[processing] Allow parameter metadata to set the number of decimal places

to show in numeric/distance widgets

E.g. to only show 2 decimal places:

  # only show two decimal places in parameter's widgets, not 6:
  param.setMetadata( {'widget_wrapper':
    { 'decimals': 2 }
  })

python/core/auto_generated/processing/qgsprocessingparameters.sip.in

@@ -1364,6 +1364,17 @@ class QgsProcessingParameterNumber : QgsProcessingParameterDefinition
 %Docstring
 A numeric parameter for processing algorithms.
 
+For numeric parameters with a dataType() of Double, the number of decimals places
+shown in the parameter's widget can be specified by setting the parameter's metadata. For example:
+
+.. code-block:: python
+
+       param = QgsProcessingParameterNumber( 'VAL', 'Threshold', type=QgsProcessingParameter.Double)
+       # only show two decimal places in parameter's widgets, not 6:
+       param.setMetadata( {'widget_wrapper':
+         { 'decimals': 2 }
+       })
+
 .. versionadded:: 3.0
 %End
 
@@ -1463,6 +1474,17 @@ class QgsProcessingParameterDistance : QgsProcessingParameterNumber
 A double numeric parameter for distance values. Linked to a source layer or CRS parameter
 to determine what units the distance values are in.
 
+The number of decimals places shown in a distance parameter's widget can be specified by
+setting the parameter's metadata. For example:
+
+.. code-block:: python
+
+       param = QgsProcessingParameterDistance( 'VAL', 'Threshold')
+       # only show two decimal places in parameter's widgets, not 6:
+       param.setMetadata( {'widget_wrapper':
+         { 'decimals': 2 }
+       })
+
 .. versionadded:: 3.2
 %End
 

src/core/processing/qgsprocessingparameters.h

@@ -1359,7 +1359,19 @@ class CORE_EXPORT QgsProcessingParameterMultipleLayers : public QgsProcessingPar
  * \class QgsProcessingParameterNumber
  * \ingroup core
  * A numeric parameter for processing algorithms.
-  * \since QGIS 3.0
+ *
+ * For numeric parameters with a dataType() of Double, the number of decimals places
+ * shown in the parameter's widget can be specified by setting the parameter's metadata. For example:
+ *
+ * * \code{.py}
+ *   param = QgsProcessingParameterNumber( 'VAL', 'Threshold', type=QgsProcessingParameter.Double)
+ *   # only show two decimal places in parameter's widgets, not 6:
+ *   param.setMetadata( {'widget_wrapper':
+ *     { 'decimals': 2 }
+ *   })
+ * \endcode
+ *
+ * \since QGIS 3.0
  */
 class CORE_EXPORT QgsProcessingParameterNumber : public QgsProcessingParameterDefinition
 {
@@ -1449,7 +1461,19 @@ class CORE_EXPORT QgsProcessingParameterNumber : public QgsProcessingParameterDe
  * \ingroup core
  * A double numeric parameter for distance values. Linked to a source layer or CRS parameter
  * to determine what units the distance values are in.
-  * \since QGIS 3.2
+ *
+ * The number of decimals places shown in a distance parameter's widget can be specified by
+ * setting the parameter's metadata. For example:
+ *
+ * * \code{.py}
+ *   param = QgsProcessingParameterDistance( 'VAL', 'Threshold')
+ *   # only show two decimal places in parameter's widgets, not 6:
+ *   param.setMetadata( {'widget_wrapper':
+ *     { 'decimals': 2 }
+ *   })
+ * \endcode
+ *
+ * \since QGIS 3.2
  */
 class CORE_EXPORT QgsProcessingParameterDistance : public QgsProcessingParameterNumber
 {

src/gui/processing/qgsprocessingwidgetwrapperimpl.cpp

@@ -420,6 +420,8 @@ QgsProcessingNumericWidgetWrapper::QgsProcessingNumericWidgetWrapper( const QgsP
 QWidget *QgsProcessingNumericWidgetWrapper::createWidget()
 {
   const QgsProcessingParameterNumber *numberDef = static_cast< const QgsProcessingParameterNumber * >( parameterDefinition() );
+  const QVariantMap metadata = numberDef->metadata();
+  const int decimals = metadata.value( QStringLiteral( "widget_wrapper" ) ).toMap().value( QStringLiteral( "decimals" ), 6 ).toInt();
   switch ( type() )
   {
     case QgsProcessingGui::Standard:
@@ -433,13 +435,14 @@ QWidget *QgsProcessingNumericWidgetWrapper::createWidget()
         case QgsProcessingParameterNumber::Double:
           mDoubleSpinBox = new QgsDoubleSpinBox();
           mDoubleSpinBox->setExpressionsEnabled( true );
-          mDoubleSpinBox->setDecimals( 6 );
+          mDoubleSpinBox->setDecimals( decimals );
 
           // guess reasonable step value for double spin boxes
           if ( !qgsDoubleNear( numberDef->maximum(), std::numeric_limits<double>::max() ) &&
                !qgsDoubleNear( numberDef->minimum(), std::numeric_limits<double>::lowest() + 1 ) )
           {
-            const double singleStep = calculateStep( numberDef->minimum(), numberDef->maximum() );
+            double singleStep = calculateStep( numberDef->minimum(), numberDef->maximum() );
+            singleStep = std::max( singleStep, std::pow( 10, -decimals ) );
             mDoubleSpinBox->setSingleStep( singleStep );
           }
 

tests/src/gui/testprocessinggui.cpp

@@ -1005,6 +1005,19 @@ void TestProcessingGui::testNumericWrapperDouble()
     QCOMPARE( wrapperOptionalDefault.parameterValue().toDouble(), 5.0 );
 
     delete w;
+
+    // with decimals
+    QgsProcessingParameterNumber paramDecimals( QStringLiteral( "num" ), QStringLiteral( "num" ), QgsProcessingParameterNumber::Double, QVariant(), true, 1, 1.02 );
+    QVariantMap metadata;
+    QVariantMap wrapperMetadata;
+    wrapperMetadata.insert( QStringLiteral( "decimals" ), 2 );
+    metadata.insert( QStringLiteral( "widget_wrapper" ), wrapperMetadata );
+    paramDecimals.setMetadata( metadata );
+    QgsProcessingNumericWidgetWrapper wrapperDecimals( &paramDecimals, type );
+    w = wrapperDecimals.createWrappedWidget( context );
+    QCOMPARE( static_cast< QgsDoubleSpinBox * >( wrapperDecimals.wrappedWidget() )->decimals(), 2 );
+    QCOMPARE( static_cast< QgsDoubleSpinBox * >( wrapperDecimals.wrappedWidget() )->singleStep(), 0.01 ); // single step should never be less than set number of decimals
+    delete w;
   };
 
   // standard wrapper
@@ -1280,6 +1293,19 @@ void TestProcessingGui::testDistanceWrapper()
 
   delete w;
 
+  // with decimals
+  QgsProcessingParameterDistance paramDecimals( QStringLiteral( "num" ), QStringLiteral( "num" ), QVariant(), QString(), true, 1, 1.02 );
+  QVariantMap metadata;
+  QVariantMap wrapperMetadata;
+  wrapperMetadata.insert( QStringLiteral( "decimals" ), 2 );
+  metadata.insert( QStringLiteral( "widget_wrapper" ), wrapperMetadata );
+  paramDecimals.setMetadata( metadata );
+  QgsProcessingDistanceWidgetWrapper wrapperDecimals( &paramDecimals, QgsProcessingGui::Standard );
+  w = wrapperDecimals.createWrappedWidget( context );
+  QCOMPARE( wrapperDecimals.mDoubleSpinBox->decimals(), 2 );
+  QCOMPARE( wrapperDecimals.mDoubleSpinBox->singleStep(), 0.01 ); // single step should never be less than set number of decimals
+  delete w;
+
   // batch wrapper
   QgsProcessingDistanceWidgetWrapper wrapperB( &param, QgsProcessingGui::Batch );