Add support for Markdown rendering for GFM task list items

CHANGELOG.md

@@ -9,6 +9,7 @@ with the exception that 0.x versions can break between minor versions.
 ## [Unreleased]
 ### Added
 - Allow customizing HTML attributes for alert title `<p>` tag via `AttributeProvider`
+- Support rendering GFM task list items to Markdown
 
 ## [0.28.0] - 2026-03-31
 ### Added

commonmark-ext-task-list-items/src/main/java/org/commonmark/ext/task/list/items/TaskListItemsExtension.java

@@ -1,10 +1,16 @@
 package org.commonmark.ext.task.list.items;
 
+import java.util.Set;
 import org.commonmark.Extension;
 import org.commonmark.ext.task.list.items.internal.TaskListItemHtmlNodeRenderer;
+import org.commonmark.ext.task.list.items.internal.TaskListItemMarkdownNodeRenderer;
 import org.commonmark.ext.task.list.items.internal.TaskListItemPostProcessor;
 import org.commonmark.parser.Parser;
+import org.commonmark.renderer.NodeRenderer;
 import org.commonmark.renderer.html.HtmlRenderer;
+import org.commonmark.renderer.markdown.MarkdownNodeRendererContext;
+import org.commonmark.renderer.markdown.MarkdownNodeRendererFactory;
+import org.commonmark.renderer.markdown.MarkdownRenderer;
 
 /**
  * Extension for adding task list items.
@@ -16,7 +22,8 @@
  *
  * @since 0.15.0
  */
-public class TaskListItemsExtension implements Parser.ParserExtension, HtmlRenderer.HtmlRendererExtension {
+public class TaskListItemsExtension implements Parser.ParserExtension, HtmlRenderer.HtmlRendererExtension,
+        MarkdownRenderer.MarkdownRendererExtension {
 
     private TaskListItemsExtension() {
     }
@@ -34,4 +41,19 @@ public void extend(Parser.Builder parserBuilder) {
     public void extend(HtmlRenderer.Builder rendererBuilder) {
         rendererBuilder.nodeRendererFactory(TaskListItemHtmlNodeRenderer::new);
     }
+
+    @Override
+    public void extend(MarkdownRenderer.Builder rendererBuilder) {
+        rendererBuilder.nodeRendererFactory(new MarkdownNodeRendererFactory() {
+            @Override
+            public NodeRenderer create(MarkdownNodeRendererContext context) {
+                return new TaskListItemMarkdownNodeRenderer(context);
+            }
+
+            @Override
+            public Set<Character> getSpecialCharacters() {
+                return Set.of();
+            }
+        });
+    }
 }

commonmark-ext-task-list-items/src/main/java/org/commonmark/ext/task/list/items/internal/TaskListItemHtmlNodeRenderer.java

@@ -2,15 +2,13 @@
 
 import org.commonmark.ext.task.list.items.TaskListItemMarker;
 import org.commonmark.node.Node;
-import org.commonmark.renderer.NodeRenderer;
 import org.commonmark.renderer.html.HtmlNodeRendererContext;
 import org.commonmark.renderer.html.HtmlWriter;
 
 import java.util.LinkedHashMap;
 import java.util.Map;
-import java.util.Set;
 
-public class TaskListItemHtmlNodeRenderer implements NodeRenderer {
+public class TaskListItemHtmlNodeRenderer extends TaskListItemNodeRenderer {
 
     private final HtmlNodeRendererContext context;
     private final HtmlWriter html;
@@ -20,11 +18,6 @@ public TaskListItemHtmlNodeRenderer(HtmlNodeRendererContext context) {
         this.html = context.getWriter();
     }
 
-    @Override
-    public Set<Class<? extends Node>> getNodeTypes() {
-        return Set.of(TaskListItemMarker.class);
-    }
-
     @Override
     public void render(Node node) {
         if (node instanceof TaskListItemMarker) {

commonmark-ext-task-list-items/src/main/java/org/commonmark/ext/task/list/items/internal/TaskListItemMarkdownNodeRenderer.java

@@ -0,0 +1,36 @@
+package org.commonmark.ext.task.list.items.internal;
+
+import org.commonmark.ext.task.list.items.TaskListItemMarker;
+import org.commonmark.node.Node;
+import org.commonmark.renderer.markdown.MarkdownNodeRendererContext;
+import org.commonmark.renderer.markdown.MarkdownWriter;
+
+public class TaskListItemMarkdownNodeRenderer extends TaskListItemNodeRenderer {
+
+    private final MarkdownNodeRendererContext context;
+    private final MarkdownWriter writer;
+
+    public TaskListItemMarkdownNodeRenderer(MarkdownNodeRendererContext context) {
+        this.context = context;
+        this.writer = context.getWriter();
+    }
+
+    @Override
+    public void render(Node node) {
+        if (node instanceof TaskListItemMarker) {
+            var taskListItemNode = (TaskListItemMarker) node;
+            var checkboxFill = taskListItemNode.isChecked() ? "x" : " ";
+            writer.raw("[" + checkboxFill + "] ");
+            renderChildren(node);
+        }
+    }
+
+    private void renderChildren(Node parent) {
+        Node node = parent.getFirstChild();
+        while (node != null) {
+            Node next = node.getNext();
+            context.render(node);
+            node = next;
+        }
+    }
+}

commonmark-ext-task-list-items/src/main/java/org/commonmark/ext/task/list/items/internal/TaskListItemNodeRenderer.java

@@ -0,0 +1,13 @@
+package org.commonmark.ext.task.list.items.internal;
+
+import java.util.Set;
+import org.commonmark.ext.task.list.items.TaskListItemMarker;
+import org.commonmark.node.Node;
+import org.commonmark.renderer.NodeRenderer;
+
+public abstract class TaskListItemNodeRenderer implements NodeRenderer {
+    @Override
+    public Set<Class<? extends Node>> getNodeTypes() {
+        return Set.of(TaskListItemMarker.class);
+    }
+}

commonmark-ext-task-list-items/src/test/java/org/commonmark/ext/task/list/items/TaskListItemMarkdownRendererTest.java

@@ -0,0 +1,84 @@
+package org.commonmark.ext.task.list.items;
+
+import java.util.Set;
+import org.commonmark.Extension;
+import org.commonmark.node.BulletList;
+import org.commonmark.node.Document;
+import org.commonmark.node.ListItem;
+import org.commonmark.node.Node;
+import org.commonmark.node.Paragraph;
+import org.commonmark.node.Text;
+import org.commonmark.parser.Parser;
+import org.commonmark.renderer.markdown.MarkdownRenderer;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class TaskListItemMarkdownRendererTest {
+
+    private static final Set<Extension> EXTENSIONS = Set.of(TaskListItemsExtension.create());
+    private static final Parser PARSER = Parser.builder().extensions(EXTENSIONS).build();
+    private static final MarkdownRenderer RENDERER = MarkdownRenderer.builder().extensions(EXTENSIONS).build();
+
+    @Test
+    public void testCheckedRoundTrip() {
+        assertRoundTrip("- [x] I am checked\n");
+    }
+
+    @Test
+    public void testUncheckedRoundTrip() {
+        assertRoundTrip("- [ ] I am unchecked\n");
+    }
+
+    @Test
+    public void testMixedRoundTrip() {
+        assertRoundTrip("- [x] I am checked\n- [ ] I am unchecked\n");
+    }
+
+    @Test
+    public void testNestedRoundTrip() {
+        assertRoundTrip("- [ ] I am unchecked\n  - [x] I am a checked child\n");
+    }
+
+    @Test
+    public void testFormattingRoundTrip() {
+        assertRoundTrip("- [x] I am **boldly** checked\n- [ ] I am *italicly* unchecked\n");
+    }
+
+    @Test
+    public void testNonTaskListItemRoundTrip() {
+        assertRoundTrip("- [x] I am checked\n- [ ] I am unchecked\n- I am not a task item\n");
+    }
+
+    @Test
+    public void testOrderedListRoundTrip() {
+        assertRoundTrip("1. [x] I am checked\n2. [ ] I am unchecked\n");
+    }
+
+    @Test
+    public void testProgrammaticallyBuilt() {
+        var doc = new Document();
+        var list = new BulletList();
+        var item = new ListItem();
+        var taskMarker = new TaskListItemMarker(false);
+        var para = new Paragraph();
+        var text = new Text("I am a task");
+        para.appendChild(text);
+        item.appendChild(taskMarker);
+        item.appendChild(para);
+        list.appendChild(item);
+        doc.appendChild(list);
+
+        assertRenderedEquals(doc, "- [ ] I am a task\n");
+    }
+
+    private void assertRoundTrip(String input) {
+        String rendered = RENDERER.render(PARSER.parse(input));
+        assertThat(rendered).isEqualTo(input);
+    }
+
+    private void assertRenderedEquals(Node inputNode, String expectedOutput) {
+        var renderedOutput = RENDERER.render(inputNode);
+        assertThat(renderedOutput).isEqualTo(expectedOutput);
+    }
+}