[needs-docs] Tweak custom dash button appearance

- remove "Change" label and replace with larger dash preview icon.
The "change" text is unnecessary and adds to dialog clutter, better
to use the space for a wider preview icon (especially given that
the previous narrow icon never really showed enough of the pattern
to be useful!)

- don't offset the line in the preview if the symbol has an offset
set

- respond correctly to dash pattern, line width unit changes, cap
style changes

- show a nice big preview tooltip on hover

python/gui/auto_generated/symbology/qgssymbollayerwidget.sip.in

@@ -125,6 +125,9 @@ Creates a new QgsSimpleLineSymbolLayerWidget.
 
     void updatePatternIcon();
 
+    virtual void resizeEvent( QResizeEvent *event );
+
+
 };
 
 

src/gui/symbology/qgssymbollayerwidget.cpp

@@ -298,6 +298,11 @@ void QgsSimpleLineSymbolLayerWidget::setSymbolLayer( QgsSymbolLayer *layer )
   mCustomCheckBox->setCheckState( useCustomDashPattern ? Qt::Checked : Qt::Unchecked );
   mCustomCheckBox->blockSignals( false );
 
+  //make sure height of custom dash button looks good under different platforms
+  QSize size = mChangePatternButton->minimumSizeHint();
+  int fontHeight = static_cast< int >( Qgis::UI_SCALE_FACTOR * fontMetrics().height() * 1.4 );
+  mChangePatternButton->setMinimumSize( QSize( size.width(), std::max( size.height(), fontHeight ) ) );
+
   //draw inside polygon?
   const bool drawInsidePolygon = mLayer->drawInsidePolygon();
   whileBlocking( mDrawInsideCheckBox )->setCheckState( drawInsidePolygon ? Qt::Checked : Qt::Unchecked );
@@ -332,7 +337,6 @@ void QgsSimpleLineSymbolLayerWidget::penWidthChanged()
 void QgsSimpleLineSymbolLayerWidget::colorChanged( const QColor &color )
 {
   mLayer->setColor( color );
-  updatePatternIcon();
   emit changed();
 }
 
@@ -341,6 +345,7 @@ void QgsSimpleLineSymbolLayerWidget::penStyleChanged()
   mLayer->setPenStyle( cboPenStyle->penStyle() );
   mLayer->setPenJoinStyle( cboJoinStyle->penJoinStyle() );
   mLayer->setPenCapStyle( cboCapStyle->penCapStyle() );
+  updatePatternIcon();
   emit changed();
 }
 
@@ -396,6 +401,7 @@ void QgsSimpleLineSymbolLayerWidget::mPenWidthUnitWidget_changed()
   {
     mLayer->setWidthUnit( mPenWidthUnitWidget->unit() );
     mLayer->setWidthMapUnitScale( mPenWidthUnitWidget->getMapUnitScale() );
+    updatePatternIcon();
     emit changed();
   }
 }
@@ -416,6 +422,7 @@ void QgsSimpleLineSymbolLayerWidget::mDashPatternUnitWidget_changed()
   {
     mLayer->setCustomDashPatternUnit( mDashPatternUnitWidget->unit() );
     mLayer->setCustomDashPatternMapUnitScale( mDashPatternUnitWidget->getMapUnitScale() );
+    updatePatternIcon();
     emit changed();
   }
 }
@@ -434,17 +441,53 @@ void QgsSimpleLineSymbolLayerWidget::updatePatternIcon()
   {
     return;
   }
-  QgsSimpleLineSymbolLayer *layerCopy = mLayer->clone();
+  std::unique_ptr< QgsSimpleLineSymbolLayer > layerCopy( mLayer->clone() );
   if ( !layerCopy )
   {
     return;
   }
   QColor color = qApp->palette().color( QPalette::WindowText );
   layerCopy->setColor( color );
+  // reset offset, we don't want to show that in the preview
+  layerCopy->setOffset( 0 );
   layerCopy->setUseCustomDashPattern( true );
-  QIcon buttonIcon = QgsSymbolLayerUtils::symbolLayerPreviewIcon( layerCopy, QgsUnitTypes::RenderMillimeters, mChangePatternButton->iconSize() );
-  mChangePatternButton->setIcon( buttonIcon );
-  delete layerCopy;
+
+  QSize currentIconSize;
+  //icon size is button size with a small margin
+#ifdef Q_OS_WIN
+  currentIconSize = QSize( mChangePatternButton->width() - 10, mChangePatternButton->height() - 6 );
+#else
+  currentIconSize = QSize( mChangePatternButton->width() - 10, mChangePatternButton->height() - 12 );
+#endif
+
+  if ( !currentIconSize.isValid() || currentIconSize.width() <= 0 || currentIconSize.height() <= 0 )
+  {
+    return;
+  }
+
+  //create an icon pixmap
+  std::unique_ptr< QgsLineSymbol > previewSymbol = qgis::make_unique< QgsLineSymbol >( QgsSymbolLayerList() << layerCopy.release() );
+  const QIcon icon = QgsSymbolLayerUtils::symbolPreviewIcon( previewSymbol.get(), currentIconSize );
+  mChangePatternButton->setIconSize( currentIconSize );
+  mChangePatternButton->setIcon( icon );
+
+  // set tooltip
+  // create very large preview image
+  int width = static_cast< int >( Qgis::UI_SCALE_FACTOR * fontMetrics().width( 'X' ) * 23 );
+  int height = static_cast< int >( width / 1.61803398875 ); // golden ratio
+
+  QPixmap pm = QgsSymbolLayerUtils::symbolPreviewPixmap( previewSymbol.get(), QSize( width, height ), height / 20 );
+  QByteArray data;
+  QBuffer buffer( &data );
+  pm.save( &buffer, "PNG", 100 );
+  mChangePatternButton->setToolTip( QStringLiteral( "<img src='data:image/png;base64, %3'>" ).arg( QString( data.toBase64() ) ) );
+}
+
+void QgsSimpleLineSymbolLayerWidget::resizeEvent( QResizeEvent *event )
+{
+  QgsSymbolLayerWidget::resizeEvent( event );
+  // redraw custom dash pattern icon -- the button size has changed
+  updatePatternIcon();
 }
 
 

src/gui/symbology/qgssymbollayerwidget.h

@@ -153,6 +153,8 @@ class GUI_EXPORT QgsSimpleLineSymbolLayerWidget : public QgsSymbolLayerWidget, p
     //creates a new icon for the 'change pattern' button
     void updatePatternIcon();
 
+    void resizeEvent( QResizeEvent *event ) override;
+
   private slots:
 
     void updateAssistantSymbol();

src/ui/symbollayer/widget_simpleline.ui

@@ -225,30 +225,6 @@
      </property>
     </widget>
    </item>
-   <item row="7" column="2">
-    <layout class="QHBoxLayout" name="horizontalLayout_12">
-     <item>
-      <widget class="QPushButton" name="mChangePatternButton">
-       <property name="sizePolicy">
-        <sizepolicy hsizetype="Expanding" vsizetype="Fixed">
-         <horstretch>0</horstretch>
-         <verstretch>0</verstretch>
-        </sizepolicy>
-       </property>
-       <property name="text">
-        <string>Change</string>
-       </property>
-      </widget>
-     </item>
-     <item>
-      <widget class="QgsUnitSelectionWidget" name="mDashPatternUnitWidget" native="true">
-       <property name="focusPolicy">
-        <enum>Qt::StrongFocus</enum>
-       </property>
-      </widget>
-     </item>
-    </layout>
-   </item>
    <item row="4" column="3">
     <widget class="QgsPropertyOverrideButton" name="mJoinStyleDDBtn">
      <property name="text">
@@ -287,6 +263,46 @@
    <item row="9" column="2" colspan="2">
     <widget class="QComboBox" name="mRingFilterComboBox"/>
    </item>
+   <item row="7" column="0" colspan="3">
+    <layout class="QHBoxLayout" name="horizontalLayout_12">
+     <item>
+      <spacer name="horizontalSpacer">
+       <property name="orientation">
+        <enum>Qt::Horizontal</enum>
+       </property>
+       <property name="sizeType">
+        <enum>QSizePolicy::Maximum</enum>
+       </property>
+       <property name="sizeHint" stdset="0">
+        <size>
+         <width>20</width>
+         <height>1</height>
+        </size>
+       </property>
+      </spacer>
+     </item>
+     <item>
+      <widget class="QPushButton" name="mChangePatternButton">
+       <property name="sizePolicy">
+        <sizepolicy hsizetype="Expanding" vsizetype="Fixed">
+         <horstretch>0</horstretch>
+         <verstretch>0</verstretch>
+        </sizepolicy>
+       </property>
+       <property name="text">
+        <string/>
+       </property>
+      </widget>
+     </item>
+     <item>
+      <widget class="QgsUnitSelectionWidget" name="mDashPatternUnitWidget" native="true">
+       <property name="focusPolicy">
+        <enum>Qt::StrongFocus</enum>
+       </property>
+      </widget>
+     </item>
+    </layout>
+   </item>
   </layout>
  </widget>
  <customwidgets>