',
   'h2',
@@ -190,6 +190,12 @@ class MarkdownBuilder implements md.NodeVisitor {
     _linkHandlers.clear();
     _isInBlockquote = false;
 
+    builders.forEach((String key, MarkdownElementBuilder value) {
+      if (value.isBlockElement()) {
+        _kBlockTags.add(key);
+      }
+    });
+
     _blocks.add(_BlockElement(null));
 
     for (final md.Node node in nodes) {

packages/flutter_markdown/lib/src/widget.dart

@@ -70,6 +70,11 @@ abstract class SyntaxHighlighter {
 
 /// An interface for an element builder.
 abstract class MarkdownElementBuilder {
+  /// For block syntax has to return true.
+  ///
+  /// By default returns false.
+  bool isBlockElement() => false;
+
   /// Called when an Element has been reached, before its children have been
   /// visited.
   void visitElementBefore(md.Element element) {}

packages/flutter_markdown/pubspec.yaml

@@ -4,7 +4,7 @@ description: A Markdown renderer for Flutter. Create rich text output,
   formatted with simple Markdown tags.
 repository: https://github.com/flutter/packages/tree/main/packages/flutter_markdown
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+flutter_markdown%22
-version: 0.6.21+1
+version: 0.6.22
 
 environment:
   sdk: ^3.3.0

packages/flutter_markdown/test/custom_syntax_test.dart

@@ -35,6 +35,30 @@ void defineTests() {
       },
     );
 
+    testWidgets(
+      'Custom block element',
+      (WidgetTester tester) async {
+        const String blockContent = 'note block';
+        await tester.pumpWidget(
+          boilerplate(
+            Markdown(
+              data: '[!NOTE] $blockContent',
+              extensionSet: md.ExtensionSet.none,
+              blockSyntaxes: <md.BlockSyntax>[NoteSyntax()],
+              builders: <String, MarkdownElementBuilder>{
+                'note': NoteBuilder(),
+              },
+            ),
+          ),
+        );
+        final ColoredBox container =
+            tester.widgetList(find.byType(ColoredBox)).first as ColoredBox;
+        expect(container.color, Colors.red);
+        expect(container.child, isInstanceOf<Text>());
+        expect((container.child! as Text).data, blockContent);
+      },
+    );
+
     testWidgets(
       'link for wikistyle',
       (WidgetTester tester) async {
@@ -331,3 +355,28 @@ class ImgBuilder extends MarkdownElementBuilder {
     return Text('foo', style: preferredStyle);
   }
 }
+
+class NoteBuilder extends MarkdownElementBuilder {
+  @override
+  Widget? visitText(md.Text text, TextStyle? preferredStyle) {
+    return ColoredBox(
+        color: Colors.red, child: Text(text.text, style: preferredStyle));
+  }
+
+  @override
+  bool isBlockElement() {
+    return true;
+  }
+}
+
+class NoteSyntax extends md.BlockSyntax {
+  @override
+  md.Node? parse(md.BlockParser parser) {
+    final md.Line line = parser.current;
+    parser.advance();
+    return md.Element('note', <md.Node>[md.Text(line.content.substring(8))]);
+  }
+
+  @override
+  RegExp get pattern => RegExp(r'^\[!NOTE] ');
+}