Micro-optimise screenpool event filter by caching xcb extension offset

Summary: BUG: 390499

Test Plan:
See callgrind in bug report
Added debug in the relevant section, unplugged a monitor. Saw my output

Reviewers: #plasma, broulik

Subscribers: jtamate, mart, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10538

shell/CMakeLists.txt

@@ -73,6 +73,7 @@ target_link_libraries(plasmashell
  KF5::XmlGui
  KF5::Package
  KF5::WaylandClient
+ KF5::WindowSystem
 )
 target_include_directories(plasmashell PRIVATE "${CMAKE_BINARY_DIR}")
 target_compile_definitions(plasmashell PRIVATE -DPROJECT_VERSION="${PROJECT_VERSION}")

shell/autotests/CMakeLists.txt

@@ -9,6 +9,7 @@ MACRO(PLASMASHELL_UNIT_TESTS)
                             Qt5::Test
                             Qt5::Gui
                             KF5::Service
+                            KF5::WindowSystem
                                      )
                if(HAVE_X11)
                   target_link_libraries(${_testname} ${X11_LIBRARIES} ${XCB_LIBRARIES} )

shell/screenpool.cpp

@@ -22,6 +22,7 @@
 
 #include <QGuiApplication>
 #include <QScreen>
+#include <KWindowSystem>
 
 #if HAVE_X11
 #include <QX11Info>
@@ -34,12 +35,19 @@ ScreenPool::ScreenPool(KSharedConfig::Ptr config, QObject *parent)
     : QObject(parent),
       m_configGroup(KConfigGroup(config, QStringLiteral("ScreenConnectors")))
 {
-    qApp->installNativeEventFilter(this);
 
     m_configSaveTimer.setSingleShot(true);
     connect(&m_configSaveTimer, &QTimer::timeout, this, [this](){
         m_configGroup.sync();
     });
+
+#if HAVE_X11
+    if (KWindowSystem::isPlatformX11()) {
+        qApp->installNativeEventFilter(this);
+        const xcb_query_extension_reply_t* reply = xcb_get_extension_data(QX11Info::connection(), &xcb_randr_id);
+        m_xrandrExtensionOffset = reply->first_event;
+    }
+#endif
 }
 
 void ScreenPool::load()
@@ -185,9 +193,8 @@ bool ScreenPool::nativeEventFilter(const QByteArray& eventType, void* message, l
 
     const auto responseType = XCB_EVENT_RESPONSE_TYPE(ev);
 
-    const xcb_query_extension_reply_t* reply = xcb_get_extension_data(QX11Info::connection(), &xcb_randr_id);
 
-    if (responseType == reply->first_event + XCB_RANDR_SCREEN_CHANGE_NOTIFY) {
+    if (responseType == m_xrandrExtensionOffset + XCB_RANDR_SCREEN_CHANGE_NOTIFY) {
         if (qGuiApp->primaryScreen()->name() != primaryConnector()) {
             //new screen?
             if (id(qGuiApp->primaryScreen()->name()) < 0) {

shell/screenpool.h

@@ -65,6 +65,7 @@ private:
     QHash<QString, int> m_idForConnector;
 
     QTimer m_configSaveTimer;
+    int m_xrandrExtensionOffset;
 };
 
 #endif // SCREENPOOL_H