Label: support painting background with rounded corners (issue #842)

Demo: added rounded panels and labels to "More Components" tab

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

@@ -638,14 +638,18 @@ class UIDefaultsLoader
 			// Syntax: top,left,bottom,right[,lineColor[,lineThickness[,arc]]]
 			List<String> parts = splitFunctionParams( value, ',' );
 			Insets insets = parseInsets( value );
-			ColorUIResource lineColor = (parts.size() >= 5)
+			ColorUIResource lineColor = (parts.size() >= 5 && !parts.get( 4 ).isEmpty())
 				? (ColorUIResource) parseColorOrFunction( resolver.apply( parts.get( 4 ) ), resolver )
 				: null;
-			float lineThickness = (parts.size() >= 6 && !parts.get( 5 ).isEmpty()) ? parseFloat( parts.get( 5 ) ) : 1f;
-			int arc = (parts.size() >= 7) ? parseInteger( parts.get( 6 ) ) : 0;
+			float lineThickness = (parts.size() >= 6 && !parts.get( 5 ).isEmpty())
+				? parseFloat( parts.get( 5 ) )
+				: 1f;
+			int arc = (parts.size() >= 7) && !parts.get( 6 ).isEmpty()
+				? parseInteger( parts.get( 6 ) )
+				: 0;
 
 			return (LazyValue) t -> {
-				return (lineColor != null)
+				return (lineColor != null || arc > 0)
 					? new FlatLineBorder( insets, lineColor, lineThickness, arc )
 					: new FlatEmptyBorder( insets );
 			};

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatBorder.java

@@ -277,7 +277,7 @@ public class FlatBorder
 	}
 
 	/**
-	 * Returns the (unscaled) arc diameter of the border.
+	 * Returns the (unscaled) arc diameter of the border corners.
 	 */
 	protected int getArc( Component c ) {
 		return 0;

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatLabelUI.java

@@ -64,6 +64,9 @@ public class FlatLabelUI
 {
 	@Styleable protected Color disabledForeground;
 
+	// only used via styling (not in UI defaults)
+	/** @since 3.5 */ @Styleable protected int arc = -1;
+
 	private final boolean shared;
 	private boolean defaults_initialized = false;
 	private Map<String, Object> oldStyleValues;
@@ -244,6 +247,12 @@ public class FlatLabelUI
 		return false;
 	}
 
+	@Override
+	public void update( Graphics g, JComponent c ) {
+		FlatPanelUI.fillRoundedBackground( g, c, arc );
+		paint( g, c );
+	}
+
 	static Graphics createGraphicsHTMLTextYCorrection( Graphics g, JComponent c ) {
 		return (c.getClientProperty( BasicHTML.propertyKey ) != null)
 			? HiDPIUtils.createGraphicsTextYCorrection( (Graphics2D) g )

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatLineBorder.java

@@ -26,10 +26,14 @@ import javax.swing.JComponent;
 
 /**
  * Line border for various components.
- *
+ * <p>
  * Paints a scaled (usually 1px thick) line around the component.
  * The line thickness is not added to the border insets.
  * The insets should be at least have line thickness (usually 1,1,1,1).
+ * <p>
+ * For {@link javax.swing.JPanel} and {@link javax.swing.JLabel}, this border
+ * can be used paint rounded background (if line color is {@code null}) or
+ * paint rounded line border with rounded background.
  *
  * @author Karl Tauber
  */
@@ -52,15 +56,28 @@ public class FlatLineBorder
 		this.arc = arc;
 	}
 
+	/** @since 3.5 */
+	public FlatLineBorder( Insets insets, int arc ) {
+		this( insets, null, 0, arc );
+	}
+
 	public Color getLineColor() {
 		return lineColor;
 	}
 
+	/**
+	 * Returns the (unscaled) line thickness used to paint the border.
+	 * The line thickness does not affect the border insets.
+	 */
 	public float getLineThickness() {
 		return lineThickness;
 	}
 
-	/** @since 2 */
+	/**
+	 * Returns the (unscaled) arc diameter of the border corners.
+	 *
+	 * @since 2
+	 */
 	public int getArc() {
 		return arc;
 	}
@@ -70,11 +87,16 @@ public class FlatLineBorder
 		if( c instanceof JComponent && ((JComponent)c).getClientProperty( FlatPopupFactory.KEY_POPUP_USES_NATIVE_BORDER ) != null )
 			return;
 
+		Color lineColor = getLineColor();
+		float lineThickness = getLineThickness();
+		if( lineColor == null || lineThickness <= 0 )
+			return;
+
 		Graphics2D g2 = (Graphics2D) g.create();
 		try {
 			FlatUIUtils.setRenderingHints( g2 );
 			FlatUIUtils.paintOutlinedComponent( g2, x, y, width, height,
-				0, 0, 0, scale( getLineThickness() ), scale( getArc() ), null, getLineColor(), null );
+				0, 0, 0, scale( lineThickness ), scale( getArc() ), null, lineColor, null );
 		} finally {
 			g2.dispose();
 		}

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatPanelUI.java

@@ -25,6 +25,7 @@ import java.util.Map;
 import javax.swing.JComponent;
 import javax.swing.JPanel;
 import javax.swing.LookAndFeel;
+import javax.swing.border.Border;
 import javax.swing.plaf.ComponentUI;
 import javax.swing.plaf.basic.BasicPanelUI;
 import com.formdev.flatlaf.FlatClientProperties;
@@ -160,11 +161,18 @@ public class FlatPanelUI
 
 	@Override
 	public void update( Graphics g, JComponent c ) {
-		int arc = (this.arc >= 0)
-			? this.arc
-			: ((c.getBorder() instanceof FlatLineBorder)
-				? ((FlatLineBorder)c.getBorder()).getArc()
+		fillRoundedBackground( g, c, arc );
+		paint( g, c );
+	}
+
+	/** @since 3.5 */
+	public static void fillRoundedBackground( Graphics g, JComponent c, int arc ) {
+		if( arc < 0 ) {
+			Border border = c.getBorder();
+			arc = ((border instanceof FlatLineBorder)
+				? ((FlatLineBorder)border).getArc()
 				: 0);
+		}
 
 		// fill background
 		if( c.isOpaque() ) {
@@ -185,8 +193,6 @@ public class FlatPanelUI
 				0, UIScale.scale( arc ) );
 			FlatUIUtils.resetRenderingHints( g, oldRenderingHints );
 		}
-
-		paint( g, c );
 	}
 
 	@Override

flatlaf-core/src/test/java/com/formdev/flatlaf/TestUIDefaultsLoader.java

@@ -84,10 +84,12 @@ public class TestUIDefaultsLoader
 	void parseBorders() {
 		Insets insets = new Insets( 1,2,3,4 );
 		assertBorderEquals( new FlatEmptyBorder( insets ), "1,2,3,4" );
+		assertBorderEquals( new FlatEmptyBorder( insets ), "1,2,3,4,,," );
 		assertBorderEquals( new FlatLineBorder( insets, Color.red ), "1,2,3,4,#f00" );
 		assertBorderEquals( new FlatLineBorder( insets, Color.red, 2.5f, 0 ), "1,2,3,4,#f00,2.5" );
 		assertBorderEquals( new FlatLineBorder( insets, Color.red, 2.5f, 6 ), "1,2,3,4,#f00,2.5,6" );
 		assertBorderEquals( new FlatLineBorder( insets, Color.red, 1, 6 ), "1,2,3,4,#f00,,6" );
+		assertBorderEquals( new FlatLineBorder( insets, null, 1, 6 ), "1,2,3,4,,,6" );
 	}
 
 	private void assertBorderEquals( Border expected, String actualStyle ) {

flatlaf-core/src/test/java/com/formdev/flatlaf/ui/TestFlatStyleableInfo.java

@@ -253,7 +253,8 @@ public class TestFlatStyleableInfo
 		FlatLabelUI ui = (FlatLabelUI) c.getUI();
 
 		Map<String, Class<?>> expected = expectedMap(
-			"disabledForeground", Color.class
+			"disabledForeground", Color.class,
+			"arc", int.class
 		);
 
 		assertMapEquals( expected, ui.getStyleableInfos( c ) );

flatlaf-core/src/test/java/com/formdev/flatlaf/ui/TestFlatStyleableValue.java

@@ -355,6 +355,7 @@ public class TestFlatStyleableValue
 		FlatLabelUI ui = (FlatLabelUI) c.getUI();
 
 		testColor( c, ui, "disabledForeground", 0x123456 );
+		testInteger( c, ui, "arc", 123 );
 	}
 
 	@Test

flatlaf-core/src/test/java/com/formdev/flatlaf/ui/TestFlatStyling.java

@@ -412,6 +412,7 @@ public class TestFlatStyling
 		FlatLabelUI ui = (FlatLabelUI) c.getUI();
 
 		ui.applyStyle( c, "disabledForeground: #fff" );
+		ui.applyStyle( c, "arc: 8" );
 
 		// JComponent properties
 		ui.applyStyle( c, "background: #fff" );