qgsprojectelevationsettingswidget: Check elevation range values

python/PyQt6/gui/auto_generated/editorwidgets/qgsdoublespinbox.sip.in

@@ -113,6 +113,16 @@ Defines if the clear value should be the minimum or maximum values of the widget
 Returns the value used when :py:func:`~QgsDoubleSpinBox.clear` is called.
 
 .. seealso:: :py:func:`setClearValue`
+%End
+
+    bool isCleared() const;
+%Docstring
+
+:return: ``True`` if the value is equal to the clear value.
+
+.. seealso:: :py:func:`clearValue`
+
+.. versionadded:: 3.42
 %End
 
     void setLineEditAlignment( Qt::Alignment alignment );

python/gui/auto_generated/editorwidgets/qgsdoublespinbox.sip.in

@@ -113,6 +113,16 @@ Defines if the clear value should be the minimum or maximum values of the widget
 Returns the value used when :py:func:`~QgsDoubleSpinBox.clear` is called.
 
 .. seealso:: :py:func:`setClearValue`
+%End
+
+    bool isCleared() const;
+%Docstring
+
+:return: ``True`` if the value is equal to the clear value.
+
+.. seealso:: :py:func:`clearValue`
+
+.. versionadded:: 3.42
 %End
 
     void setLineEditAlignment( Qt::Alignment alignment );

src/app/project/qgsprojectelevationsettingswidget.cpp

@@ -113,6 +113,9 @@ QgsProjectElevationSettingsWidget::QgsProjectElevationSettingsWidget( QWidget *p
   else
     whileBlocking( mElevationUpperSpin )->clear();
 
+  connect( mElevationLowerSpin, qOverload<double>( &QgsDoubleSpinBox::valueChanged ), this, &QgsProjectElevationSettingsWidget::validate );
+  connect( mElevationUpperSpin, qOverload<double>( &QgsDoubleSpinBox::valueChanged ), this, &QgsProjectElevationSettingsWidget::validate );
+
   updateVerticalCrsOptions();
   connect( QgsProject::instance(), &QgsProject::crsChanged, this, &QgsProjectElevationSettingsWidget::updateVerticalCrsOptions );
 
@@ -241,6 +244,15 @@ bool QgsProjectElevationSettingsWidget::validate()
       mMessageBar->pushMessage( tr( "An elevation layer must be selected for a mesh terrain" ), Qgis::MessageLevel::Critical );
     }
   }
+
+  // Show an error message if the lower value is greater than the upper one
+  // However, do not show the error message if one of the values is not set
+  if ( !mElevationLowerSpin->isCleared() && !mElevationUpperSpin->isCleared() && ( mElevationLowerSpin->value() >= mElevationUpperSpin->value() ) )
+  {
+    valid = false;
+    mMessageBar->pushMessage( tr( "Upper elevation range must be greater than the lower one" ), Qgis::MessageLevel::Critical );
+  }
+
   return valid;
 }
 

src/gui/editorwidgets/qgsdoublespinbox.cpp

@@ -282,3 +282,8 @@ bool QgsDoubleSpinBox::shouldShowClearForValue( const double value ) const
   }
   return value != clearValue();
 }
+
+bool QgsDoubleSpinBox::isCleared() const
+{
+  return value() == clearValue();
+}

src/gui/editorwidgets/qgsdoublespinbox.h

@@ -124,6 +124,14 @@ class GUI_EXPORT QgsDoubleSpinBox : public QDoubleSpinBox
      */
     double clearValue() const;
 
+    /**
+     * \returns TRUE if the value is equal to the clear value.
+     * \see clearValue()
+     *
+     * \since QGIS 3.42
+     */
+    bool isCleared() const;
+
     /**
      * Set alignment in the embedded line edit widget
      * \param alignment

tests/src/gui/testqgsdoublespinbox.cpp

@@ -59,15 +59,21 @@ void TestQgsDoubleSpinBox::clear()
   spinBox->setMinimum( 1.0 );
   spinBox->setValue( 5.0 );
   spinBox->setClearValueMode( QgsDoubleSpinBox::MinimumValue );
+  QVERIFY( !spinBox->isCleared() );
   spinBox->clear();
+  QVERIFY( spinBox->isCleared() );
   QCOMPARE( spinBox->value(), 1.0 );
   QCOMPARE( spinBox->clearValue(), 1.0 );
   spinBox->setClearValueMode( QgsDoubleSpinBox::MaximumValue );
+  QVERIFY( !spinBox->isCleared() );
   spinBox->clear();
+  QVERIFY( spinBox->isCleared() );
   QCOMPARE( spinBox->value(), 10.0 );
   QCOMPARE( spinBox->clearValue(), 10.0 );
   spinBox->setClearValue( 7.0 );
+  QVERIFY( !spinBox->isCleared() );
   spinBox->clear();
+  QVERIFY( spinBox->isCleared() );
   QCOMPARE( spinBox->value(), 7.0 );
   QCOMPARE( spinBox->clearValue(), 7.0 );
   delete spinBox;