Add more context to the legend filter expressions in layouts

Fix #53229

python/gui/auto_generated/qgslegendfilterbutton.sip.in

@@ -39,6 +39,14 @@ Returns the current text used as filter expression
 %Docstring
 Sets the current text used as filter expression.
 This will update the menu
+%End
+
+    void registerExpressionContextGenerator( QgsExpressionContextGenerator *generator );
+%Docstring
+Register an expression context generator class that will be used to retrieve
+an expression context for the button when required.
+
+.. versionadded:: 3.32
 %End
 
     QgsVectorLayer *vectorLayer() const;

src/gui/layout/qgslayoutlegendwidget.cpp

@@ -203,6 +203,7 @@ QgsLayoutLegendWidget::QgsLayoutLegendWidget( QgsLayoutItemLegend *legend, QgsMa
   mGroupFontButton->registerExpressionContextGenerator( this );
   mLayerFontButton->registerExpressionContextGenerator( this );
   mItemFontButton->registerExpressionContextGenerator( this );
+  mExpressionFilterButton->registerExpressionContextGenerator( this );
 
   mTitleFontButton->setLayer( coverageLayer() );
   mGroupFontButton->setLayer( coverageLayer() );

src/gui/qgslegendfilterbutton.cpp

@@ -20,6 +20,7 @@
 
 #include "qgsapplication.h"
 #include "qgsexpressionbuilderdialog.h"
+#include "qgsexpressioncontextutils.h"
 
 QgsLegendFilterButton::QgsLegendFilterButton( QWidget *parent )
   : QToolButton( parent )
@@ -58,7 +59,14 @@ void QgsLegendFilterButton::onToggle( bool checked )
 
 void QgsLegendFilterButton::onSetLegendFilterExpression()
 {
-  QgsExpressionBuilderDialog dlg( mLayer, mExpression );
+  QgsExpressionContext context;
+  if ( mExpressionContextGenerator )
+    context  = mExpressionContextGenerator->createExpressionContext();
+  else
+  {
+    context.appendScopes( QgsExpressionContextUtils::globalProjectLayerScopes( mLayer ) );
+  }
+  QgsExpressionBuilderDialog dlg( mLayer, mExpression, nullptr, QStringLiteral( "generic" ), context );
   if ( dlg.exec() )
   {
     setExpressionText( dlg.expressionText() );
@@ -79,6 +87,11 @@ void QgsLegendFilterButton::onSetLegendFilterExpression()
   }
 }
 
+void QgsLegendFilterButton::registerExpressionContextGenerator( QgsExpressionContextGenerator *generator )
+{
+  mExpressionContextGenerator = generator;
+}
+
 void QgsLegendFilterButton::onClearFilterExpression()
 {
   mClearExpressionAction->setEnabled( false );

src/gui/qgslegendfilterbutton.h

@@ -17,8 +17,10 @@
 
 #include <QToolButton>
 #include "qgis_gui.h"
+#include "qgsexpressioncontext.h"
 
 class QgsVectorLayer;
+class QgsExpressionContextGenerator;
 
 /**
  * \ingroup gui
@@ -52,6 +54,13 @@ class GUI_EXPORT QgsLegendFilterButton: public QToolButton
      */
     void setExpressionText( const QString &expression );
 
+    /**
+     * Register an expression context generator class that will be used to retrieve
+     * an expression context for the button when required.
+     * \since QGIS 3.32
+     */
+    void registerExpressionContextGenerator( QgsExpressionContextGenerator *generator );
+
     /**
      * Returns the current associated vectorLayer
      * May be NULLPTR
@@ -76,6 +85,7 @@ class GUI_EXPORT QgsLegendFilterButton: public QToolButton
     QAction *mSetExpressionAction = nullptr;
     QAction *mClearExpressionAction = nullptr;
     QString mExpression;
+    QgsExpressionContextGenerator *mExpressionContextGenerator = nullptr;
 
     void updateMenu();