[flutter_markdown] Unable to use MarkdownElementBuilder to act those tags without children. (#3952)

fix https://github.com/flutter/flutter/issues/126402

- [  ] I updated `pubspec.yaml` with an appropriate new version according to the [pub versioning philosophy], or this PR is [exempt from version changes].
- [  ] I updated `CHANGELOG.md` to add a description of the change, [following repository CHANGELOG style].
- [  ] I updated/added relevant documentation (doc comments with `///`).
- [  ] I added new tests to check the change I am making, or this PR is [test-exempt].
- [  ] All existing and new tests are passing.

packages/flutter_markdown/CHANGELOG.md

@@ -1,9 +1,10 @@
-## NEXT
+## 0.6.15
 
 * Fixes unawaited_futures violations.
 * Updates minimum Flutter version to 3.3.
 * Aligns Dart and Flutter SDK constraints.
 * Replace `describeEnum` with the `name` getter.
+* Supports custom rendering of tags without children.
 
 ## 0.6.14
 

packages/flutter_markdown/lib/src/builder.dart

@@ -454,7 +454,11 @@ class MarkdownBuilder implements md.NodeVisitor {
         final Widget? child =
             builders[tag]!.visitElementAfter(element, styleSheet.styles[tag]);
         if (child != null) {
-          current.children[0] = child;
+          if (current.children.isEmpty) {
+            current.children.add(child);
+          } else {
+            current.children[0] = child;
+          }
         }
       } else if (tag == 'img') {
         // create an image widget for this image

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: 0.6.14
+version: 0.6.15
 
 environment:
   sdk: ">=2.18.0 <4.0.0"

packages/flutter_markdown/test/custom_syntax_test.dart

@@ -112,6 +112,30 @@ void defineTests() {
       expect(widgetSpan.child, isInstanceOf<Container>());
     },
   );
+
+  testWidgets(
+    'Custom rendering of tags without children',
+    (WidgetTester tester) async {
+      const String data = '![alt](/assets/images/logo.png)';
+      await tester.pumpWidget(
+        boilerplate(
+          Markdown(
+            data: data,
+            builders: <String, MarkdownElementBuilder>{
+              'img': ImgBuilder(),
+            },
+          ),
+        ),
+      );
+
+      final Finder imageFinder = find.byType(Image);
+      expect(imageFinder, findsNothing);
+      final Finder textFinder = find.byType(Text);
+      expect(textFinder, findsOneWidget);
+      final Text textWidget = tester.widget(find.byType(Text));
+      expect(textWidget.data, 'foo');
+    },
+  );
 }
 
 class SubscriptSyntax extends md.InlineSyntax {
@@ -225,3 +249,10 @@ class ContainerBuilder2 extends MarkdownElementBuilder {
     );
   }
 }
+
+class ImgBuilder extends MarkdownElementBuilder {
+  @override
+  Widget? visitElementAfter(md.Element element, TextStyle? preferredStyle) {
+    return Text('foo', style: preferredStyle);
+  }
+}