Allow setting project for QgsExpressionBuilderWidget

(still defaults to QgsProject::instance())

python/gui/qgsexpressionbuilderwidget.sip

@@ -244,6 +244,22 @@ Sets the expression string for the widget
  :rtype: QStandardItemModel
 %End
 
+    QgsProject *project();
+%Docstring
+ Returns the project currently associated with the widget.
+.. seealso:: setProject()
+.. versionadded:: 3.0
+ :rtype: QgsProject
+%End
+
+    void setProject( QgsProject *project );
+%Docstring
+ Sets the ``project`` currently associated with the widget. This
+ controls which layers and relations and other project-specific items are shown in the widget.
+.. seealso:: project()
+.. versionadded:: 3.0
+%End
+
   public slots:
 
     void loadSampleValues();

src/gui/qgsexpressionbuilderwidget.cpp

@@ -45,6 +45,7 @@ QgsExpressionBuilderWidget::QgsExpressionBuilderWidget( QWidget *parent )
   , mLayer( nullptr )
   , highlighter( nullptr )
   , mExpressionValid( false )
+  , mProject( QgsProject::instance() )
 {
   setupUi( this );
 
@@ -445,7 +446,10 @@ void QgsExpressionBuilderWidget::loadRecent( const QString &collection )
 
 void QgsExpressionBuilderWidget::loadLayers()
 {
-  QMap<QString, QgsMapLayer *> layers = QgsProject::instance()->mapLayers();
+  if ( !mProject )
+    return;
+
+  QMap<QString, QgsMapLayer *> layers = mProject->mapLayers();
   QMap<QString, QgsMapLayer *>::const_iterator layerIt = layers.constBegin();
   for ( ; layerIt != layers.constEnd(); ++layerIt )
   {
@@ -455,7 +459,10 @@ void QgsExpressionBuilderWidget::loadLayers()
 
 void QgsExpressionBuilderWidget::loadRelations()
 {
-  QMap<QString, QgsRelation> relations = QgsProject::instance()->relationManager()->relations();
+  if ( !mProject )
+    return;
+
+  QMap<QString, QgsRelation> relations = mProject->relationManager()->relations();
   QMap<QString, QgsRelation>::const_iterator relIt = relations.constBegin();
   for ( ; relIt != relations.constEnd(); ++relIt )
   {
@@ -662,6 +669,17 @@ QStandardItemModel *QgsExpressionBuilderWidget::model()
   return mModel;
 }
 
+QgsProject *QgsExpressionBuilderWidget::project()
+{
+  return mProject;
+}
+
+void QgsExpressionBuilderWidget::setProject( QgsProject *project )
+{
+  mProject = project;
+  updateFunctionTree();
+}
+
 void QgsExpressionBuilderWidget::showEvent( QShowEvent *e )
 {
   QWidget::showEvent( e );

src/gui/qgsexpressionbuilderwidget.h

@@ -233,6 +233,21 @@ class GUI_EXPORT QgsExpressionBuilderWidget : public QWidget, private Ui::QgsExp
      */
     QStandardItemModel *model();
 
+    /**
+     * Returns the project currently associated with the widget.
+     * \see setProject()
+     * \since QGIS 3.0
+     */
+    QgsProject *project();
+
+    /**
+     * Sets the \a project currently associated with the widget. This
+     * controls which layers and relations and other project-specific items are shown in the widget.
+     * \see project()
+     * \since QGIS 3.0
+     */
+    void setProject( QgsProject *project );
+
   public slots:
 
     /**
@@ -338,6 +353,7 @@ class GUI_EXPORT QgsExpressionBuilderWidget : public QWidget, private Ui::QgsExp
     QString mRecentKey;
     QMap<QString, QStringList> mFieldValues;
     QgsExpressionContext mExpressionContext;
+    QPointer< QgsProject > mProject;
 };
 
 #endif // QGSEXPRESSIONBUILDER_H

tests/src/python/test_qgsexpressionbuilderwidget.py

@@ -122,6 +122,19 @@ class TestQgsExpressionBuilderWidget(unittest.TestCase):
         items = m.findItems('layer2', Qt.MatchRecursive)
         self.assertEqual(len(items), 1)
 
+        # change project
+        p2 = QgsProject()
+        layer3 = QgsVectorLayer("Point", "layer3", "memory")
+        p2.addMapLayers([layer3])
+        w.setProject(p2)
+        m = w.model()
+        items = m.findItems('layer1', Qt.MatchRecursive)
+        self.assertEqual(len(items), 0)
+        items = m.findItems('layer2', Qt.MatchRecursive)
+        self.assertEqual(len(items), 0)
+        items = m.findItems('layer3', Qt.MatchRecursive)
+        self.assertEqual(len(items), 1)
+
     def testRelations(self):
         """ check that layers are shown in widget model"""
         p = QgsProject.instance()