Gui: Add ParameterDefinition to Style Parameters

This adds a way to define parameters within code, that allows developers
to easily encapsulate the parameters and their default values.

src/Gui/StyleParameters/ParameterManager.h

@@ -37,6 +37,14 @@
 #include <Base/Bitmask.h>
 #include <Base/Parameter.h>
 
+// That macro uses inline const because some older compilers to not properly support constexpr
+// for std::string. It should be changed into static constepxr once we migrate to newer compiler.
+#define DEFINE_STYLE_PARAMETER(_name_, _defaultValue_) \
+    static inline const Gui::StyleParameters::ParameterDefinition _name_ { \
+        .name = #_name_, \
+        .defaultValue = (_defaultValue_), \
+    }
+
 namespace Gui::StyleParameters
 {
 
@@ -56,7 +64,7 @@ struct Numeric
     /// Numeric value of the length.
     double value;
     /// Unit of the length, empty if the value is dimensionless.
-    std::string unit;
+    std::string unit = "";
 
     /**
      * @name Operators
@@ -119,6 +127,33 @@ struct Value : std::variant<Numeric, Base::Color, std::string>
     std::string toString() const;
 };
 
+/**
+ * @brief A structure to define parameters which can be referenced in the code.
+ *
+ * @tparam T The type of the parameter.
+ *
+ * This structure allows defining parameters which encapsulate a name and a corresponding
+ * default value. Parameters defined this way can be reused across the codebase for consistency.
+ * The supported value types include:
+ *  - Numeric types.
+ *  - Colors using `Base::Color`.
+ *  - Strings.
+ *
+ * Example Usage:
+ * @code{.cpp}
+ * DEFINE_STYLE_PARAMETER(SomeParameter, Numeric { 10 });
+ * DEFINE_STYLE_PARAMETER(TextColor, Base::Color(0.5F, 0.2F, 0.8F));
+ * @endcode
+ */
+template <typename T>
+struct ParameterDefinition
+{
+    /// The name of the parameter, must be unique.
+    const char* name;
+    /// The default value of the parameter.
+    T defaultValue;
+};
+
 /**
  * @struct Parameter
  *
@@ -413,7 +448,29 @@ public:
      * @param context Resolution context for handling circular references
      * @return The resolved value
      */
-    Value resolve(const std::string& name, ResolveContext context = {}) const;
+    std::optional<Value> resolve(const std::string& name, ResolveContext context = {}) const;
+
+    /**
+     * @brief Resolves a parameter to its final value, based on definition.
+     *
+     * This method evaluates the parameter's expression and returns the computed
+     * value or default one from definition if the parameter is not available.
+     *
+     * @param definition Definition of the parameter to resolve
+     * @param context Resolution context for handling circular references
+     * @return The resolved value
+     */
+    template <typename T>
+    T resolve(const ParameterDefinition<T>& definition, ResolveContext context = {}) const
+    {
+        auto value = resolve(definition.name, std::move(context));
+
+        if (!value || !std::holds_alternative<T>(*value)) {
+            return definition.defaultValue;
+        }
+
+        return std::get<T>(*value);
+    }
 
     /**
      * @brief Evaluates an expression string and returns the result.

tests/src/Gui/StyleParameters/ParameterManagerTest.cpp

@@ -328,3 +328,22 @@ TEST_F(ParameterManagerTest, ErrorHandling)
     EXPECT_TRUE(invalidResult.has_value());
     EXPECT_TRUE(std::holds_alternative<std::string>(*invalidResult));
 }
+
+DEFINE_STYLE_PARAMETER(BaseSize, Numeric(8, "px"));
+
+TEST_F(ParameterManagerTest, ResolveParameterDefinition)
+{
+    auto result = manager.resolve(BaseSize);
+    EXPECT_DOUBLE_EQ(result.value, 16);
+    EXPECT_EQ(result.unit, "px");
+}
+
+
+DEFINE_STYLE_PARAMETER(MarginSize, Numeric(16, "px"));
+
+TEST_F(ParameterManagerTest, ResolveParameterDefinitionDefault)
+{
+    auto result = manager.resolve(MarginSize);
+    EXPECT_DOUBLE_EQ(result.value, 16);
+    EXPECT_EQ(result.unit, "px");
+}