Update sample (toc plugin)

sample/src/main/java/io/noties/markwon/sample/basicplugins/AnchorHeadingPlugin.java

@@ -89,7 +89,7 @@ public class AnchorHeadingPlugin extends AbstractMarkwonPlugin {
     }
 
     @NonNull
-    private static String createAnchor(@NonNull CharSequence content) {
+    public static String createAnchor(@NonNull CharSequence content) {
         return String.valueOf(content)
                 .replaceAll("[^\\w]", "")
                 .toLowerCase();

sample/src/main/java/io/noties/markwon/sample/basicplugins/BasicPluginsActivity.java

@@ -58,7 +58,8 @@ public class BasicPluginsActivity extends ActivityWithMenuOptions {
                 .add("headingNoSpaceBlockHandler", this::headingNoSpaceBlockHandler)
                 .add("allBlocksNoForcedLine", this::allBlocksNoForcedLine)
                 .add("anchor", this::anchor)
-                .add("letterOrderedList", this::letterOrderedList);
+                .add("letterOrderedList", this::letterOrderedList)
+                .add("tableOfContents", this::tableOfContents);
     }
 
     @Override
@@ -411,4 +412,26 @@ public class BasicPluginsActivity extends ActivityWithMenuOptions {
 
         markwon.setMarkdown(textView, md);
     }
+
+    private void tableOfContents() {
+        final String lorem = getString(R.string.lorem);
+        final String md = "" +
+                "# First\n" +
+                "" + lorem + "\n\n" +
+                "# Second\n" +
+                "" + lorem + "\n\n" +
+                "## Second level\n\n" +
+                "" + lorem + "\n\n" +
+                "### Level 3\n\n" +
+                "" + lorem + "\n\n" +
+                "# First again\n" +
+                "" + lorem + "\n\n";
+
+        final Markwon markwon = Markwon.builder(this)
+                .usePlugin(new TableOfContentsPlugin())
+                .usePlugin(new AnchorHeadingPlugin((view, top) -> scrollView.smoothScrollTo(0, top)))
+                .build();
+
+        markwon.setMarkdown(textView, md);
+    }
 }

sample/src/main/java/io/noties/markwon/sample/basicplugins/TableOfContentsPlugin.java

@@ -0,0 +1,115 @@
+package io.noties.markwon.sample.basicplugins;
+
+import androidx.annotation.NonNull;
+
+import org.commonmark.node.AbstractVisitor;
+import org.commonmark.node.BulletList;
+import org.commonmark.node.CustomBlock;
+import org.commonmark.node.Heading;
+import org.commonmark.node.Link;
+import org.commonmark.node.ListItem;
+import org.commonmark.node.Node;
+import org.commonmark.node.Text;
+
+import io.noties.markwon.AbstractMarkwonPlugin;
+import io.noties.markwon.MarkwonVisitor;
+import io.noties.markwon.core.SimpleBlockNodeVisitor;
+
+public class TableOfContentsPlugin extends AbstractMarkwonPlugin {
+    @Override
+    public void configure(@NonNull Registry registry) {
+        // just to make it explicit
+        registry.require(AnchorHeadingPlugin.class);
+    }
+
+    @Override
+    public void configureVisitor(@NonNull MarkwonVisitor.Builder builder) {
+        builder.on(TableOfContentsBlock.class, new SimpleBlockNodeVisitor());
+    }
+
+    @Override
+    public void beforeRender(@NonNull Node node) {
+
+        // custom block to hold TOC
+        final TableOfContentsBlock block = new TableOfContentsBlock();
+
+        // create TOC title
+        {
+            final Text text = new Text("Table of contents");
+            final Heading heading = new Heading();
+            // important one - set TOC heading level
+            heading.setLevel(1);
+            heading.appendChild(text);
+            block.appendChild(heading);
+        }
+
+        final HeadingVisitor visitor = new HeadingVisitor(block);
+        node.accept(visitor);
+
+        // make it the very first node in rendered markdown
+        node.prependChild(block);
+    }
+
+    private static class HeadingVisitor extends AbstractVisitor {
+
+        private final BulletList bulletList = new BulletList();
+        private final StringBuilder builder = new StringBuilder();
+        private boolean isInsideHeading;
+
+        HeadingVisitor(@NonNull Node node) {
+            node.appendChild(bulletList);
+        }
+
+        @Override
+        public void visit(Heading heading) {
+            this.isInsideHeading = true;
+            try {
+                // reset build from previous content
+                builder.setLength(0);
+
+                // obtain level (can additionally filter by level, to skip lower ones)
+                final int level = heading.getLevel();
+
+                // build heading title
+                visitChildren(heading);
+
+                // initial list item
+                final ListItem listItem = new ListItem();
+
+                Node parent = listItem;
+                Node node = listItem;
+
+                for (int i = 1; i < level; i++) {
+                    final ListItem li = new ListItem();
+                    final BulletList bulletList = new BulletList();
+                    bulletList.appendChild(li);
+                    parent.appendChild(bulletList);
+                    parent = li;
+                    node = li;
+                }
+
+                final String content = builder.toString();
+                final Link link = new Link("#" + AnchorHeadingPlugin.createAnchor(content), null);
+                final Text text = new Text(content);
+                link.appendChild(text);
+                node.appendChild(link);
+                bulletList.appendChild(listItem);
+
+
+            } finally {
+                isInsideHeading = false;
+            }
+        }
+
+        @Override
+        public void visit(Text text) {
+            // can additionally check if we are building heading (to skip all other texts)
+            if (isInsideHeading) {
+                builder.append(text.getLiteral());
+            }
+        }
+    }
+
+    private static class TableOfContentsBlock extends CustomBlock {
+    }
+}