feat: resolve css variables

itext.tests/itext.svg.tests/itext/svg/css/SvgStyleResolverTest.cs

@@ -187,6 +187,54 @@ public virtual void OverrideDefaultStyleTest() {
             NUnit.Framework.Assert.AreEqual("white", resolvedStyles.Get(SvgConstants.Attributes.STROKE));
         }
 
+        [NUnit.Framework.TestCase("a", "30px", "30px")]
+        [NUnit.Framework.TestCase("b", "50px", "30px")]
+        [NUnit.Framework.TestCase("c d", "35px", "35px")]
+        public virtual void ResolveCssVariablesTest(string divClass, string expectedMargin, string expectedVarValue) {
+            iText.StyledXmlParser.Jsoup.Nodes.Element styleTag = new iText.StyledXmlParser.Jsoup.Nodes.Element(iText.StyledXmlParser.Jsoup.Parser.Tag
+                .ValueOf("style"), "");
+            TextNode styleContents = new TextNode(@"
+	div {
+		--test-var: 30px;
+	}
+    div.a {
+        margin: var(--test-var,40px);
+    }
+    div.b {
+        margin: var(--other-var,50px);
+    }
+    div.c {
+        --test-var: 35px;
+    }
+    div.c.d {
+        margin: var(--test-var,40px);
+    }
+  ");
+            JsoupElementNode jSoupStyle = new JsoupElementNode(styleTag);
+            jSoupStyle.AddChild(new JsoupTextNode(styleContents));
+            SvgProcessorContext context = new SvgProcessorContext(new SvgConverterProperties());
+
+            SvgStyleResolver resolver = new SvgStyleResolver(jSoupStyle, context);
+            AbstractCssContext svgContext = new SvgCssContext();
+
+            iText.StyledXmlParser.Jsoup.Nodes.Element div = new iText.StyledXmlParser.Jsoup.Nodes.Element(iText.StyledXmlParser.Jsoup.Parser.Tag
+                .ValueOf("div"), "");
+            JsoupElementNode jSoupDiv = new JsoupElementNode(div);
+            Attributes divAttributes = div.Attributes();
+            divAttributes.Put(new iText.StyledXmlParser.Jsoup.Nodes.Attribute("class", divClass));
+
+            IDictionary<String, String> actual = resolver.ResolveStyles(jSoupDiv, svgContext);
+            IDictionary<String, String> expected = new Dictionary<String, String>();
+            expected.Put("class", divClass);
+            expected.Put("--test-var", expectedVarValue);
+            expected.Put("margin-top", expectedMargin);
+            expected.Put("margin-right", expectedMargin);
+            expected.Put("margin-bottom", expectedMargin);
+            expected.Put("margin-left", expectedMargin);
+            expected.Put("font-size", "12pt");
+            NUnit.Framework.Assert.AreEqual(expected, actual);
+        }
+
         [NUnit.Framework.Test]
         public virtual void SvgCssResolverStyleTagTest() {
             iText.StyledXmlParser.Jsoup.Nodes.Element styleTag = new iText.StyledXmlParser.Jsoup.Nodes.Element(iText.StyledXmlParser.Jsoup.Parser.Tag

itext/itext.styledxmlparser/itext/styledxmlparser/css/resolve/CssInheritance.cs

@@ -66,6 +66,12 @@ public class CssInheritance : IStyleInheritance {
         /// <param name="cssProperty">the CSS property</param>
         /// <returns>true, if the property is inheritable</returns>
         public virtual bool IsInheritable(String cssProperty) {
+
+            // Always inherit CSS variables
+            if (cssProperty.StartsWith("--")) {
+                return true;
+            }
+
             return INHERITABLE_PROPERTIES.Contains(cssProperty);
         }
     }

itext/itext.svg/itext/svg/css/impl/SvgStyleResolver.cs

@@ -40,6 +40,7 @@ You should have received a copy of the GNU Affero General Public License
 using iText.Svg.Exceptions;
 using iText.Svg.Logs;
 using iText.Svg.Processors.Impl;
+using System.Text.RegularExpressions;
 
 namespace iText.Svg.Css.Impl {
     /// <summary>Default implementation of SVG`s styles and attribute resolver .</summary>
@@ -60,6 +61,8 @@ public class SvgStyleResolver : ICssResolver {
         private static readonly ILogger LOGGER = ITextLogManager.GetLogger(typeof(iText.Svg.Css.Impl.SvgStyleResolver
             ));
 
+        private static readonly Regex CssVarDecl = new Regex(@"^\s*var\(\s*(?<decl>--.*)\)\s*$", RegexOptions.IgnoreCase | RegexOptions.Compiled);
+
         private CssStyleSheet css;
 
         private const String DEFAULT_CSS_PATH = "iText.Svg.default.css";
@@ -262,6 +265,10 @@ private IDictionary<String, String> ResolveStyles(INode element, SvgCssContext c
                 // TODO DEVSIX-8792 SVG: implement styles appliers like in html2pdf
                 styles.Put(CommonCssConstants.FONT_FAMILY, fontFamilies[0]);
             }
+
+            // Resolve CSS variables
+            ResolveCssVariables(styles);
+
             // Set root font size
             bool isSvgElement = element is IElementNode && SvgConstants.Tags.SVG.Equals(((IElementNode)element).Name()
                 );
@@ -275,6 +282,57 @@ private IDictionary<String, String> ResolveStyles(INode element, SvgCssContext c
             return styles;
         }
 
+        private static bool IsCssVarDecl(string decl, out string innerDecl) {
+            if (decl == null) {
+                innerDecl = null;
+                return false;
+            }
+
+            var cssVarDecl = CssVarDecl.Match(decl);
+            if (cssVarDecl.Success) {
+                innerDecl = cssVarDecl.Groups["decl"].Value;
+                return true;
+            }
+
+            innerDecl = null;
+            return false;
+        }
+
+        private static void ResolveCssVariables(IDictionary<string, string> styles) {
+            var varOverrides = new Dictionary<string, string>();
+
+            foreach (KeyValuePair<String, String> entry in styles) {
+                if (IsCssVarDecl(entry.Value, out var decl)) {
+                    var value = ResolveVariable(decl);
+                    varOverrides.Add(entry.Key, value);
+                }
+                continue;
+
+                string ResolveVariable(string substring) {
+                    var hasDefault = substring.IndexOf(',');
+                    var varName = hasDefault == -1 ? substring : substring.Substring(0, hasDefault);
+                    var dfltVal = hasDefault == -1 ? null : substring.Substring(hasDefault + 1).Trim();
+                    if (styles.TryGetValue(varName, out var variable)) {
+                        return variable;
+                    }
+
+                    if (dfltVal is null) {
+                        return null;
+                    }
+
+                    if (IsCssVarDecl(dfltVal, out var innerDecl)) {
+                        return ResolveVariable(innerDecl);
+                    }
+
+                    return dfltVal;
+                }
+            }
+
+            foreach (var varOverride in varOverrides) {
+                styles.Put(varOverride.Key, varOverride.Value);
+            }
+        }
+
         /// <summary>
         /// Resolves the full path of link href attribute,
         /// thanks to the resource resolver.