Add API to allow QgsScaleWidget to accept NULL values

python/gui/auto_generated/qgsscalewidget.sip.in

@@ -74,6 +74,17 @@ Returns the selected scale as a double.
 The scale value indicates the scale denominator, e.g. 1000.0 for a 1:1000 map.
 
 .. seealso:: :py:func:`setScale`
+%End
+
+    bool isNull() const;
+%Docstring
+Returns true if the widget is currently set to a "null" value.
+
+.. seealso:: :py:func:`setAllowNull`
+
+.. seealso:: :py:func:`clear`
+
+.. versionadded:: 3.8
 %End
 
     double minScale() const;
@@ -101,6 +112,32 @@ The returned value indicates the scale denominator, e.g. 1000.0 for a 1:1000 map
 If specified, ``ok`` will be set to ``True`` if the string was successfully interpreted as a scale.
 
 .. seealso:: :py:func:`toString`
+%End
+
+    void setAllowNull( bool allowNull );
+%Docstring
+Sets whether the scale widget can be set to a NULL value.
+
+.. seealso:: :py:func:`allowNull`
+
+.. seealso:: :py:func:`isNull`
+
+.. seealso:: :py:func:`clear`
+
+.. versionadded:: 3.8
+%End
+
+    bool allowNull() const;
+%Docstring
+Returns ``True`` if the widget can be set to a NULL value.
+
+.. seealso:: :py:func:`setAllowNull`
+
+.. seealso:: :py:func:`isNull`
+
+.. seealso:: :py:func:`clear`
+
+.. versionadded:: 3.8
 %End
 
   public slots:
@@ -132,6 +169,19 @@ Set the minimum allowed ``scale``. Set to 0 to disable the minimum scale.
 The ``scale`` value indicates the scale denominator, e.g. 1000.0 for a 1:1000 map.
 Any scale lower than the minimum scale will automatically be converted to the minimum scale.
 Except for 0 which is always allowed.
+%End
+
+    void clear();
+%Docstring
+Sets the widget to the null value.
+
+This only has an effect if allowNull() is true.
+
+.. seealso:: :py:func:`allowNull`
+
+.. seealso:: :py:func:`isNull`
+
+.. versionadded:: 3.8
 %End
 
   signals:

src/gui/qgsscalewidget.cpp

@@ -51,6 +51,21 @@ void QgsScaleWidget::setMapCanvas( QgsMapCanvas *canvas )
   mCurrentScaleButton->setVisible( mShowCurrentScaleButton && mCanvas );
 }
 
+bool QgsScaleWidget::isNull() const
+{
+  return mScaleComboBox->isNull();
+}
+
+void QgsScaleWidget::setAllowNull( bool allowNull )
+{
+  mScaleComboBox->setAllowNull( allowNull );
+}
+
+bool QgsScaleWidget::allowNull() const
+{
+  return mScaleComboBox->allowNull();
+}
+
 void QgsScaleWidget::setScaleFromCanvas()
 {
   if ( !mCanvas )
@@ -59,6 +74,11 @@ void QgsScaleWidget::setScaleFromCanvas()
   setScale( mCanvas->scale() );
 }
 
+void QgsScaleWidget::clear()
+{
+  mScaleComboBox->clear();
+}
+
 void QgsScaleWidget::setScale( double scale )
 {
   mScaleComboBox->setScale( scale );

src/gui/qgsscalewidget.h

@@ -84,6 +84,15 @@ class GUI_EXPORT QgsScaleWidget : public QWidget
      */
     double scale() const { return mScaleComboBox->scale(); }
 
+    /**
+     * Returns true if the widget is currently set to a "null" value.
+     *
+     * \see setAllowNull()
+     * \see clear()
+     * \since QGIS 3.8
+     */
+    bool isNull() const;
+
     /**
      * Returns the minimum scale, or 0 if no minimum scale set.
      * The \a scale value indicates the scale denominator, e.g. 1000.0 for a 1:1000 map.
@@ -109,6 +118,24 @@ class GUI_EXPORT QgsScaleWidget : public QWidget
      */
     static double toDouble( const QString &scaleString, bool *ok = nullptr ) { return QgsScaleComboBox::toDouble( scaleString, ok ); }
 
+    /**
+     * Sets whether the scale widget can be set to a NULL value.
+     * \see allowNull()
+     * \see isNull()
+     * \see clear()
+     * \since QGIS 3.8
+     */
+    void setAllowNull( bool allowNull );
+
+    /**
+     * Returns TRUE if the widget can be set to a NULL value.
+     * \see setAllowNull()
+     * \see isNull()
+     * \see clear()
+     * \since QGIS 3.8
+     */
+    bool allowNull() const;
+
   public slots:
 
     /**
@@ -138,6 +165,17 @@ class GUI_EXPORT QgsScaleWidget : public QWidget
      */
     void setMinScale( double scale ) { mScaleComboBox->setMinScale( scale ); }
 
+    /**
+     * Sets the widget to the null value.
+     *
+     * This only has an effect if allowNull() is true.
+     *
+     * \see allowNull()
+     * \see isNull()
+     * \since QGIS 3.8
+     */
+    void clear();
+
   signals:
 
     /**

tests/src/python/CMakeLists.txt

@@ -178,6 +178,7 @@ ADD_PYTHON_TEST(PyQgsRenderContext test_qgsrendercontext.py)
 ADD_PYTHON_TEST(PyQgsRenderer test_qgsrenderer.py)
 ADD_PYTHON_TEST(PyQgsReport test_qgsreport.py)
 ADD_PYTHON_TEST(PyQgsRulebasedRenderer test_qgsrulebasedrenderer.py)
+ADD_PYTHON_TEST(PyQgsScaleWidget test_qgsscalewidget.py)
 ADD_PYTHON_TEST(PyQgsSingleSymbolRenderer test_qgssinglesymbolrenderer.py)
 ADD_PYTHON_TEST(PyQgsShapefileProvider test_provider_shapefile.py)
 ADD_PYTHON_TEST(PyQgsSvgCache test_qgssvgcache.py)

tests/src/python/test_qgsscalewidget.py

@@ -0,0 +1,99 @@
+# -*- coding: utf-8 -*-
+"""QGIS Unit tests for QgsScaleWidget
+
+.. note:: This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2 of the License, or
+(at your option) any later version.
+"""
+__author__ = 'Nyall Dawson'
+__date__ = '13/03/2019'
+__copyright__ = 'Copyright 2019, The QGIS Project'
+# This will get replaced with a git SHA1 when you do a git archive
+__revision__ = '$Format:%H$'
+
+import qgis  # NOQA
+import math
+from qgis.PyQt.QtCore import Qt
+from qgis.PyQt.QtTest import QSignalSpy
+
+from qgis.gui import QgsScaleWidget
+from qgis.testing import start_app, unittest
+
+start_app()
+
+
+class TestQgsScaleWidget(unittest.TestCase):
+
+    def testBasic(self):
+        w = QgsScaleWidget()
+        spy = QSignalSpy(w.scaleChanged)
+        w.setScaleString('1:2345')
+        self.assertEqual(w.scaleString(), '1:2,345')
+        self.assertEqual(w.scale(), 2345)
+        self.assertEqual(len(spy), 1)
+        self.assertEqual(spy[-1][0], 2345)
+
+        w.setScaleString('0.02')
+        self.assertEqual(w.scaleString(), '1:50')
+        self.assertEqual(w.scale(), 50)
+        self.assertEqual(len(spy), 2)
+        self.assertEqual(spy[-1][0], 50)
+
+        w.setScaleString('1:4,000')
+        self.assertEqual(w.scaleString(), '1:4,000')
+        self.assertEqual(w.scale(), 4000)
+        self.assertEqual(len(spy), 3)
+        self.assertEqual(spy[-1][0], 4000)
+
+    def testNull(self):
+        w = QgsScaleWidget()
+
+        w.setScale(50)
+        self.assertFalse(w.allowNull())
+        w.clear() # no effect
+        self.assertEqual(w.scale(), 50.0)
+        self.assertFalse(w.isNull())
+
+        spy = QSignalSpy(w.scaleChanged)
+        w.setAllowNull(True)
+        self.assertTrue(w.allowNull())
+
+        w.setScaleString('')
+        self.assertEqual(len(spy), 1)
+        self.assertTrue(math.isnan(w.scale()))
+        self.assertTrue(math.isnan(spy[-1][0]))
+        self.assertTrue(w.isNull())
+        w.setScaleString("    ")
+        self.assertTrue(math.isnan(w.scale()))
+        self.assertTrue(w.isNull())
+
+        w.setScaleString('0.02')
+        self.assertEqual(w.scale(), 50.0)
+        self.assertEqual(len(spy), 2)
+        self.assertEqual(spy[-1][0], 50.0)
+        self.assertFalse(w.isNull())
+
+        w.setScaleString('')
+        self.assertTrue(math.isnan(w.scale()))
+        self.assertEqual(len(spy), 3)
+        self.assertTrue(math.isnan(spy[-1][0]))
+        self.assertTrue(w.isNull())
+
+        w.setScaleString('0.02')
+        self.assertEqual(w.scale(), 50.0)
+        self.assertEqual(len(spy), 4)
+        self.assertEqual(spy[-1][0], 50.0)
+        self.assertFalse(w.isNull())
+        w.clear()
+        self.assertTrue(math.isnan(w.scale()))
+        self.assertEqual(len(spy), 5)
+        self.assertTrue(math.isnan(spy[-1][0]))
+        self.assertTrue(w.isNull())
+
+        w.setAllowNull(False)
+        self.assertFalse(w.allowNull())
+
+
+if __name__ == '__main__':
+    unittest.main()