Introduce Units singleton

Every single (Frame)SvgItem would creates its own Units instance which in turn would
create a property map for icon sizes and lots of other stuff. Avoid this.

CHANGELOG: There is now a global "Units" instance reducing memory consumption and creation time of SVG items

Differential Revision: https://phabricator.kde.org/D4012

src/declarativeimports/core/corebindingsplugin.cpp

@@ -59,8 +59,7 @@ void CoreBindingsPlugin::initializeEngine(QQmlEngine *engine, const char *uri)
     Plasma::QuickTheme *theme = new Plasma::QuickTheme(engine);
     context->setContextProperty(QStringLiteral("theme"), theme);
 
-    Units *units = new Units(context);
-    context->setContextProperty(QStringLiteral("units"), units);
+    context->setContextProperty(QStringLiteral("units"), &Units::instance());
 
     if (!engine->rootContext()->contextObject()) {
         KDeclarative::KDeclarative kdeclarative;

src/declarativeimports/core/framesvgitem.cpp

@@ -265,7 +265,7 @@ FrameSvgItem::FrameSvgItem(QQuickItem *parent)
     m_fixedMargins->setFixed(true);
     setFlag(ItemHasContents, true);
     connect(m_frameSvg, SIGNAL(repaintNeeded()), this, SLOT(doUpdate()));
-    connect(&m_units, &Units::devicePixelRatioChanged, this, &FrameSvgItem::updateDevicePixelRatio);
+    connect(&Units::instance(), &Units::devicePixelRatioChanged, this, &FrameSvgItem::updateDevicePixelRatio);
     connect(m_frameSvg, &Svg::fromCurrentThemeChanged, this, &FrameSvgItem::fromCurrentThemeChanged);
     connect(m_frameSvg, &Svg::statusChanged, this, &FrameSvgItem::statusChanged);
 }
@@ -532,7 +532,7 @@ void FrameSvgItem::updateDevicePixelRatio()
     } else {
         m_frameSvg->setDevicePixelRatio(qMax<qreal>(1.0, floor(qApp->devicePixelRatio())));
     }
-    m_frameSvg->setScaleFactor(qMax<qreal>(1.0, floor(m_units.devicePixelRatio())));
+    m_frameSvg->setScaleFactor(qMax<qreal>(1.0, floor(Units::instance().devicePixelRatio())));
     m_textureChanged = true;
 }
 

src/declarativeimports/core/framesvgitem.h

@@ -236,7 +236,6 @@ private:
     FrameSvgItemMargins *m_margins;
     FrameSvgItemMargins *m_fixedMargins;
     QString m_prefix;
-    Units m_units;
     bool m_textureChanged;
     bool m_sizeChanged;
     bool m_fastPath;

src/declarativeimports/core/svgitem.cpp

@@ -40,7 +40,7 @@ SvgItem::SvgItem(QQuickItem *parent)
       m_textureChanged(false)
 {
     setFlag(QQuickItem::ItemHasContents, true);
-    connect(&m_units, &Units::devicePixelRatioChanged, this, &SvgItem::updateDevicePixelRatio);
+    connect(&Units::instance(), &Units::devicePixelRatioChanged, this, &SvgItem::updateDevicePixelRatio);
 }
 
 SvgItem::~SvgItem()
@@ -197,7 +197,7 @@ void SvgItem::updateDevicePixelRatio()
         } else {
             m_svg.data()->setDevicePixelRatio(qMax<qreal>(1.0, floor(qApp->devicePixelRatio())));
         }
-        m_svg.data()->setScaleFactor(qMax<qreal>(1.0, floor(m_units.devicePixelRatio())));
+        m_svg.data()->setScaleFactor(qMax<qreal>(1.0, floor(Units::instance().devicePixelRatio())));
     }
 }
 

src/declarativeimports/core/svgitem.h

@@ -106,7 +106,6 @@ private:
     QString m_elementID;
     bool m_smooth;
     bool m_textureChanged;
-    Units m_units;
     QImage m_image;
 };
 }

src/declarativeimports/core/units.cpp

@@ -91,6 +91,13 @@ Units::Units(QObject *parent)
 
 Units::~Units()
 {
+
+}
+
+Units &Units::instance()
+{
+    static Units units;
+    return units;
 }
 
 void Units::settingsFileChanged(const QString &file)

src/declarativeimports/core/units.h

@@ -117,9 +117,14 @@ class Units : public QObject
 public:
 /// @cond INTERNAL_DOCS
 
-    Units(QObject *parent = 0);
     ~Units();
 
+    /**
+     * @return a reference to the global Units instance
+     * @since 5.31
+     */
+    static Units &instance();
+
     /**
      * @return pixel value for a grid Unit. Depends on DPI and font size.
      */
@@ -181,6 +186,12 @@ private Q_SLOTS:
     void updateSpacing();
 
 private:
+    Units(QObject *parent = 0);
+    Units(Units const&) = delete; // Copy construct
+    Units(Units&&) = delete; // Move construct
+    Units& operator=(Units const&) = delete; // Copy assign
+    Units& operator=(Units &&) = delete; // Move assign
+
     void updateDevicePixelRatio();
     void updatePlasmaRCSettings();
     /**