[FEATURE] Add an eval expression

Funded by
 * Regional Council of Picardy
 * ADUGA
 * Ville de Nyon
 * Wetu GIT cc

Author: m-kuhn

resources/function_help/json/eval

@@ -0,0 +1,10 @@
+{
+  "name": "eval",
+  "type": "function",
+  "description": "Evaluates an expression which is passed in a string. Useful to expand dynamic parameters passed as context variables or fields.",
+  "arguments": [ {"arg":"expression","description":"an expression string"}],
+  "examples": [
+    { "expression":"eval('\\'nice\\'')", "returns":"'nice'"},
+    { "expression":"eval(@expression_var)", "returns":"[whatever the result of evaluating @expression_var might be...]"}
+  ]
+}

src/core/qgsexpression.cpp

@@ -409,6 +409,16 @@ static QVariant fcnGetVariable( const QVariantList& values, const QgsExpressionC
   return context->variable( name );
 }
 
+static QVariant fcnEval( const QVariantList& values, const QgsExpressionContext* context, QgsExpression* parent )
+{
+  if ( !context )
+    return QVariant();
+
+  QString expString = getStringValue( values.at( 0 ), parent );
+  QgsExpression expression( expString );
+  return expression.evaluate( context );
+}
+
 static QVariant fcnSqrt( const QVariantList& values, const QgsExpressionContext*, QgsExpression* parent )
 {
   double x = getDoubleValue( values.at( 0 ), parent );
@@ -2522,6 +2532,7 @@ const QList<QgsExpression::Function*>& QgsExpression::Functions()
     //return all attributes string for referencedColumns - this is caught by
     // QgsFeatureRequest::setSubsetOfAttributes and causes all attributes to be fetched by the
     // feature request
+    << new StaticFunction( "eval", 1, fcnEval, "General", QString(), true, QStringList( QgsFeatureRequest::AllAttributes ) )
     << new StaticFunction( "attribute", 2, fcnAttribute, "Record", QString(), false, QStringList( QgsFeatureRequest::AllAttributes ) )
 
     << new StaticFunction( "_specialcol_", 1, fcnSpecialColumn, "Special" )

tests/src/core/testqgsexpression.cpp

@@ -1530,6 +1530,36 @@ class TestQgsExpression: public QObject
       Q_NOWARN_DEPRECATED_POP
     }
 
+    void eval_eval()
+    {
+      QgsFeature f( 100 );
+      QgsFields fields;
+      fields.append( QgsField( "col1" ) );
+      fields.append( QgsField( "second_column", QVariant::Int ) );
+      f.setFields( fields, true );
+      f.setAttribute( QString( "col1" ), QString( "test value" ) );
+      f.setAttribute( QString( "second_column" ), 5 );
+
+      QgsExpressionContext context = QgsExpressionContextUtils::createFeatureBasedContext( f, QgsFields() );
+
+      QgsExpression exp1( "eval()" );
+      QVariant v1 = exp1.evaluate( &context );
+
+      Q_ASSERT( !v1.isValid() );
+
+      QgsExpression exp2( "eval('4')" );
+      QVariant v2 = exp2.evaluate( &context );
+      QCOMPARE( v2, QVariant( 4 ) );
+
+      QgsExpression exp3( "eval('\"second_column\" * 2')" );
+      QVariant v3 = exp3.evaluate( &context );
+      QCOMPARE( v3, QVariant( 10 ) );
+
+      QgsExpression exp4( "eval('\"col1\"')" );
+      QVariant v4 = exp4.evaluate( &context );
+      QCOMPARE( v4, QVariant( "test value" ) );
+    }
+
     void expression_from_expression_data()
     {
       QTest::addColumn<QString>( "string" );