Add integration for QtQuick Controls 2 applications (#130)

This enables the QtQuick Controls 2 desktop style when it is available.
Since the experience without it is really broken, QGnomePlatform now
warns on the console when the style is not installed.

Author: jbruechert

CMakeLists.txt

@@ -36,6 +36,7 @@ find_package(Qt${QT_VERSION_MAJOR} ${QT_MIN_VERSION} CONFIG REQUIRED COMPONENTS
     DBus
     Gui
     Widgets
+    QuickControls2
 )
 
 find_package(Qt${QT_VERSION_MAJOR}Gui ${QT_MIN_VERSION} CONFIG REQUIRED Private)
@@ -51,6 +52,17 @@ else()
     find_package(AdwaitaQt "1.4.2" REQUIRED)
 endif()
 
+
+if (NOT USE_QT6)
+    find_package(KF5QQC2DesktopStyle)
+    set_package_properties(KF5QQC2DesktopStyle PROPERTIES
+        DESCRIPTION "Styling for QtQuick Controls 2 applications"
+        TYPE RUNTIME)
+else()
+    # TODO, the desktop style for Qt6 is not yet released
+endif()
+
+
 find_package(PkgConfig REQUIRED)
 pkg_check_modules(GTK+3 REQUIRED IMPORTED_TARGET gtk+-3.0)
 

src/theme/CMakeLists.txt

@@ -16,6 +16,7 @@ target_link_libraries(qgnomeplatformtheme
     Qt${QT_VERSION_MAJOR}::DBus
     Qt${QT_VERSION_MAJOR}::Gui
     Qt${QT_VERSION_MAJOR}::GuiPrivate
+    Qt${QT_VERSION_MAJOR}::QuickControls2
     ${ADWAITAQT_LIBRARIES}
     PkgConfig::GTK+3
 )

src/theme/qgnomeplatformtheme.cpp

@@ -25,6 +25,8 @@
 
 #include <QApplication>
 #include <QGuiApplication>
+#include <QLoggingCategory>
+#include <QQuickStyle>
 #include <QStyleFactory>
 
 #undef signals
@@ -43,6 +45,8 @@
 #include <QtGui/private/qgenericunixthemes_p.h>
 #endif
 
+Q_LOGGING_CATEGORY(QGnomePlatformThemeLog, "qt.qpa.qgnomeplatform.theme")
+
 void gtkMessageHandler(const gchar *log_domain, GLogLevelFlags log_level, const gchar *message, gpointer unused_data)
 {
     /* Silence false-positive Gtk warnings (we are using Xlib to set
@@ -87,6 +91,29 @@ QGnomePlatformTheme::QGnomePlatformTheme()
     // Load QGnomeTheme
     m_platformTheme = QGenericUnixTheme::createUnixTheme(QLatin1String("gnome"));
 #endif
+
+    // Configure the Qt Quick Controls 2 style to the KDE desktop style,
+    // Which passes the QtWidgets theme through to Qt Quick Controls.
+    // From https://invent.kde.org/plasma/plasma-integration/-/blob/02fe12a55522a43de3efa6de2185a695ff2a576a/src/platformtheme/kdeplatformtheme.cpp#L582
+
+    // if the user has explicitly set something else, don't meddle
+    // Also ignore the default Fusion style
+    if (!QQuickStyle::name().isEmpty() && QQuickStyle::name() != QLatin1String("Fusion")) {
+        return;
+    }
+
+    // Unfortunately we only have a way to check this on Qt5
+    // On Qt6 this should just fall back to the Fusion style automatically.
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
+    if (!QQuickStyle::availableStyles().contains(QStringLiteral("org.kde.desktop"))) {
+        qCWarning(QGnomePlatformThemeLog) << "The desktop style for QtQuick Controls 2 applications"
+                                          << "is not available on the system (qqc2-desktop-style)."
+                                          << "The application may look broken.";
+        return;
+    }
+#endif
+
+    QQuickStyle::setStyle(QStringLiteral("org.kde.desktop"));
 }
 
 QGnomePlatformTheme::~QGnomePlatformTheme()