[feature][layouts] Add map_credits function

This function collates a list of all the layer metadata attribution
strings for the layers shown inside a specified map item.

For example:

  array_to_string( map_credits( 'Main Map' ) )

Returns a comma separated list of layer credits for layers shown in the
'Main Map' layout item, e.g 'CC-BY-NC, CC-BY-SA'

There's an optional include_layer_names argument you can use
to include layer names before their attribution:

  array_to_string( map_credits( 'Main Map',
     include_layer_names := true,
     layer_name_separator := ': ' ) )

Returns a comma separated list of layer names and their credits for layers
shown in the 'Main Map' layout item,
e.g. 'Railway lines: CC-BY-NC, Basemap: CC-BY-SA'"

resources/function_help/json/map_credits

@@ -0,0 +1,12 @@
+{
+  "name": "map_credits",
+  "type": "function",
+  "groups": ["Layout"],
+  "description": "Returns a list of credit (usage rights) strings for the layers shown in a layout map item.",
+  "arguments": [ {"arg":"id","description":"map item ID"},
+                 {"arg":"include_layer_names","description":"Set to true to include layer names before their credit strings", "optional": true, "default": "false"},
+                 {"arg":"layer_name_separator","description":"String to insert between layer names and their credit strings, if include_layer_names is true", "optional": true, "default": "': '"} ],
+  "examples": [ { "expression":"array_to_string( map_credits( 'Main Map' ) )", "returns":"comma separated list of layer credits for layers shown in the 'Main Map' layout item, e.g 'CC-BY-NC, CC-BY-SA'" },
+  { "expression":"array_to_string( map_credits( 'Main Map', include_layer_names := true, layer_name_separator := ': ' ) )", "returns":"comma separated list of layer names and their credits for layers shown in the 'Main Map' layout item, e.g. 'Railway lines: CC-BY-NC, Basemap: CC-BY-SA'" }
+  ]
+}

src/core/expression/qgsexpressioncontextutils.cpp

@@ -30,6 +30,7 @@
 #include "qgslayoutatlas.h"
 #include "qgslayoutmultiframe.h"
 #include "qgsfeatureid.h"
+#include "qgslayoutitemmap.h"
 
 QgsExpressionContextScope *QgsExpressionContextUtils::globalScope()
 {
@@ -110,6 +111,69 @@ class GetLayoutItemVariables : public QgsScopedExpressionFunction
 
 };
 
+
+class GetLayoutMapLayerCredits : public QgsScopedExpressionFunction
+{
+  public:
+    GetLayoutMapLayerCredits( const QgsLayout *c )
+      : QgsScopedExpressionFunction( QStringLiteral( "map_credits" ),
+                                     QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "id" ) )
+                                     << QgsExpressionFunction::Parameter( QStringLiteral( "include_layer_names" ), true, false )
+                                     << QgsExpressionFunction::Parameter( QStringLiteral( "layer_name_separator" ), true, QStringLiteral( ": " ) ), QStringLiteral( "Layout" ) )
+      , mLayout( c )
+    {}
+
+    QVariant func( const QVariantList &values, const QgsExpressionContext *, QgsExpression *, const QgsExpressionNodeFunction * ) override
+    {
+      if ( !mLayout )
+        return QVariant();
+
+      QString id = values.value( 0 ).toString();
+
+      if ( QgsLayoutItemMap *map = qobject_cast< QgsLayoutItemMap * >( mLayout->itemById( id ) ) )
+      {
+        QgsExpressionContext c = map->createExpressionContext();
+        const QVariantList mapLayers = c.variable( QStringLiteral( "map_layers" ) ).toList();
+
+        const bool includeLayerNames = values.value( 1 ).toBool();
+        const QString layerNameSeparator = values.value( 2 ).toString();
+
+        QVariantList res;
+        for ( const QVariant &value : mapLayers )
+        {
+          if ( const QgsMapLayer *layer = qobject_cast< const QgsMapLayer * >( value.value< QObject * >() ) )
+          {
+            const QStringList credits = !layer->metadata().rights().isEmpty() ? layer->metadata().rights() : QStringList() << layer->attribution();
+            for ( const QString &credit : credits )
+            {
+              if ( credit.trimmed().isEmpty() )
+                continue;
+
+              const QString creditString = includeLayerNames ? layer->name() + layerNameSeparator + credit
+                                           : credit;
+
+              if ( !res.contains( creditString ) )
+                res << creditString;
+            }
+          }
+        }
+
+        return res;
+      }
+      return QVariant();
+    }
+
+    QgsScopedExpressionFunction *clone() const override
+    {
+      return new GetLayoutMapLayerCredits( mLayout );
+    }
+
+  private:
+
+    const QgsLayout *mLayout = nullptr;
+
+};
+
 class GetCurrentFormFieldValue : public QgsScopedExpressionFunction
 {
   public:
@@ -523,6 +587,7 @@ QgsExpressionContextScope *QgsExpressionContextUtils::layoutScope( const QgsLayo
   scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "layout_dpi" ), layout->renderContext().dpi(), true ) );
 
   scope->addFunction( QStringLiteral( "item_variables" ), new GetLayoutItemVariables( layout ) );
+  scope->addFunction( QStringLiteral( "map_credits" ), new GetLayoutMapLayerCredits( layout ) );
 
   if ( layout->reportContext().layer() )
   {
@@ -827,6 +892,7 @@ void QgsExpressionContextUtils::registerContextFunctions()
 {
   QgsExpression::registerFunction( new GetNamedProjectColor( nullptr ) );
   QgsExpression::registerFunction( new GetLayoutItemVariables( nullptr ) );
+  QgsExpression::registerFunction( new GetLayoutMapLayerCredits( nullptr ) );
   QgsExpression::registerFunction( new GetLayerVisibility( QList<QgsMapLayer *>(), 0.0 ) );
   QgsExpression::registerFunction( new GetProcessingParameterValue( QVariantMap() ) );
   QgsExpression::registerFunction( new GetCurrentFormFieldValue( ) );

tests/src/core/testqgslayoutitem.cpp

@@ -174,6 +174,7 @@ class TestQgsLayoutItem: public QObject
     void page();
     void itemVariablesFunction();
     void variables();
+    void mapCreditsFunction();
 
   private:
 
@@ -1471,6 +1472,71 @@ void TestQgsLayoutItem::variables()
   QCOMPARE( scope->variable( QStringLiteral( "var2" ) ).toInt(), 7 );
 }
 
+void TestQgsLayoutItem::mapCreditsFunction()
+{
+  QgsRectangle extent( 2000, 2800, 2500, 2900 );
+  QgsLayout l( QgsProject::instance() );
+
+  QgsExpression e( QStringLiteral( "array_to_string( map_credits( 'Map_id' ) )" ) );
+  // no map
+  QgsExpressionContext c = l.createExpressionContext();
+  QVariant r = e.evaluate( &c );
+  QVERIFY( !r.isValid() );
+
+  QgsLayoutItemMap *map = new QgsLayoutItemMap( &l );
+  map->setCrs( QgsCoordinateReferenceSystem( QStringLiteral( "EPSG:4326" ) ) );
+  map->attemptSetSceneRect( QRectF( 30, 60, 200, 100 ) );
+  map->setExtent( extent );
+  l.addLayoutItem( map );
+  map->setId( QStringLiteral( "Map_id" ) );
+
+  c = l.createExpressionContext();
+  e.prepare( &c );
+  r = e.evaluate( &c );
+  // no layers
+  QCOMPARE( r.toString(), QString() );
+
+  // with layers
+  std::unique_ptr< QgsVectorLayer > layer = qgis::make_unique< QgsVectorLayer >( QStringLiteral( "Point?field=id_a:integer" ), QStringLiteral( "A" ), QStringLiteral( "memory" ) );
+  QgsLayerMetadata metadata;
+  metadata.setRights( QStringList() << QStringLiteral( "CC BY SA" ) );
+  layer->setMetadata( metadata );
+  std::unique_ptr< QgsVectorLayer > layer2 = qgis::make_unique< QgsVectorLayer >( QStringLiteral( "Point?field=id_a:integer" ), QStringLiteral( "B" ), QStringLiteral( "memory" ) );
+  metadata.setRights( QStringList() << QStringLiteral( "CC NC" ) );
+  layer2->setMetadata( metadata );
+  std::unique_ptr< QgsVectorLayer > layer3 = qgis::make_unique< QgsVectorLayer >( QStringLiteral( "Point?field=id_a:integer" ), QStringLiteral( "C" ), QStringLiteral( "memory" ) );
+  metadata.setRights( QStringList() << QStringLiteral( "CC BY SA" ) );
+  layer3->setMetadata( metadata );
+  std::unique_ptr< QgsVectorLayer > layer4 = qgis::make_unique< QgsVectorLayer >( QStringLiteral( "Point?field=id_a:integer" ), QStringLiteral( "C" ), QStringLiteral( "memory" ) );
+
+  map->setLayers( QList<QgsMapLayer *>() << layer.get() << layer2.get() << layer3.get()  << layer4.get() );
+  e.prepare( &c );
+  QCOMPARE( e.evaluate( &c ).toString(), QStringLiteral( "CC BY SA,CC NC" ) );
+  map->setLayers( QList<QgsMapLayer *>() << layer.get() << layer3.get()  << layer4.get() );
+  e.prepare( &c );
+  QCOMPARE( e.evaluate( &c ).toString(), QStringLiteral( "CC BY SA" ) );
+
+  QgsExpression e2( QStringLiteral( "array_to_string( map_credits( 'Map_id', include_layer_names:=true ) )" ) );
+  e2.prepare( &c );
+  QCOMPARE( e2.evaluate( &c ).toString(), QStringLiteral( "A: CC BY SA,C: CC BY SA" ) );
+  map->setLayers( QList<QgsMapLayer *>() << layer.get() << layer2.get() << layer3.get()  << layer4.get() );
+  QgsExpression e3( QStringLiteral( "array_to_string( map_credits( 'Map_id', include_layer_names:=true, layer_name_separator:='|' ) )" ) );
+  e3.prepare( &c );
+  QCOMPARE( e3.evaluate( &c ).toString(), QStringLiteral( "A|CC BY SA,B|CC NC,C|CC BY SA" ) );
+
+  // second map
+  QgsLayoutItemMap *map2 = new QgsLayoutItemMap( &l );
+  map2->setCrs( QgsCoordinateReferenceSystem( QStringLiteral( "EPSG:4326" ) ) );
+  map2->attemptSetSceneRect( QRectF( 30, 60, 200, 100 ) );
+  map2->setExtent( extent );
+  l.addLayoutItem( map2 );
+  map2->setId( QStringLiteral( "Map_2" ) );
+  map2->setLayers( QList<QgsMapLayer *>() << layer.get()  << layer4.get() );
+  QgsExpression e4( QStringLiteral( "array_to_string( map_credits( 'Map_2', include_layer_names:=true ) )" ) );
+  e4.prepare( &c );
+  QCOMPARE( e4.evaluate( &c ).toString(), QStringLiteral( "A: CC BY SA" ) );
+}
+
 void TestQgsLayoutItem::rotation()
 {
   QgsProject proj;