Implement QPlatformTheme::appearance() for iOS

Implement support for detecting Light/Dark mode on iOS. This is needed
by the QQuickIOSStyle in order to display the proper image assets
according to the theme.

A further improvement would be to react to theme changes dynamically.

Change-Id: I95e11c4a4b647614bdd78d734941d2b11546687a
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/ios/qiostheme.h

@@ -55,6 +55,8 @@ public:
     const QPalette *palette(Palette type = SystemPalette) const override;
     QVariant themeHint(ThemeHint hint) const override;
 
+    Appearance appearance() const override;
+
     QPlatformMenuItem* createPlatformMenuItem() const override;
     QPlatformMenu* createPlatformMenu() const override;
 

src/plugins/platforms/ios/qiostheme.mm

@@ -167,6 +167,19 @@ QVariant QIOSTheme::themeHint(ThemeHint hint) const
     }
 }
 
+QPlatformTheme::Appearance QIOSTheme::appearance() const
+{
+    if (@available(ios 12, *)) {
+        if (UIWindow *window = qt_apple_sharedApplication().keyWindow) {
+            return window.rootViewController.traitCollection.userInterfaceStyle
+                    == UIUserInterfaceStyleDark
+                    ? QPlatformTheme::Appearance::Dark
+                    : QPlatformTheme::Appearance::Light;
+        }
+    }
+    return QPlatformTheme::Appearance::Unknown;
+}
+
 const QFont *QIOSTheme::font(Font type) const
 {
     const auto *platformIntegration = QGuiApplicationPrivate::platformIntegration();