Allow expression widget to be empty so expression could be removed (#9032)

* fixes #20516 : Allow Constraint widget in attribute type form to be empty so expression could be removed

per widget modification

* correct tests

* Add the clear button on field expression widget when widget is allowed to be empty

* Change API since version to 3.6

python/gui/auto_generated/qgsfieldexpressionwidget.sip.in

@@ -55,6 +55,24 @@ Returns the title used for the expression dialog
     void setFilters( QgsFieldProxyModel::Filters filters );
 %Docstring
 setFilters allows fitering according to the type of field
+%End
+
+    void setAllowEmptyFieldName( bool allowEmpty );
+%Docstring
+Sets whether an optional empty field ("not set") option is shown in the combo box.
+
+.. seealso:: :py:func:`allowEmptyFieldName`
+
+.. versionadded:: 3.6
+%End
+
+    bool allowEmptyFieldName() const;
+%Docstring
+Returns true if the combo box allows the empty field ("not set") choice.
+
+.. seealso:: :py:func:`setAllowEmptyFieldName`
+
+.. versionadded:: 3.6
 %End
 
     void setLeftHandButtonStyle( bool isLeft );

src/app/qgsattributetypedialog.cpp

@@ -91,6 +91,7 @@ QgsAttributeTypeDialog::QgsAttributeTypeDialog( QgsVectorLayer *vl, int fieldIdx
   QgsSettings settings;
   restoreGeometry( settings.value( QStringLiteral( "Windows/QgsAttributeTypeDialog/geometry" ) ).toByteArray() );
 
+  constraintExpressionWidget->setAllowEmptyFieldName( true );
   constraintExpressionWidget->setLayer( vl );
 }
 

src/gui/qgsfieldexpressionwidget.cpp

@@ -83,6 +83,17 @@ void QgsFieldExpressionWidget::setFilters( QgsFieldProxyModel::Filters filters )
   mFieldProxyModel->setFilters( filters );
 }
 
+void QgsFieldExpressionWidget::setAllowEmptyFieldName( bool allowEmpty )
+{
+  mCombo->lineEdit()->setClearButtonEnabled( allowEmpty );
+  mFieldProxyModel->sourceFieldModel()->setAllowEmptyFieldName( allowEmpty );
+}
+
+bool QgsFieldExpressionWidget::allowEmptyFieldName() const
+{
+  return mFieldProxyModel->sourceFieldModel()->allowEmptyFieldName();
+}
+
 void QgsFieldExpressionWidget::setLeftHandButtonStyle( bool isLeft )
 {
   QHBoxLayout *layout = dynamic_cast<QHBoxLayout *>( this->layout() );

src/gui/qgsfieldexpressionwidget.h

@@ -48,6 +48,7 @@ class GUI_EXPORT QgsFieldExpressionWidget : public QWidget
     Q_OBJECT
     Q_PROPERTY( QString expressionDialogTitle READ expressionDialogTitle WRITE setExpressionDialogTitle )
     Q_PROPERTY( QgsFieldProxyModel::Filters filters READ filters WRITE setFilters )
+    Q_PROPERTY( bool allowEmptyFieldName READ allowEmptyFieldName WRITE setAllowEmptyFieldName )
     Q_PROPERTY( bool allowEvalErrors READ allowEvalErrors WRITE setAllowEvalErrors NOTIFY allowEvalErrorsChanged )
 
   public:
@@ -75,6 +76,20 @@ class GUI_EXPORT QgsFieldExpressionWidget : public QWidget
     //! setFilters allows fitering according to the type of field
     void setFilters( QgsFieldProxyModel::Filters filters );
 
+    /**
+     * Sets whether an optional empty field ("not set") option is shown in the combo box.
+     * \see allowEmptyFieldName()
+     * \since QGIS 3.6
+     */
+    void setAllowEmptyFieldName( bool allowEmpty );
+
+    /**
+     * Returns true if the combo box allows the empty field ("not set") choice.
+     * \see setAllowEmptyFieldName()
+     * \since QGIS 3.6
+     */
+    bool allowEmptyFieldName() const;
+
     void setLeftHandButtonStyle( bool isLeft );
 
     //! currently used filter on list of fields

tests/src/gui/testqgsfieldexpressionwidget.cpp

@@ -332,5 +332,3 @@ void TestQgsFieldExpressionWidget::setNull()
 
 QGSTEST_MAIN( TestQgsFieldExpressionWidget )
 #include "testqgsfieldexpressionwidget.moc"
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