[FEATURE] New expression function "attributes"

Returns a map containing all attributes from a feature, with field
names as map keys. We've got featureful, robust support for working
with maps in expressions now, so this allows rapid conversion
of all feature attributes to a map to use with these handy
functions.

resources/function_help/json/attributes

@@ -0,0 +1,14 @@
+{
+  "name": "attributes",
+  "type": "function",
+  "description": "Returns a map containing all attributes from a feature, with field names as map keys.",
+  "variants": [
+    { "variant": "Variant 1",
+        "variant_description": "Returns a map of all attributes from the current feature.",
+        "examples": [ { "expression":"attributes()['name']", "returns":"value stored in 'name' attribute for the current feature"}] },
+    { "variant": "Variant 2",
+        "variant_description": "Allows the target feature to be specified.",
+        "arguments": [ {"arg":"feature","description":"a feature"}],
+        "examples": [ { "expression":"attributes( @atlas_feature )['name']", "returns":"value stored in 'name' attribute for the current atlas feature"}] }
+   ]
+}

src/core/expression/qgsexpressionfunction.cpp

@@ -1394,6 +1394,7 @@ static QVariant fcnFeature( const QVariantList &, const QgsExpressionContext *co
 
   return context->feature();
 }
+
 static QVariant fcnAttribute( const QVariantList &values, const QgsExpressionContext *context, QgsExpression *parent, const QgsExpressionNodeFunction * )
 {
   QgsFeature feature;
@@ -1417,6 +1418,28 @@ static QVariant fcnAttribute( const QVariantList &values, const QgsExpressionCon
   return feature.attribute( attr );
 }
 
+static QVariant fcnAttributes( const QVariantList &values, const QgsExpressionContext *context, QgsExpression *parent, const QgsExpressionNodeFunction * )
+{
+  QgsFeature feature;
+  QString attr;
+  if ( values.size() == 0 || values.at( 0 ).isNull() )
+  {
+    feature = context->feature();
+  }
+  else
+  {
+    feature = QgsExpressionUtils::getFeature( values.at( 0 ), parent );
+  }
+
+  const QgsFields fields = feature.fields();
+  QVariantMap result;
+  for ( int i = 0; i < fields.count(); ++i )
+  {
+    result.insert( fields.at( i ).name(), feature.attribute( i ) );
+  }
+  return result;
+}
+
 static QVariant fcnIsSelected( const QVariantList &values, const QgsExpressionContext *context, QgsExpression *parent, const QgsExpressionNodeFunction * )
 {
   QgsVectorLayer *layer = nullptr;
@@ -5488,7 +5511,9 @@ const QList<QgsExpressionFunction *> &QgsExpression::Functions()
                                             fcnGetFeature, QStringLiteral( "Record and Attributes" ), QString(), false, QSet<QString>(), false, QStringList() << QStringLiteral( "QgsExpressionUtils::getFeature" ) )
         << new QgsStaticExpressionFunction( QStringLiteral( "get_feature_by_id" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "layer" ) )
                                             << QgsExpressionFunction::Parameter( QStringLiteral( "feature_id" ) ),
-                                            fcnGetFeatureById, QStringLiteral( "Record and Attributes" ), QString(), false, QSet<QString>(), false );
+                                            fcnGetFeatureById, QStringLiteral( "Record and Attributes" ), QString(), false, QSet<QString>(), false )
+        << new QgsStaticExpressionFunction( QStringLiteral( "attributes" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "feature" ), true ),
+                                            fcnAttributes, QStringLiteral( "Record and Attributes" ), QString(), false, QSet<QString>() << QgsFeatureRequest::ALL_ATTRIBUTES );
 
     QgsStaticExpressionFunction *isSelectedFunc = new QgsStaticExpressionFunction(
       QStringLiteral( "is_selected" ),

tests/src/core/testqgsexpression.cpp

@@ -1602,6 +1602,36 @@ class TestQgsExpression: public QObject
       QCOMPARE( v.toString(), QString( "test value" ) );
     }
 
+    void eval_feature_attributes()
+    {
+      QgsFeature f( 100 );
+      QgsFields fields;
+      fields.append( QgsField( QStringLiteral( "col1" ) ) );
+      fields.append( QgsField( QStringLiteral( "second_column" ), QVariant::Int ) );
+      f.setFields( fields, true );
+      f.setAttribute( QStringLiteral( "col1" ), QStringLiteral( "test value" ) );
+      f.setAttribute( QStringLiteral( "second_column" ), 5 );
+      QgsExpression exp( QStringLiteral( "attributes()['col1']" ) );
+      QgsExpressionContext context = QgsExpressionContextUtils::createFeatureBasedContext( f, QgsFields() );
+      QVariant v = exp.evaluate( &context );
+      QCOMPARE( v.toString(), QString( "test value" ) );
+      QgsExpression exp2( QStringLiteral( "attributes()['second_column']" ) );
+      v = exp2.evaluate( &context );
+      QCOMPARE( v.toInt(), 5 );
+
+      QgsExpression exp3( QStringLiteral( "attributes($currentfeature)['col1']" ) );
+      v = exp.evaluate( &context );
+      QCOMPARE( v.toString(), QString( "test value" ) );
+      QgsExpression exp4( QStringLiteral( "attributes($currentfeature)['second_column']" ) );
+      v = exp4.evaluate( &context );
+      QCOMPARE( v.toInt(), 5 );
+
+      QgsExpression exp5( QStringLiteral( "attributes('a')" ) );
+      v = exp5.evaluate( &context );
+      QVERIFY( v.isNull() );
+      QVERIFY( exp5.hasEvalError() );
+    }
+
     void eval_get_feature_data()
     {
       QTest::addColumn<QString>( "string" );