Add DingusApp for quickly testing out renderings

Named Dingus because of https://spec.commonmark.org/dingus/

commonmark-integration-test/src/test/java/org/commonmark/ui/DingusApp.java

@@ -0,0 +1,114 @@
+package org.commonmark.ui;
+
+import org.commonmark.parser.Parser;
+import org.commonmark.renderer.html.HtmlRenderer;
+import org.commonmark.renderer.text.TextContentRenderer;
+
+import java.awt.*;
+import javax.swing.*;
+import javax.swing.event.ChangeEvent;
+import javax.swing.event.ChangeListener;
+import javax.swing.event.DocumentEvent;
+import javax.swing.event.DocumentListener;
+
+/**
+ * Simple UI to quickly test out different rendering of CommonMark inputs.
+ * Similar to <a href="https://spec.commonmark.org/dingus/">commonmark.js dingus</a>.
+ **/
+public class DingusApp {
+
+    private final Parser parser = Parser.builder().build();
+    private final TextContentRenderer textRenderer = TextContentRenderer.builder().build();
+    private final HtmlRenderer htmlRenderer = HtmlRenderer.builder().build();
+
+    private final JTabbedPane tabbedPane;
+    private final JEditorPane htmlVisualRendererOutput;
+    private final JTextArea htmlSourceRendererOutput;
+    private final JTextArea textRendererOutput;
+
+    public static void main(String[] args) {
+        new DingusApp().run();
+    }
+
+    private DingusApp() {
+        tabbedPane = new JTabbedPane();
+
+        htmlVisualRendererOutput = new JEditorPane();
+        htmlVisualRendererOutput.setEnabled(false);
+        htmlVisualRendererOutput.setContentType("text/html");
+
+        htmlSourceRendererOutput = new JTextArea();
+        htmlSourceRendererOutput.setEnabled(false);
+        htmlSourceRendererOutput.setLineWrap(true);
+        htmlSourceRendererOutput.setFont(new Font(Font.MONOSPACED, Font.PLAIN, 12));
+
+        textRendererOutput = new JTextArea();
+        textRendererOutput.setEnabled(false);
+        textRendererOutput.setLineWrap(true);
+        textRendererOutput.setFont(new Font(Font.MONOSPACED, Font.PLAIN, 12));
+    }
+
+    private void run() {
+        JFrame frame = new JFrame("commonmark-java dingus");
+        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
+        frame.setMinimumSize(new Dimension(400, 300));
+        frame.setSize(new Dimension(1200, 675));
+
+        final JTextArea input = new JTextArea();
+        input.setBorder(BorderFactory.createTitledBorder("Input"));
+        input.setLineWrap(true);
+        input.setFont(new Font(Font.MONOSPACED, Font.PLAIN, 12));
+
+        input.getDocument().addDocumentListener(new DocumentListener() {
+            @Override
+            public void insertUpdate(DocumentEvent e) {
+                updateOutput(input.getText());
+            }
+
+            @Override
+            public void removeUpdate(DocumentEvent e) {
+                updateOutput(input.getText());
+            }
+
+            @Override
+            public void changedUpdate(DocumentEvent e) {
+            }
+        });
+
+        tabbedPane.addTab("HTML rendered", htmlVisualRendererOutput);
+        tabbedPane.addTab("HTML source", htmlSourceRendererOutput);
+        tabbedPane.addTab("Plain text", textRendererOutput);
+
+        tabbedPane.addChangeListener(new ChangeListener() {
+            @Override
+            public void stateChanged(ChangeEvent e) {
+                updateOutput(input.getText());
+            }
+        });
+
+        input.setText("# Example\n" +
+                "Enter text *here* and see how it renders on the right.\n\n" +
+                "* Try\n* this\n\n" +
+                "```\nor this\n```");
+        updateOutput(input.getText());
+
+        frame.setLayout(new GridLayout());
+        frame.add(input);
+        frame.add(tabbedPane);
+
+        frame.setVisible(true);
+    }
+
+    private void updateOutput(String inputText) {
+        if (tabbedPane.getSelectedComponent() == htmlVisualRendererOutput) {
+            String rendered = htmlRenderer.render(parser.parse(inputText));
+            htmlVisualRendererOutput.setText(rendered);
+        } else if (tabbedPane.getSelectedComponent() == htmlSourceRendererOutput) {
+            String rendered = htmlRenderer.render(parser.parse(inputText));
+            htmlSourceRendererOutput.setText(rendered);
+        } else if (tabbedPane.getSelectedComponent() == textRendererOutput) {
+            String rendered = textRenderer.render(parser.parse(inputText));
+            textRendererOutput.setText(rendered);
+        }
+    }
+}