Render group templates with new styles

- Can now render ST group with hardcoded
  main template name
- Add additional style tags
- Add sample to list various styles

spring-shell-core/src/main/java/org/springframework/shell/style/TemplateExecutor.java

@@ -23,6 +23,7 @@ import org.slf4j.LoggerFactory;
 import org.stringtemplate.v4.ST;
 import org.stringtemplate.v4.STErrorListener;
 import org.stringtemplate.v4.STGroup;
+import org.stringtemplate.v4.STGroupString;
 import org.stringtemplate.v4.misc.STMessage;
 
 /**
@@ -32,6 +33,7 @@ import org.stringtemplate.v4.misc.STMessage;
  */
 public class TemplateExecutor {
 
+	private final static Logger log = LoggerFactory.getLogger(TemplateExecutor.class);
 	private final static STErrorListener ERROR_LISTENER = new LoggingSTErrorListener();
 	private final ThemeResolver themeResolver;
 	private StringToStyleExpressionRenderer renderer;
@@ -61,6 +63,31 @@ public class TemplateExecutor {
 		return themeResolver.evaluateExpression(templateRendered);
 	}
 
+	/**
+	 * Render template group with a given attributes expecting to find instance
+	 * named {@code main}.
+	 *
+	 * @param template   the ST template
+	 * @param attributes the ST template attributes
+	 * @return a rendered template
+	 */
+	public AttributedString renderGroup(String template, Map<String, Object> attributes) {
+		STGroup group = new STGroupString(template);
+		group.setListener(ERROR_LISTENER);
+		group.registerRenderer(String.class, renderer);
+
+		ST st = group.getInstanceOf("main");
+		if (st == null) {
+			throw new IllegalArgumentException("template instance 'main' not found from a group");
+		}
+		if (attributes != null) {
+			attributes.entrySet().stream().forEach(e -> st.add(e.getKey(), e.getValue()));
+		}
+		String templateRendered = st.render();
+		log.debug("Rendered template {}", templateRendered);
+		return themeResolver.evaluateExpression(templateRendered);
+	}
+
 	private static class LoggingSTErrorListener implements STErrorListener {
 
 		private final static Logger log = LoggerFactory.getLogger(LoggingSTErrorListener.class);

spring-shell-core/src/main/java/org/springframework/shell/style/ThemeSettings.java

@@ -49,8 +49,48 @@ public abstract class ThemeSettings {
 	 */
 	public final static String TAG_LIST_VALUE = "list-value";
 
+	/**
+	 * Styling for some arbitrary content indicating {@code INFO} level.
+	 */
+	public final static String TAG_LEVEL_INFO = "level-info";
+
+	/**
+	 * Styling for some arbitrary content indicating {@code WARN} level.
+	 */
+	public final static String TAG_LEVEL_WARN = "level-warn";
+
+	/**
+	 * Styling for some arbitrary content indicating {@code ERROR} level.
+	 */
+	public final static String TAG_LEVEL_ERROR = "level-error";
+
+	/**
+	 * Styling for something i.e. in selectors when item is selectable.
+	 */
+	public final static String TAG_ITEM_ENABLED = "item-enabled";
+
+	/**
+	 * Styling for something i.e. in selectors when item can't be selected.
+	 */
+	public final static String TAG_ITEM_DISABLED = "item-disabled";
+
+	/**
+	 * Styling for something i.e. in selectors when item is selected.
+	 */
+	public final static String TAG_ITEM_SELECTED = "item-selected";
+
+	/**
+	 * Styling for something i.e. in selectors when item is not selected.
+	 */
+	public final static String TAG_ITEM_UNSELECTED = "item-unselected";
+
+	/**
+	 * Styling for selector i.e. arrow in selectors.
+	 */
+	public final static String TAG_ITEM_SELECTOR = "item-selector";
+
 	public String title() {
-		return "bold";
+		return "bold,fg:bright-white";
 	}
 
 	public String value() {
@@ -58,13 +98,45 @@ public abstract class ThemeSettings {
 	}
 
 	public String listKey() {
-		return null;
+		return "default";
 	}
 
 	public String listValue() {
 		return "bold,fg:green";
 	}
 
+	public String listLevelInfo() {
+		return "fg:green";
+	}
+
+	public String listLevelWarn() {
+		return "fg:yellow";
+	}
+
+	public String listLevelError() {
+		return "fg:red";
+	}
+
+	public String itemEnabled() {
+		return "bold";
+	}
+
+	public String itemDisabled() {
+		return "faint";
+	}
+
+	public String itemSelected() {
+		return "fg:green";
+	}
+
+	public String itemUnselected() {
+		return "bold";
+	}
+
+	public String itemSelector() {
+		return "bold,fg:bright-cyan";
+	}
+
 	/**
 	 * Resolve a theme setting from a given tag.
 	 *
@@ -81,6 +153,22 @@ public abstract class ThemeSettings {
 				return listKey();
 			case TAG_LIST_VALUE:
 				return listValue();
+			case TAG_LEVEL_INFO:
+				return listLevelInfo();
+			case TAG_LEVEL_WARN:
+				return listLevelWarn();
+			case TAG_LEVEL_ERROR:
+				return listLevelError();
+			case TAG_ITEM_ENABLED:
+				return itemEnabled();
+			case TAG_ITEM_DISABLED:
+				return itemDisabled();
+			case TAG_ITEM_SELECTED:
+				return itemSelected();
+			case TAG_ITEM_UNSELECTED:
+				return itemUnselected();
+			case TAG_ITEM_SELECTOR:
+				return itemSelector();
 		}
 		throw new IllegalArgumentException(String.format("Unknown tag '%s'", tag));
 	}