Add QStyleHints::useHoverEffects

The delivery of hover events creates unnecessary overhead on touch
platforms. This allows Qt Quick Controls 2 to determine whether the
underlying platform wants hover effects. The hover effects are enabled
by default for the classic desktop platforms: Linux, Windows & macOS.

Change-Id: Ia4e7b5c0fcb7af8f1c47e06fb28086cffdf35976
Task-number: QTBUG-50003
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/gui/kernel/qplatformintegration.cpp

@@ -402,6 +402,8 @@ QVariant QPlatformIntegration::styleHint(StyleHint hint) const
         return true;
     case ItemViewActivateItemOnSingleClick:
         return QPlatformTheme::defaultThemeHint(QPlatformTheme::ItemViewActivateItemOnSingleClick);
+    case UiEffects:
+        return QPlatformTheme::defaultThemeHint(QPlatformTheme::UiEffects);
     }
 
     return 0;

src/gui/kernel/qplatformintegration.h

@@ -156,7 +156,8 @@ public:
         MousePressAndHoldInterval,
         TabFocusBehavior,
         ReplayMousePressOutsidePopup,
-        ItemViewActivateItemOnSingleClick
+        ItemViewActivateItemOnSingleClick,
+        UiEffects
     };
 
     virtual QVariant styleHint(StyleHint hint) const;

src/gui/kernel/qplatformtheme.cpp

@@ -444,6 +444,8 @@ QVariant QPlatformTheme::themeHint(ThemeHint hint) const
         return QGuiApplicationPrivate::platformIntegration()->styleHint(QPlatformIntegration::MousePressAndHoldInterval);
     case QPlatformTheme::ItemViewActivateItemOnSingleClick:
         return QGuiApplicationPrivate::platformIntegration()->styleHint(QPlatformIntegration::ItemViewActivateItemOnSingleClick);
+    case QPlatformTheme::UiEffects:
+        return QGuiApplicationPrivate::platformIntegration()->styleHint(QPlatformIntegration::UiEffects);
     default:
         return QPlatformTheme::defaultThemeHint(hint);
     }

src/gui/kernel/qplatformtheme.h

@@ -270,7 +270,8 @@ public:
         AnimateComboUiEffect = 0x8,
         AnimateTooltipUiEffect = 0x10,
         FadeTooltipUiEffect = 0x20,
-        AnimateToolBoxUiEffect = 0x40
+        AnimateToolBoxUiEffect = 0x40,
+        HoverEffect = 0x80
     };
 
     enum IconOption {

src/gui/kernel/qstylehints.cpp

@@ -77,6 +77,7 @@ public:
         , m_keyboardInputInterval(-1)
         , m_cursorFlashTime(-1)
         , m_tabFocusBehavior(-1)
+        , m_uiEffects(-1)
         {}
 
     int m_mouseDoubleClickInterval;
@@ -86,6 +87,7 @@ public:
     int m_keyboardInputInterval;
     int m_cursorFlashTime;
     int m_tabFocusBehavior;
+    int m_uiEffects;
 };
 
 /*!
@@ -451,4 +453,34 @@ bool QStyleHints::singleClickActivation() const
     return themeableHint(QPlatformTheme::ItemViewActivateItemOnSingleClick, QPlatformIntegration::ItemViewActivateItemOnSingleClick).toBool();
 }
 
+/*!
+    \property QStyleHints::useHoverEffects
+    \brief \c true if UI elements should use hover effects. This is the
+    standard behavior on desktop platforms with a mouse pointer, whereas
+    on touch platforms the overhead of hover event delivery can be avoided.
+
+    \since 5.8
+*/
+bool QStyleHints::useHoverEffects() const
+{
+    Q_D(const QStyleHints);
+    return (d->m_uiEffects >= 0 ?
+            d->m_uiEffects :
+            themeableHint(QPlatformTheme::UiEffects, QPlatformIntegration::UiEffects).toInt()) & QPlatformTheme::HoverEffect;
+}
+
+void QStyleHints::setUseHoverEffects(bool useHoverEffects)
+{
+    Q_D(QStyleHints);
+    if (d->m_uiEffects >= 0 && useHoverEffects == bool(d->m_uiEffects & QPlatformTheme::HoverEffect))
+        return;
+    if (d->m_uiEffects == -1)
+        d->m_uiEffects = 0;
+    if (useHoverEffects)
+        d->m_uiEffects |= QPlatformTheme::HoverEffect;
+    else
+        d->m_uiEffects &= ~QPlatformTheme::HoverEffect;
+    emit useHoverEffectsChanged(useHoverEffects);
+}
+
 QT_END_NAMESPACE

src/gui/kernel/qstylehints.h

@@ -70,6 +70,7 @@ class Q_GUI_EXPORT QStyleHints : public QObject
     Q_PROPERTY(bool useRtlExtensions READ useRtlExtensions STORED false CONSTANT FINAL)
     Q_PROPERTY(Qt::TabFocusBehavior tabFocusBehavior READ tabFocusBehavior NOTIFY tabFocusBehaviorChanged FINAL)
     Q_PROPERTY(bool singleClickActivation READ singleClickActivation STORED false CONSTANT FINAL)
+    Q_PROPERTY(bool useHoverEffects READ useHoverEffects WRITE setUseHoverEffects NOTIFY useHoverEffectsChanged FINAL)
 
 public:
     void setMouseDoubleClickInterval(int mouseDoubleClickInterval);
@@ -96,6 +97,8 @@ public:
     Qt::TabFocusBehavior tabFocusBehavior() const;
     void setTabFocusBehavior(Qt::TabFocusBehavior tabFocusBehavior);
     bool singleClickActivation() const;
+    bool useHoverEffects() const;
+    void setUseHoverEffects(bool useHoverEffects);
 
 Q_SIGNALS:
     void cursorFlashTimeChanged(int cursorFlashTime);
@@ -105,6 +108,7 @@ Q_SIGNALS:
     void startDragDistanceChanged(int startDragDistance);
     void startDragTimeChanged(int startDragTime);
     void tabFocusBehaviorChanged(Qt::TabFocusBehavior tabFocusBehavior);
+    void useHoverEffectsChanged(bool useHoverEffects);
 
 private:
     friend class QGuiApplication;

src/platformsupport/themes/genericunix/qgenericunixthemes.cpp

@@ -666,6 +666,8 @@ QVariant QGnomeTheme::themeHint(QPlatformTheme::ThemeHint hint) const
         return QVariant(int(GnomeKeyboardScheme));
     case QPlatformTheme::PasswordMaskCharacter:
         return QVariant(QChar(0x2022));
+    case QPlatformTheme::UiEffects:
+        return QVariant(int(HoverEffect));
     default:
         break;
     }

src/plugins/platforms/cocoa/qcocoatheme.mm

@@ -329,6 +329,8 @@ QVariant QCocoaTheme::themeHint(ThemeHint hint) const
     }
     case QPlatformTheme::PasswordMaskCharacter:
         return QVariant(QChar(kBulletUnicode));
+    case QPlatformTheme::UiEffects:
+        return QVariant(int(HoverEffect));
     default:
         break;
     }

src/plugins/platforms/windows/qwindowstheme.cpp

@@ -347,7 +347,7 @@ static inline QStringList styleNames()
 
 static inline int uiEffects()
 {
-    int result = 0;
+    int result = QPlatformTheme::HoverEffect;
     if (booleanSystemParametersInfo(SPI_GETUIEFFECTS, false))
         result |= QPlatformTheme::GeneralUiEffect;
     if (booleanSystemParametersInfo(SPI_GETMENUANIMATION, false))