Typography: support deriving font from any other font (was always from defaultFont) (issue #384)

flatlaf-core/src/main/java/com/formdev/flatlaf/FlatLaf.java

@@ -527,7 +527,7 @@ public abstract class FlatLaf
 		// use active value for all fonts to allow changing fonts in all components with:
 		//     UIManager.put( "defaultFont", myFont );
 		// (this is similar as in Nimbus L&F)
-		Object activeFont = new ActiveFont( null, -1, 0, 0, 0, 0 );
+		Object activeFont = new ActiveFont( null, null, -1, 0, 0, 0, 0 );
 
 		// override fonts
 		for( Object key : defaults.keySet() ) {
@@ -611,7 +611,7 @@ public abstract class FlatLaf
 
 	/** @since 1.1 */
 	public static ActiveValue createActiveFontValue( float scaleFactor ) {
-		return new ActiveFont( null, -1, 0, 0, 0, scaleFactor );
+		return new ActiveFont( null, null, -1, 0, 0, 0, scaleFactor );
 	}
 
 	/**
@@ -1182,6 +1182,7 @@ public abstract class FlatLaf
 	static class ActiveFont
 		implements ActiveValue
 	{
+		private final String baseFontKey;
 		private final List<String> families;
 		private final int style;
 		private final int styleChange;
@@ -1191,7 +1192,9 @@ public abstract class FlatLaf
 
 		// cache (scaled/derived) font
 		private FontUIResource font;
-		private Font lastDefaultFont;
+		private Font lastBaseFont;
+
+		private boolean inCreateValue;
 
 		/**
 		 * @param families list of font families, or {@code null}
@@ -1202,9 +1205,10 @@ public abstract class FlatLaf
 		 * @param relativeSize added to size of base font, or {@code 0}
 		 * @param scaleSize multiply size of base font, or {@code 0}
 		 */
-		ActiveFont( List<String> families, int style, int styleChange,
+		ActiveFont( String baseFontKey, List<String> families, int style, int styleChange,
 			int absoluteSize, int relativeSize, float scaleSize )
 		{
+			this.baseFontKey = baseFontKey;
 			this.families = families;
 			this.style = style;
 			this.styleChange = styleChange;
@@ -1215,16 +1219,30 @@ public abstract class FlatLaf
 
 		@Override
 		public Object createValue( UIDefaults table ) {
-			Font defaultFont = UIManager.getFont( "defaultFont" );
+			if( inCreateValue )
+				throw new IllegalStateException( "FlatLaf: endless recursion in font" );
 
-			// fallback (to avoid NPE in case that this is used in another Laf)
+			Font baseFont = null;
-			if( defaultFont == null )
-				defaultFont = UIManager.getFont( "Label.font" );
 
-			if( lastDefaultFont != defaultFont ) {
+			inCreateValue = true;
-				lastDefaultFont = defaultFont;
+			try {
+				if( baseFontKey != null )
+					baseFont = (Font) UIDefaultsLoader.lazyUIManagerGet( baseFontKey );
 
-				font = derive( defaultFont, fontSize -> UIScale.scale( fontSize ) );
+				if( baseFont == null )
+					baseFont = UIManager.getFont( "defaultFont" );
+
+				// fallback (to avoid NPE in case that this is used in another Laf)
+				if( baseFont == null )
+					baseFont = UIManager.getFont( "Label.font" );
+			} finally {
+				inCreateValue = false;
+			}
+
+			if( lastBaseFont != baseFont ) {
+				lastBaseFont = baseFont;
+
+				font = derive( baseFont, fontSize -> UIScale.scale( fontSize ) );
 			}
 
 			return font;

flatlaf-core/src/main/java/com/formdev/flatlaf/UIDefaultsLoader.java

@@ -1000,7 +1000,7 @@ class UIDefaultsLoader
 	}
 
 	/**
-	 * Syntax: [normal] [bold|+bold|-bold] [italic|+italic|-italic] [<size>|+<incr>|-<decr>|<percent>%] [family[, family]]
+	 * Syntax: [normal] [bold|+bold|-bold] [italic|+italic|-italic] [<size>|+<incr>|-<decr>|<percent>%] [family[, family]] [$baseFontKey]
 	 */
 	private static Object parseFont( String value ) {
 		Object font = fontCache.get( value );
@@ -1013,6 +1013,7 @@ class UIDefaultsLoader
 		int relativeSize = 0;
 		float scaleSize = 0;
 		List<String> families = null;
+		String baseFontKey = null;
 
 		// use StreamTokenizer to split string because it supports quoted strings
 		StreamTokenizer st = new StreamTokenizer( new StringReader( value ) );
@@ -1062,6 +1063,12 @@ class UIDefaultsLoader
 								scaleSize = parseInteger( param.substring( 0, param.length() - 1 ), true ) / 100f;
 							else
 								absoluteSize = parseInteger( param, true );
+						} else if( firstChar == '$' ) {
+							// reference to base font
+							if( baseFontKey != null )
+								throw new IllegalArgumentException( "baseFontKey specified more than once in '" + value + "'" );
+
+							baseFontKey = param.substring( 1 );
 						} else {
 							// font family
 							if( families == null )
@@ -1088,7 +1095,7 @@ class UIDefaultsLoader
 				throw new IllegalArgumentException( "can not use '+italic' and '-italic' in '" + value + "'" );
 		}
 
-		font = new FlatLaf.ActiveFont( families, style, styleChange, absoluteSize, relativeSize, scaleSize );
+		font = new FlatLaf.ActiveFont( baseFontKey, families, style, styleChange, absoluteSize, relativeSize, scaleSize );
 		fontCache.put( value, font );
 		return font;
 	}
@@ -1227,7 +1234,7 @@ class UIDefaultsLoader
 	 * For use in LazyValue to get value for given key from UIManager and report error
 	 * if not found. If key is prefixed by '?', then no error is reported.
 	 */
-	private static Object lazyUIManagerGet( String uiKey ) {
+	static Object lazyUIManagerGet( String uiKey ) {
 		boolean optional = false;
 		if( uiKey.startsWith( OPTIONAL_PREFIX ) ) {
 			uiKey = uiKey.substring( OPTIONAL_PREFIX.length() );