[FEATURE] item_variables expression function inside compositions

This adds a new item_variables expression function when expressions
are used inside a composition context.

The function takes a single argument, the id for an item inside
the composition, and returns a map of variable name to value
for that item.

This allows you to do things like insert text in a label fetching
properties of another item in the composition, eg

Insert scale of map into a label:

map_get( item_variables( 'map'),'map_scale')

Insert x coordinate of map center into a label:

x(map_get( item_variables( 'map'),'map_extent_center'))

python/core/qgsexpressioncontext.sip

@@ -22,6 +22,21 @@ class QgsScopedExpressionFunction : QgsExpression::Function
                                  bool handlesNull = false,
                                  bool isContextual = true );
 
+    /**
+     * Create a new QgsScopedExpressionFunction using named parameters.
+     *
+     * @note Added in QGIS 3.0
+     */
+    QgsScopedExpressionFunction( const QString& fnname,
+                                 const QgsExpression::ParameterList& params,
+                                 const QString& group,
+                                 const QString& helpText = QString(),
+                                 bool usesGeometry = false,
+                                 const QSet<QString>& referencedColumns = QSet<QString>(),
+                                 bool lazyEval = false,
+                                 bool handlesNull = false,
+                                 bool isContextual = true );
+
     virtual ~QgsScopedExpressionFunction();
 
     virtual QVariant func( const QVariantList& values, const QgsExpressionContext* context, QgsExpression* parent ) = 0;

resources/function_help/json/item_variables

@@ -0,0 +1,8 @@
+{
+  "name": "item_variables",
+  "type": "function",
+  "description": "Returns a map of variables from a composer item inside this composition.",
+  "arguments": [ {"arg":"id","description":"composer item ID"}],
+  "examples": [ { "expression":"map_get(item_variables('main_map'), 'map_scale')", "returns":"2000"}
+  ]
+}

src/core/qgsexpressioncontext.cpp

@@ -609,6 +609,41 @@ class GetNamedProjectColor : public QgsScopedExpressionFunction
 
 };
 
+class GetComposerItemVariables : public QgsScopedExpressionFunction
+{
+  public:
+    GetComposerItemVariables( const QgsComposition* c )
+        : QgsScopedExpressionFunction( QStringLiteral( "item_variables" ), QgsExpression::ParameterList() << QgsExpression::Parameter( QStringLiteral( "id" ) ), QStringLiteral( "Composition" ) )
+        , mComposition( c )
+    {}
+
+    virtual QVariant func( const QVariantList& values, const QgsExpressionContext*, QgsExpression* ) override
+    {
+      if ( !mComposition )
+        return QVariant();
+
+      QString id = values.at( 0 ).toString().toLower();
+
+      const QgsComposerItem* item = mComposition->getComposerItemById( id );
+      if ( !item )
+        return QVariant();
+
+      QgsExpressionContext c = item->createExpressionContext();
+
+      return c.variablesToMap();
+    }
+
+    QgsScopedExpressionFunction* clone() const override
+    {
+      return new GetComposerItemVariables( mComposition );
+    }
+
+  private:
+
+    const QgsComposition* mComposition;
+
+};
+
 ///@endcond
 
 QgsExpressionContextScope* QgsExpressionContextUtils::projectScope( const QgsProject* project )
@@ -818,6 +853,9 @@ QgsExpressionContextScope *QgsExpressionContextUtils::compositionScope( const Qg
   scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "layout_pagewidth" ), composition->paperWidth(), true ) );
   scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "layout_dpi" ), composition->printResolution(), true ) );
 
+
+  scope->addFunction( QStringLiteral( "item_variables" ), new GetComposerItemVariables( composition ) );
+
   return scope;
 }
 
@@ -970,6 +1008,7 @@ QgsExpressionContext QgsExpressionContextUtils::createFeatureBasedContext( const
 void QgsExpressionContextUtils::registerContextFunctions()
 {
   QgsExpression::registerFunction( new GetNamedProjectColor( nullptr ) );
+  QgsExpression::registerFunction( new GetComposerItemVariables( nullptr ) );
 }
 
 bool QgsScopedExpressionFunction::usesGeometry( const QgsExpression::NodeFunction* node ) const

src/core/qgsexpressioncontext.h

@@ -63,6 +63,25 @@ class CORE_EXPORT QgsScopedExpressionFunction : public QgsExpression::Function
         , mReferencedColumns( referencedColumns )
     {}
 
+    /**
+     * Create a new QgsScopedExpressionFunction using named parameters.
+     *
+     * @note Added in QGIS 3.0
+     */
+    QgsScopedExpressionFunction( const QString& fnname,
+                                 const QgsExpression::ParameterList& params,
+                                 const QString& group,
+                                 const QString& helpText = QString(),
+                                 bool usesGeometry = false,
+                                 const QSet<QString>& referencedColumns = QSet<QString>(),
+                                 bool lazyEval = false,
+                                 bool handlesNull = false,
+                                 bool isContextual = true )
+        : QgsExpression::Function( fnname, params, group, helpText, lazyEval, handlesNull, isContextual )
+        , mUsesGeometry( usesGeometry )
+        , mReferencedColumns( referencedColumns )
+    {}
+
     virtual QVariant func( const QVariantList& values, const QgsExpressionContext* context, QgsExpression* parent ) override = 0;
 
     /** Returns a clone of the function.

tests/src/core/testqgscomposition.cpp

@@ -30,6 +30,7 @@
 
 #include <QObject>
 #include "qgstest.h"
+#include "qgstestutils.h"
 
 class TestQgsComposition : public QObject
 {
@@ -55,6 +56,7 @@ class TestQgsComposition : public QObject
     void resizeToContentsMultiPage();
     void georeference();
     void variablesEdited();
+    void itemVariablesFunction();
 
   private:
     QgsComposition *mComposition;
@@ -599,5 +601,31 @@ void TestQgsComposition::variablesEdited()
   QVERIFY( spyVariablesChanged.count() == 2 );
 }
 
+void TestQgsComposition::itemVariablesFunction()
+{
+  QgsRectangle extent( 2000, 2800, 2500, 2900 );
+  QgsMapSettings ms;
+  ms.setExtent( extent );
+  QgsComposition* composition = new QgsComposition( ms, QgsProject::instance() );
+
+  QgsExpression e( "map_get( item_variables( 'map_id' ), 'map_scale' )" );
+  // no map
+  QgsExpressionContext c = composition->createExpressionContext();
+  QVariant r = e.evaluate( &c );
+  QVERIFY( !r.isValid() );
+
+  QgsComposerMap* map = new QgsComposerMap( composition );
+  map->setNewExtent( extent );
+  map->setSceneRect( QRectF( 30, 60, 200, 100 ) );
+  composition->addComposerMap( map );
+  map->setId( "map_id" );
+
+  c = composition->createExpressionContext();
+  r = e.evaluate( &c );
+  QGSCOMPARENEAR( r.toDouble(), 1.38916e+08, 100 );
+
+  delete composition;
+}
+
 QGSTEST_MAIN( TestQgsComposition )
 #include "testqgscomposition.moc"