Add wxXmlResourceHandler::GetNodeAttribute()

Add another helper function that can be used in XRC handlers without
using wxXmlNode directly.

include/wx/xrc/xmlres.h

@@ -511,6 +511,11 @@ public:
     // Returns the name of the node, e.g. "object" or "sizeritem".
     wxString GetNodeName(const wxXmlNode *node) const override;
 
+    // Returns the value of the given attribute under the node.
+    wxString GetNodeAttribute(const wxXmlNode *node,
+                              const wxString& attrName,
+                              const wxString& defaultValue) const override;
+
     // Gets node content from wxXML_ENTITY_NODE
     // The problem is, <tag>content<tag> is represented as
     // wxXML_ENTITY_NODE name="tag", content=""

include/wx/xrc/xmlreshandler.h

@@ -62,6 +62,9 @@ public:
     virtual bool IsOfClass(wxXmlNode *node, const wxString& classname) const = 0;
     virtual bool IsObjectNode(const wxXmlNode *node) const = 0;
     virtual wxString GetNodeName(const wxXmlNode *node) const = 0;
+    virtual wxString GetNodeAttribute(const wxXmlNode *node,
+                                      const wxString& attrName,
+                                      const wxString& defaultValue) const = 0;
     virtual wxString GetNodeContent(const wxXmlNode *node) const = 0;
     virtual wxXmlNode *GetNodeParent(const wxXmlNode *node) const = 0;
     virtual wxXmlNode *GetNodeNext(const wxXmlNode *node) const = 0;
@@ -247,6 +250,13 @@ protected:
         return GetImpl()->GetNodeName(node);
     }
 
+    wxString GetNodeAttribute(const wxXmlNode *node,
+                              const wxString& attrName,
+                              const wxString& defaultValue = {}) const
+    {
+        return GetImpl()->GetNodeAttribute(node, attrName, defaultValue);
+    }
+
     wxString GetNodeContent(const wxXmlNode *node) const
     {
         return GetImpl()->GetNodeContent(node);

interface/wx/xrc/xmlres.h

@@ -770,6 +770,17 @@ protected:
     */
     wxString GetNodeName(wxXmlNode* node) const;
 
+    /**
+        Gets the node attribute value.
+
+        If @a node is @NULL or the attribute is not present, returns @a defaultValue.
+
+        @since 3.3.0
+    */
+    wxString GetNodeAttribute(const wxXmlNode *node,
+                              const wxString& attrName,
+                              const wxString& defaultValue = {}) const;
+
     /**
         Gets node content from wxXML_ENTITY_NODE.
     */

src/xrc/xmlres.cpp

@@ -2265,6 +2265,14 @@ wxString wxXmlResourceHandlerImpl::GetNodeName(const wxXmlNode *node) const
     return node ? node->GetName() : wxString{};
 }
 
+wxString
+wxXmlResourceHandlerImpl::GetNodeAttribute(const wxXmlNode *node,
+                                           const wxString& attrName,
+                                           const wxString& defaultValue) const
+{
+    return node ? node->GetAttribute(attrName, defaultValue) : defaultValue;
+}
+
 wxString wxXmlResourceHandlerImpl::GetNodeContent(const wxXmlNode *node) const
 {
     return node ? node->GetNodeContent() : wxString{};