[wayland] Better track if WaylandBackend is ready

The WaylandBackend emits a signal when the backend is ready. If a user
connects to it after it became ready, it will never get notified.
Therefore the WaylandBackend tracks also whether it is ready and
implements connectNotify to emit the signal again if a user connects and
the backend is already ready.

Users of the signal need to disconnect if they cannot handle it being
invoked multiple times. So far the only user does handle this properly.

wayland_backend.cpp

@@ -44,6 +44,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include <QAbstractEventDispatcher>
 #include <QCoreApplication>
 #include <QDebug>
+#include <QMetaMethod>
 #include <QThread>
 // xcb
 #include <xcb/xtest.h>
@@ -552,6 +553,7 @@ void WaylandBackend::initConnection()
         Qt::QueuedConnection);
     connect(m_connectionThreadObject, &ConnectionThread::connectionDied, this,
         [this]() {
+            m_ready = false;
             emit systemCompositorDied();
             m_cursorTracker.reset();
             m_seat.reset();
@@ -652,9 +654,18 @@ void WaylandBackend::checkBackendReady()
         return;
     }
     disconnect(this, &WaylandBackend::shellSurfaceSizeChanged, this, &WaylandBackend::checkBackendReady);
+    m_ready = true;
     emit backendReady();
 }
 
+void WaylandBackend::connectNotify(const QMetaMethod &signal)
+{
+    if (m_ready && signal == QMetaMethod::fromSignal(&WaylandBackend::backendReady)) {
+        // backend is already ready, let's emit the signal
+        signal.invoke(this, Qt::QueuedConnection);
+    }
+}
+
 }
 
 } // KWin

wayland_backend.h

@@ -190,6 +190,10 @@ public:
     KWayland::Client::Surface *surface() const;
     QSize shellSurfaceSize() const;
     void installCursorImage(Qt::CursorShape shape);
+
+protected:
+    void connectNotify(const QMetaMethod &signal) override;
+
 Q_SIGNALS:
     void shellSurfaceSizeChanged(const QSize &size);
     void systemCompositorDied();
@@ -217,6 +221,7 @@ private:
     KWayland::Client::FullscreenShell *m_fullscreenShell;
     KWayland::Client::SubCompositor *m_subCompositor;
     WaylandCursor *m_cursor;
+    bool m_ready = false;
 
     KWIN_SINGLETON(WaylandBackend)
 };