Add KDE theming support

KDE applications are a notable part of the popular Qt software in the Linux world.
Sadly, they have their own theming add-ons that are applied through QPlatformTheme.

This adds support for all aspects of the KDE applications theming present in the plasma-integration QPlatformTheme:
* KDE's QtQuick style bridging the currently set QtWidgets style (you may want to get QtQuick style choice in the settings UI in the future instead)
* KDE's color schemes (the most important part as KDE applications don't fallback to QtQuick style/QtWidgets style/QPalette for colors)
* KDE's icon engine applying KDE's color schemes to icons (to not to have black icons on black background)

CMakeLists.txt

@@ -24,7 +24,10 @@ set(CMAKE_BUILD_RPATH_USE_ORIGIN ON)
 
 add_definitions(-DQT_DISABLE_DEPRECATED_BEFORE=0x060000 -DUSE_WIDGETS)
 
-find_package(Qt6 REQUIRED COMPONENTS BuildInternals Core Widgets OPTIONAL_COMPONENTS LinguistTools)
+find_package(Qt6 REQUIRED COMPONENTS BuildInternals Core Widgets QuickControls2 OPTIONAL_COMPONENTS LinguistTools)
+find_package(KF6Config REQUIRED)
+find_package(KF6ColorScheme REQUIRED)
+find_package(KF6IconThemes REQUIRED)
 
 get_target_property(QT_QTPATHS_EXECUTABLE Qt6::qtpaths IMPORTED_LOCATION)
 

src/qt6ct-common/CMakeLists.txt

@@ -26,5 +26,5 @@ include_directories(${CMAKE_CURRENT_SOURCE_DIR}/../)
 
 add_library(qt6ct-common SHARED ${app_SRCS})
 set_target_properties(qt6ct-common PROPERTIES VERSION ${QT6CT_VERSION})
-target_link_libraries(qt6ct-common PRIVATE Qt6::Gui)
+target_link_libraries(qt6ct-common PRIVATE Qt6::Gui KF6::ConfigCore KF6::ColorScheme)
 install(TARGETS qt6ct-common DESTINATION ${CMAKE_INSTALL_LIBDIR})

src/qt6ct-common/qt6ct.cpp

@@ -34,6 +34,8 @@
 #include <QFile>
 #include <QSettings>
 #include <QtDebug>
+#include <KSharedConfig>
+#include <KColorScheme>
 #include "qt6ct.h"
 
 #ifndef QT6CT_DATADIR
@@ -116,6 +118,7 @@ QStringList Qt6CT::sharedColorSchemePaths()
     for(const QString &p : QStandardPaths::standardLocations(QStandardPaths::GenericDataLocation))
     {
         paths << (p + QLatin1String("/qt6ct/colors"));
+        paths << (p + QLatin1String("/color-schemes"));
     }
     paths << QLatin1String(QT6CT_DATADIR"/qt6ct/colors");
     paths.removeDuplicates();
@@ -146,8 +149,16 @@ QString Qt6CT::resolvePath(const QString &path)
     return tmp;
 }
 
+bool Qt6CT::isKColorScheme(const QString &filePath)
+{
+    return filePath.toLower().endsWith(".colors");
+}
+
 QPalette Qt6CT::loadColorScheme(const QString &filePath, const QPalette &fallback)
 {
+    if(isKColorScheme(filePath))
+        return KColorScheme::createApplicationPalette(KSharedConfig::openConfig(filePath));
+
     QPalette customPalette;
     QSettings settings(filePath, QSettings::IniFormat);
     settings.beginGroup("ColorScheme");

src/qt6ct-common/qt6ct.h

@@ -67,6 +67,7 @@ class QT6CT_EXPORT Qt6CT
     static QString userColorSchemePath();
     static QStringList sharedColorSchemePaths();
     static QString resolvePath(const QString &path);
+    static bool isKColorScheme(const QString &filePath);
     static QPalette loadColorScheme(const QString &filePath, const QPalette &fallback);
 
     static void registerStyleInstance(StyleInstance *instance);

src/qt6ct-qtplugin/CMakeLists.txt

@@ -9,5 +9,5 @@ include_directories(${CMAKE_CURRENT_SOURCE_DIR}/../)
 
 add_library(qt6ct-qtplugin MODULE ${app_SRCS})
 set_target_properties(qt6ct-qtplugin PROPERTIES OUTPUT_NAME qt6ct)
-target_link_libraries(qt6ct-qtplugin PRIVATE Qt6::Widgets Qt6::GuiPrivate qt6ct-common)
+target_link_libraries(qt6ct-qtplugin PRIVATE Qt6::Widgets Qt6::GuiPrivate Qt6::QuickControls2 KF6::IconThemes qt6ct-common)
 install(TARGETS qt6ct-qtplugin DESTINATION ${PLUGINDIR}/platformthemes)

src/qt6ct-qtplugin/qt6ctplatformtheme.cpp

@@ -44,6 +44,7 @@
 #endif
 #include <QFile>
 #include <QFileSystemWatcher>
+#include <QQuickStyle>
 #include <private/qiconloader_p.h>
 
 #include "qt6ct.h"
@@ -52,6 +53,9 @@
 #include <QStringList>
 #include <qpa/qplatformthemefactory_p.h>
 
+#include <KIconEngine>
+#include <KIconLoader>
+
 Q_LOGGING_CATEGORY(lqt6ct, "qt6ct", QtWarningMsg)
 
 //QT_QPA_PLATFORMTHEME=qt6ct
@@ -67,12 +71,16 @@ Qt6CTPlatformTheme::Qt6CTPlatformTheme()
         QMetaObject::invokeMethod(this, "createFSWatcher", Qt::QueuedConnection);
 #endif
         QGuiApplication::setFont(m_generalFont);
+        //don't override the value explicitly set by the user
+        if(QQuickStyle::name().isEmpty() || QQuickStyle::name() == QLatin1String("Fusion"))
+            QQuickStyle::setStyle(QLatin1String("org.kde.desktop"));
     }
     qCDebug(lqt6ct) << "using qt6ct plugin";
 #ifdef QT_WIDGETS_LIB
     if(!QStyleFactory::keys().contains("qt6ct-style"))
         qCCritical(lqt6ct) << "unable to find qt6ct proxy style";
 #endif
+	QCoreApplication::instance()->installEventFilter(this);
 }
 
 Qt6CTPlatformTheme::~Qt6CTPlatformTheme()
@@ -146,6 +154,11 @@ QIcon Qt6CTPlatformTheme::fileIcon(const QFileInfo &fileInfo, QPlatformTheme::Ic
     return QIcon::fromTheme(type.iconName());
 }
 
+QIconEngine *Qt6CTPlatformTheme::createIconEngine(const QString &iconName) const
+{
+    return new KIconEngine(iconName, KIconLoader::global());
+}
+
 void Qt6CTPlatformTheme::applySettings()
 {
     if(!QGuiApplication::desktopSettingsAware() || m_isIgnored)
@@ -186,6 +199,9 @@ void Qt6CTPlatformTheme::applySettings()
         if(m_update && m_usePalette)
             qApp->setPalette(*m_palette);
 
+        if(Qt6CT::isKColorScheme(m_schemePath))
+            qApp->setProperty("KDE_COLOR_SCHEME_PATH", m_schemePath);
+
         if(m_userStyleSheet != m_prevStyleSheet)
         {
             // prepend our stylesheet to that of the application
@@ -252,11 +268,11 @@ void Qt6CTPlatformTheme::readSettings()
 
     settings.beginGroup("Appearance");
     m_style = settings.value("style", "Fusion").toString();
-    QString schemePath = settings.value("custom_palette", false).toBool()
+    m_schemePath = settings.value("custom_palette", false).toBool()
         ? Qt6CT::resolvePath(settings.value("color_scheme_path").toString()) //replace environment variables
         : QString();
-    m_palette = !schemePath.isEmpty()
-        ? std::make_unique<QPalette>(Qt6CT::loadColorScheme(schemePath, *QPlatformTheme::palette(SystemPalette)))
+    m_palette = !m_schemePath.isEmpty()
+        ? std::make_unique<QPalette>(Qt6CT::loadColorScheme(m_schemePath, *QPlatformTheme::palette(SystemPalette)))
         : nullptr;
     m_iconTheme = settings.value("icon_theme").toString();
     //load dialogs
@@ -365,3 +381,19 @@ QString Qt6CTPlatformTheme::loadStyleSheets(const QStringList &paths)
     content.replace(regExp, "\n");
     return content;
 }
+
+//There's such a thing as KColorSchemeManager that lets the user to change the color scheme
+//application-wide and we should re-apply the color scheme if KCSM resets it to the default
+//which leads KColorScheme to get the color scheme from kdeglobals which won't help us.
+bool Qt6CTPlatformTheme::eventFilter(QObject *obj, QEvent *e)
+{
+    if(obj == qApp && e->type() == QEvent::DynamicPropertyChange)
+    {
+        QDynamicPropertyChangeEvent *ee = static_cast<QDynamicPropertyChangeEvent*>(e);
+        if(ee->propertyName() == "KDE_COLOR_SCHEME_PATH" &&
+                qApp->property("KDE_COLOR_SCHEME_PATH").toString().isEmpty() &&
+                Qt6CT::isKColorScheme(m_schemePath))
+            applySettings();
+    }
+    return QObject::eventFilter(obj, e);
+}

src/qt6ct-qtplugin/qt6ctplatformtheme.h

@@ -62,10 +62,13 @@ class Qt6CTPlatformTheme : public QObject, public QGenericUnixTheme
     //virtual QPixmap fileIconPixmap(const QFileInfo &fileInfo, const QSizeF &size,
     //                               QPlatformTheme::IconOptions iconOptions = 0) const;
 
-    //virtual QIconEngine *createIconEngine(const QString &iconName) const;
+    virtual QIconEngine *createIconEngine(const QString &iconName) const;
     //virtual QList<QKeySequence> keyBindings(QKeySequence::StandardKey key) const;
     //virtual QString standardButtonText(int button) const;
 
+protected:
+	bool eventFilter(QObject *obj, QEvent *e) override;
+
 private slots:
     void applySettings();
 #ifdef QT_WIDGETS_LIB
@@ -79,7 +82,7 @@ private slots:
     bool hasWidgets();
 #endif
     QString loadStyleSheets(const QStringList &paths);
-    QString m_style, m_iconTheme, m_userStyleSheet, m_prevStyleSheet;
+    QString m_style, m_schemePath, m_iconTheme, m_userStyleSheet, m_prevStyleSheet;
     std::unique_ptr<QPalette> m_palette;
     QFont m_generalFont, m_fixedFont;
     int m_doubleClickInterval;

src/qt6ct/CMakeLists.txt

@@ -31,6 +31,6 @@ if(Qt6LinguistTools_FOUND)
 endif()
 
 add_executable(qt6ct ${app_SRCS})
-target_link_libraries(qt6ct PRIVATE Qt6::Widgets Qt6::WidgetsPrivate qt6ct-common)
+target_link_libraries(qt6ct PRIVATE Qt6::Widgets Qt6::WidgetsPrivate KF6::ConfigCore qt6ct-common)
 install(TARGETS qt6ct DESTINATION ${CMAKE_INSTALL_BINDIR})
 install(FILES qt6ct.desktop DESTINATION ${CMAKE_INSTALL_DATADIR}/applications)

src/qt6ct/appearancepage.cpp

@@ -35,6 +35,8 @@
 #include <QMenu>
 #include <QIcon>
 #include <QStringList>
+#include <KSharedConfig>
+#include <KConfigGroup>
 #include <qpa/qplatformthemefactory_p.h>
 #include "qt6ct.h"
 #include "appearancepage.h"
@@ -71,7 +73,7 @@ AppearancePage::AppearancePage(QWidget *parent) :
     QMenu *menu = new QMenu(this);
     menu->addAction(QIcon::fromTheme("document-new"), tr("Create"), this, SLOT(createColorScheme()));
     m_changeColorSchemeAction = menu->addAction(QIcon::fromTheme("accessories-text-editor"), tr("Edit"), this, SLOT(changeColorScheme()));
-    menu->addAction(QIcon::fromTheme("edit-copy"), tr("Create a Copy"), this, SLOT(copyColorScheme()));
+    m_copyColorSchemeAction = menu->addAction(QIcon::fromTheme("edit-copy"), tr("Create a Copy"), this, SLOT(copyColorScheme()));
     m_renameColorSchemeAction = menu->addAction(tr("Rename"), this, SLOT(renameColorScheme()));
     menu->addSeparator();
     m_removeColorSchemeAction = menu->addAction(QIcon::fromTheme("edit-delete"), tr("Remove"), this, SLOT(removeColorScheme()));
@@ -303,6 +305,7 @@ void AppearancePage::setPreviewPalette(const QPalette &p)
 
 void AppearancePage::updateActions()
 {
+    m_copyColorSchemeAction->setVisible(!Qt6CT::isKColorScheme(m_ui->colorSchemeComboBox->currentData().toString()));
     if(m_ui->colorSchemeComboBox->count() == 0 ||
             !QFileInfo(m_ui->colorSchemeComboBox->currentData().toString()).isWritable())
     {
@@ -380,11 +383,23 @@ void AppearancePage::findColorSchemes(const QString &path)
 {
     QDir dir(path);
     dir.setFilter(QDir::Files);
-    dir.setNameFilters(QStringList() << "*.conf");
+    dir.setNameFilters(QStringList() << "*.conf" << "*.colors");
 
     for(const QFileInfo &info : dir.entryInfoList())
     {
-        m_ui->colorSchemeComboBox->addItem(info.baseName(), info.filePath());
+        QString name;
+        QString path = info.filePath();
+        if(Qt6CT::isKColorScheme(path))
+        {
+            KSharedConfigPtr config = KSharedConfig::openConfig(path, KConfig::SimpleConfig);
+            KConfigGroup group(config, "General");
+            name = group.readEntry("Name", info.baseName()) + " (KColorScheme)";
+        }
+        else
+        {
+            name = info.baseName();
+        }
+        m_ui->colorSchemeComboBox->addItem(name, path);
     }
 }
 

src/qt6ct/appearancepage.h

@@ -72,7 +72,7 @@ private slots:
     QStyle *m_selectedStyle = nullptr;
     QPalette m_customPalette;
     QWidget *m_previewWidget;
-    QAction *m_changeColorSchemeAction, *m_renameColorSchemeAction, *m_removeColorSchemeAction;
+    QAction *m_changeColorSchemeAction, *m_copyColorSchemeAction, *m_renameColorSchemeAction, *m_removeColorSchemeAction;
     Ui::PreviewForm *m_previewUi;
 };