Add "help themes" command. And ThemeUtils class.

src/net/sourceforge/plantuml/help/CommandHelp.java

@@ -69,6 +69,7 @@ public class CommandHelp extends SingleLineCommand2<Help> {
 		diagram.add("* help colors");
 		diagram.add("* help font");
 		diagram.add("* help skinparams");
+		diagram.add("* help themes");
 
 		return CommandExecutionResult.ok();
 	}

src/net/sourceforge/plantuml/help/CommandHelpTheme.java

@@ -0,0 +1,85 @@
+/* ========================================================================
+ * PlantUML : a free UML diagram generator
+ * ========================================================================
+ *
+ * (C) Copyright 2009-2021, Arnaud Roques
+ *
+ * Project Info:  http://plantuml.com
+ *
+ * If you like this project or if you find it useful, you can support us at:
+ *
+ * http://plantuml.com/patreon (only 1$ per month!)
+ * http://plantuml.com/paypal
+ *
+ * This file is part of PlantUML.
+ *
+ * PlantUML is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * PlantUML distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+ * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public
+ * License for more details.
+ *
+ * You should have received a copy of the GNU General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301,
+ * USA.
+ *
+ *
+ * Original Author:  Matthew Leather
+ *
+ *
+ */
+package net.sourceforge.plantuml.help;
+
+import java.io.IOException;
+
+import net.sourceforge.plantuml.LineLocation;
+import net.sourceforge.plantuml.Log;
+import net.sourceforge.plantuml.command.CommandExecutionResult;
+import net.sourceforge.plantuml.command.SingleLineCommand2;
+import net.sourceforge.plantuml.command.regex.IRegex;
+import net.sourceforge.plantuml.command.regex.RegexConcat;
+import net.sourceforge.plantuml.command.regex.RegexLeaf;
+import net.sourceforge.plantuml.command.regex.RegexResult;
+import net.sourceforge.plantuml.theme.ThemeUtils;
+
+public class CommandHelpTheme extends SingleLineCommand2<Help> {
+
+	public CommandHelpTheme() {
+		super(getRegexConcat());
+	}
+
+	static IRegex getRegexConcat() {
+		return RegexConcat.build(CommandHelpTheme.class.getName(), RegexLeaf.start(), //
+				new RegexLeaf("help"), //
+				RegexLeaf.spaceOneOrMore(), //
+				new RegexLeaf("themes?"), RegexLeaf.end());
+	}
+
+	@Override
+	protected CommandExecutionResult executeArg(Help diagram, LineLocation location, RegexResult arg) {
+		diagram.add("<b>Help on themes");
+		diagram.add(" ");
+		diagram.add("The code of this command is located in <i>net.sourceforge.plantuml.help</i> package.");
+		diagram.add("You may improve it on <i>https://github.com/plantuml/plantuml/tree/master/src/net/sourceforge/plantuml/help</i>");
+		diagram.add(" ");
+		diagram.add(" The possible themes are :");
+
+		try {
+			for (String theme : ThemeUtils.getAllThemeNames()) {
+				diagram.add("* " + theme);
+			}
+		} catch (IOException e) {
+			final String message = "Unexpected error listing themes: " + e.getMessage();
+			Log.error(message);
+			e.printStackTrace();
+			return CommandExecutionResult.error(message);
+		}
+
+		return CommandExecutionResult.ok();
+	}
+}

src/net/sourceforge/plantuml/help/HelpFactory.java

@@ -59,6 +59,7 @@ public class HelpFactory extends PSystemCommandFactory {
 		cmds.add(new CommandHelpKeyword());
 		cmds.add(new CommandHelpSkinparam());
 		cmds.add(new CommandHelpType());
+		cmds.add(new CommandHelpTheme());
 
 		return cmds;
 	}

src/net/sourceforge/plantuml/preproc2/PreprocessorUtils.java

@@ -90,17 +90,6 @@ public class PreprocessorUtils {
 		return result;
 	}
 
-	public static ReadLine getReaderTheme(StringLocated s, String filename) {
-		Log.info("Loading theme " + filename);
-		final String res = "/themes/puml-theme-" + filename + ".puml";
-		final String description = "<" + res + ">";
-		final InputStream is = Stdlib.class.getResourceAsStream(res);
-		if (is == null) {
-			return null;
-		}
-		return ReadLineReader.create(new InputStreamReader(is), description);
-	}
-
 	public static ReadLine getReaderStdlibInclude(StringLocated s, String filename) {
 		Log.info("Loading sdlib " + filename);
 		InputStream is = getStdlibInputStream(filename);

src/net/sourceforge/plantuml/theme/ThemeUtils.java

@@ -0,0 +1,79 @@
+/* ========================================================================
+ * PlantUML : a free UML diagram generator
+ * ========================================================================
+ *
+ * (C) Copyright 2009-2021, Arnaud Roques
+ *
+ * Project Info:  http://plantuml.com
+ *
+ * If you like this project or if you find it useful, you can support us at:
+ *
+ * http://plantuml.com/patreon (only 1$ per month!)
+ * http://plantuml.com/paypal
+ *
+ * This file is part of PlantUML.
+ *
+ * PlantUML is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * PlantUML distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+ * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public
+ * License for more details.
+ *
+ * You should have received a copy of the GNU General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301,
+ * USA.
+ */
+package net.sourceforge.plantuml.theme;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+import java.util.Objects;
+
+import net.sourceforge.plantuml.Log;
+import net.sourceforge.plantuml.StringLocated;
+import net.sourceforge.plantuml.preproc.ReadLine;
+import net.sourceforge.plantuml.preproc.ReadLineReader;
+import net.sourceforge.plantuml.preproc.Stdlib;
+import net.sourceforge.plantuml.sprite.RessourcesUtils;
+
+public class ThemeUtils {
+
+	private static final String THEME_FILE_PREFIX = "puml-theme-";
+
+	private static final String THEME_FILE_SUFFIX = ".puml";
+
+	private static final String THEME_PATH = "themes";
+
+	public static List<String> getAllThemeNames() throws IOException {
+		final Collection<String> filenames = Objects.requireNonNull(RessourcesUtils.getJarFile(THEME_PATH, false));
+		final List<String> result = new ArrayList<>();
+		for (String f : filenames) {
+			if (f.startsWith(THEME_FILE_PREFIX) && f.endsWith(THEME_FILE_SUFFIX)) {
+				result.add(f.substring(THEME_FILE_PREFIX.length(), f.length() - THEME_FILE_SUFFIX.length()));
+			}
+		}
+		Collections.sort(result);
+		return result;
+	}
+
+	public static ReadLine getReaderTheme(StringLocated s, String filename) {
+		Log.info("Loading theme " + filename);
+		final String res = "/" + THEME_PATH + "/" + THEME_FILE_PREFIX + filename + THEME_FILE_SUFFIX;
+		final String description = "<" + res + ">";
+		final InputStream is = Stdlib.class.getResourceAsStream(res);
+		if (is == null) {
+			return null;
+		}
+		return ReadLineReader.create(new InputStreamReader(is), description);
+	}
+}

src/net/sourceforge/plantuml/tim/TContext.java

@@ -64,6 +64,7 @@ import net.sourceforge.plantuml.preproc2.PreprocessorIncludeStrategy;
 import net.sourceforge.plantuml.preproc2.PreprocessorUtils;
 import net.sourceforge.plantuml.security.SFile;
 import net.sourceforge.plantuml.security.SURL;
+import net.sourceforge.plantuml.theme.ThemeUtils;
 import net.sourceforge.plantuml.tim.expression.Knowledge;
 import net.sourceforge.plantuml.tim.expression.TValue;
 import net.sourceforge.plantuml.tim.iterator.CodeIterator;
@@ -559,7 +560,7 @@ public class TContext {
 		final EaterTheme include = new EaterTheme(s.getTrimmed());
 		include.analyze(this, memory);
 		final String location = include.getLocation();
-		final ReadLine reader = PreprocessorUtils.getReaderTheme(s, location);
+		final ReadLine reader = ThemeUtils.getReaderTheme(s, location);
 		if (reader == null) {
 			throw EaterException.located("No such theme " + location);
 		}

test/net/sourceforge/plantuml/help/HelpTest.java

@@ -0,0 +1,19 @@
+package net.sourceforge.plantuml.help;
+
+import static net.sourceforge.plantuml.test.TestUtils.renderUmlAsUnicode;
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class HelpTest {
+
+	@Test
+	public void test_help_themes() throws Exception {
+
+		final String output = renderUmlAsUnicode("help themes");
+		
+		assertThat(output)
+				.startsWith("Help on themes")
+				.contains("bluegray", "hacker");
+	}
+}

test/net/sourceforge/plantuml/test/TestUtils.java

@@ -2,12 +2,37 @@ package net.sourceforge.plantuml.test;
 
 import static java.nio.charset.StandardCharsets.UTF_8;
 
+import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
 
+import net.sourceforge.plantuml.FileFormat;
+import net.sourceforge.plantuml.FileFormatOption;
+import net.sourceforge.plantuml.Option;
+import net.sourceforge.plantuml.SourceStringReader;
+
 public class TestUtils {
 
+	public static String renderAsUnicode(String source, String... options) throws Exception {
+
+		final Option option = new Option(options);
+		option.setFileFormatOption(new FileFormatOption(FileFormat.UTXT));
+
+		final SourceStringReader ssr = new SourceStringReader(option.getDefaultDefines(), source, "UTF-8", option.getConfig());
+
+		final ByteArrayOutputStream os = new ByteArrayOutputStream();
+
+		ssr.getBlocks().get(0).getDiagram().exportDiagram(os, 0, option.getFileFormatOption());
+
+		return new String(os.toByteArray(), UTF_8);
+	}
+
+	public static String renderUmlAsUnicode(String source, String... options) throws Exception {
+
+		return renderAsUnicode("@startuml\n" + source + "\n@enduml\n", options);
+	}
+
 	public static void writeUtf8File(Path path, String string) throws IOException {
 
 		Files.createDirectories(path.getParent());