Add API to read/write QgsExpressionContextScope from/to XML

python/core/auto_generated/qgsexpressioncontext.sip.in

@@ -341,6 +341,24 @@ Convenience function for setting a fields for the scope. Any existing
 fields set by the scope will be overwritten.
 
 :param fields: fields for scope
+%End
+
+    void readXml( const QDomElement &element, const QgsReadWriteContext &context );
+%Docstring
+Reads scope variables from an XML element.
+
+.. seealso:: :py:func:`writeXml`
+
+.. versionadded:: 3.6
+%End
+
+    bool writeXml( QDomElement &element, QDomDocument &document, const QgsReadWriteContext &context ) const;
+%Docstring
+Writes scope variables to an XML ``element``.
+
+.. seealso:: :py:func:`readXml`
+
+.. versionadded:: 3.6
 %End
 
 };

src/core/qgsexpressioncontext.cpp

@@ -34,6 +34,7 @@
 #include "qgslayoutreportcontext.h"
 #include "qgsexpressionutils.h"
 #include "qgslayoutrendercontext.h"
+#include "qgsxmlutils.h"
 
 #include <QSettings>
 #include <QDir>
@@ -222,6 +223,31 @@ void QgsExpressionContextScope::setFields( const QgsFields &fields )
   addVariable( StaticVariable( QgsExpressionContext::EXPR_FIELDS, QVariant::fromValue( fields ), true ) );
 }
 
+void QgsExpressionContextScope::readXml( const QDomElement &element, const QgsReadWriteContext & )
+{
+  const QDomNodeList variablesNodeList = element.childNodes();
+  for ( int i = 0; i < variablesNodeList.size(); ++i )
+  {
+    const QDomElement variableElement = variablesNodeList.at( i ).toElement();
+    const QString key = variableElement.attribute( QStringLiteral( "name" ) );
+    const QVariant value = QgsXmlUtils::readVariant( variableElement.firstChildElement( QStringLiteral( "Option" ) ) );
+    setVariable( key, value );
+  }
+}
+
+bool QgsExpressionContextScope::writeXml( QDomElement &element, QDomDocument &document, const QgsReadWriteContext & ) const
+{
+  for ( auto it = mVariables.constBegin(); it != mVariables.constEnd(); ++it )
+  {
+    QDomElement varElem = document.createElement( QStringLiteral( "Variable" ) );
+    varElem.setAttribute( QStringLiteral( "name" ), it.key() );
+    QDomElement valueElem = QgsXmlUtils::writeVariant( it.value().value, document );
+    varElem.appendChild( valueElem );
+    element.appendChild( varElem );
+  }
+  return true;
+}
+
 
 //
 // QgsExpressionContext

src/core/qgsexpressioncontext.h

@@ -349,6 +349,20 @@ class CORE_EXPORT QgsExpressionContextScope
      */
     void setFields( const QgsFields &fields );
 
+    /**
+     * Reads scope variables from an XML element.
+     * \see writeXml()
+     * \since QGIS 3.6
+     */
+    void readXml( const QDomElement &element, const QgsReadWriteContext &context );
+
+    /**
+     * Writes scope variables to an XML \a element.
+     * \see readXml()
+     * \since QGIS 3.6
+     */
+    bool writeXml( QDomElement &element, QDomDocument &document, const QgsReadWriteContext &context ) const;
+
   private:
     QString mName;
     QHash<QString, StaticVariable> mVariables;

tests/src/core/testqgsexpressioncontext.cpp

@@ -55,6 +55,7 @@ class TestQgsExpressionContext : public QObject
 
     void valuesAsMap();
     void description();
+    void readWriteScope();
 
   private:
 
@@ -854,5 +855,37 @@ void TestQgsExpressionContext::description()
   QCOMPARE( context.description( "project_title" ), QStringLiteral( "my desc" ) );
 }
 
+void TestQgsExpressionContext::readWriteScope()
+{
+  QDomImplementation DomImplementation;
+  QDomDocumentType documentType =
+    DomImplementation.createDocumentType(
+      QStringLiteral( "qgis" ), QStringLiteral( "http://mrcc.com/qgis.dtd" ), QStringLiteral( "SYSTEM" ) );
+  QDomDocument doc( documentType );
+  QDomElement rootNode = doc.createElement( QStringLiteral( "qgis" ) );
+
+  QgsExpressionContextScope s1;
+  s1.setVariable( QStringLiteral( "v1" ), "t1" );
+  s1.setVariable( QStringLiteral( "v2" ), 55 );
+  QDomElement e = doc.createElement( QStringLiteral( "scope_test" ) );
+  s1.writeXml( e, doc, QgsReadWriteContext() );
+  doc.appendChild( e );
+
+  QgsExpressionContextScope s2;
+  QCOMPARE( s2.variableCount(), 0 );
+
+  // invalid xml element
+  QDomElement e2 = doc.createElement( QStringLiteral( "empty" ) );
+  s2.readXml( e2, QgsReadWriteContext() );
+  QCOMPARE( s2.variableCount(), 0 );
+
+  // valid element
+  s2.readXml( e, QgsReadWriteContext() );
+  QCOMPARE( s2.variableCount(), 2 );
+  QCOMPARE( s2.variableNames().toSet(), QSet< QString >() << QStringLiteral( "v1" ) << QStringLiteral( "v2" ) );
+  QCOMPARE( s2.variable( QStringLiteral( "v1" ) ).toString(), QStringLiteral( "t1" ) );
+  QCOMPARE( s2.variable( QStringLiteral( "v2" ) ).toInt(), 55 );
+}
+
 QGSTEST_MAIN( TestQgsExpressionContext )
 #include "testqgsexpressioncontext.moc"