xdgdesktopportal theme: Implement appearance()

We can use the new org.freedesktop.appearance color-scheme setting in
the XDG Desktop Portal Settings to implement this in a DE-independent
way.

In the future we can also use the related "SettingChanged" signal in the
Desktop portal to support changing the theme during runtime.

Useful links:
Spec: https://github.com/flatpak/xdg-desktop-portal/blob/d7a304a00697d7d608821253cd013f3b97ac0fb6/data/org.freedesktop.impl.portal.Settings.xml#L33-L45
Blog post: https://blogs.gnome.org/alexm/2021/10/04/dark-style-preference/

Task-number: QTBUG-93955
Pick-to: 6.2 6.3
Change-Id: Ia5df22fb215ee94d68d2787da97a1c9f8f2c6b0c
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

2 files changed, 55 insertions, 0 deletions

diff --git a/src/plugins/platformthemes/xdgdesktopportal/qxdgdesktopportaltheme.cpp b/src/plugins/platformthemes/xdgdesktopportal/qxdgdesktopportaltheme.cpp
index 2fc3167fd5..87b9f20a4c 100644
--- a/src/plugins/platformthemes/xdgdesktopportal/qxdgdesktopportaltheme.cpp
+++ b/src/plugins/platformthemes/xdgdesktopportal/qxdgdesktopportaltheme.cpp
@@ -50,12 +50,19 @@
 #include <QDBusPendingCall>
 #include <QDBusPendingCallWatcher>
 #include <QDBusPendingReply>
+#include <QDBusReply>
 
 QT_BEGIN_NAMESPACE
 
 class QXdgDesktopPortalThemePrivate : public QPlatformThemePrivate
 {
 public:
+    enum XdgColorschemePref {
+        None,
+        PreferDark,
+        PreferLight
+    };
+
     QXdgDesktopPortalThemePrivate()
         : QPlatformThemePrivate()
     { }
@@ -65,8 +72,33 @@ public:
         delete baseTheme;
     }
 
+    /*! \internal
+
+        Converts the given Freedesktop color scheme setting \a colorschemePref to a QPlatformTheme::Appearance value.
+        Specification: https://github.com/flatpak/xdg-desktop-portal/blob/d7a304a00697d7d608821253cd013f3b97ac0fb6/data/org.freedesktop.impl.portal.Settings.xml#L33-L45
+
+        Unfortunately the enum numerical values are not defined identically, so we have to convert them.
+
+        The mapping is as follows:
+
+        Enum Index: Freedesktop definition  | Qt definition
+        ----------------------------------- | -------------
+        0: No preference                    | 0: Unknown
+        1: Prefer dark appearance           | 2: Dark
+        2: Prefer light appearance          | 1: Light
+    */
+    static QPlatformTheme::Appearance appearanceFromXdgPref(const XdgColorschemePref colorschemePref)
+    {
+        switch (colorschemePref) {
+            case PreferDark: return QPlatformTheme::Appearance::Dark;
+            case PreferLight: return QPlatformTheme::Appearance::Light;
+            default: return QPlatformTheme::Appearance::Unknown;
+        }
+    }
+
     QPlatformTheme *baseTheme = nullptr;
     uint fileChooserPortalVersion = 0;
+    QPlatformTheme::Appearance appearance = QPlatformTheme::Appearance::Unknown;
 };
 
 QXdgDesktopPortalTheme::QXdgDesktopPortalTheme()
@@ -113,6 +145,21 @@ QXdgDesktopPortalTheme::QXdgDesktopPortalTheme()
         }
         watcher->deleteLater();
     });
+
+    // Get information about system theme preference
+    message = QDBusMessage::createMethodCall(QLatin1String("org.freedesktop.portal.Desktop"),
+                                             QLatin1String("/org/freedesktop/portal/desktop"),
+                                             QLatin1String("org.freedesktop.portal.Settings"),
+                                             QLatin1String("Read"));
+    message << QLatin1String("org.freedesktop.appearance") << QLatin1String("color-scheme");
+
+    // this must not be asyncCall() because we have to set appearance now
+    QDBusReply<QVariant> reply = QDBusConnection::sessionBus().call(message);
+    if (reply.isValid()) {
+        const QDBusVariant dbusVariant = qvariant_cast<QDBusVariant>(reply.value());
+        const QXdgDesktopPortalThemePrivate::XdgColorschemePref xdgPref = static_cast<QXdgDesktopPortalThemePrivate::XdgColorschemePref>(dbusVariant.variant().toUInt());
+        d->appearance = QXdgDesktopPortalThemePrivate::appearanceFromXdgPref(xdgPref);
+    }
 }
 
 QPlatformMenuItem* QXdgDesktopPortalTheme::createPlatformMenuItem() const
@@ -191,6 +238,12 @@ QVariant QXdgDesktopPortalTheme::themeHint(ThemeHint hint) const
     return d->baseTheme->themeHint(hint);
 }
 
+QPlatformTheme::Appearance QXdgDesktopPortalTheme::appearance() const
+{
+    Q_D(const QXdgDesktopPortalTheme);
+    return d->appearance;
+}
+
 QPixmap QXdgDesktopPortalTheme::standardPixmap(StandardPixmap sp, const QSizeF &size) const
 {
     Q_D(const QXdgDesktopPortalTheme);
diff --git a/src/plugins/platformthemes/xdgdesktopportal/qxdgdesktopportaltheme.h b/src/plugins/platformthemes/xdgdesktopportal/qxdgdesktopportaltheme.h
index d38b3ddda3..0d945285a2 100644
--- a/src/plugins/platformthemes/xdgdesktopportal/qxdgdesktopportaltheme.h
+++ b/src/plugins/platformthemes/xdgdesktopportal/qxdgdesktopportaltheme.h
@@ -70,6 +70,8 @@ public:
 
     QVariant themeHint(ThemeHint hint) const override;
 
+    Appearance appearance() const override;
+
     QPixmap standardPixmap(StandardPixmap sp, const QSizeF &size) const override;
     QIcon fileIcon(const QFileInfo &fileInfo,
                    QPlatformTheme::IconOptions iconOptions = { }) const override;