[wayland] Add a base implementation for Screens in a basic setup

X11, Wayland, virtual, Framebuffer and hwcomposer have extremely
similar screens implementations. Let's add a base implementation
for them as a BasicScreens.

abstract_backend.cpp

@@ -330,4 +330,9 @@ EGLContext AbstractBackend::sceneEglContext() const
     return EGL_NO_CONTEXT;
 }
 
+QSize AbstractBackend::screenSize() const
+{
+    return QSize();
+}
+
 }

abstract_backend.h

@@ -60,6 +60,14 @@ public:
      **/
     virtual EGLContext sceneEglContext() const;
 
+    /**
+     * Implementing subclasses should provide a size in case the backend represents
+     * a basic screen and uses the BasicScreens.
+     *
+     * Base implementation returns an invalid size.
+     **/
+    virtual QSize screenSize() const;
+
     bool usesSoftwareCursor() const {
         return m_softWareCursor;
     }
@@ -114,6 +122,10 @@ Q_SIGNALS:
     void initFailed();
     void cursorChanged();
     void readyChanged(bool);
+    /**
+     * Emitted by backends using a one screen (nested window) approach and when the size of that changes.
+     **/
+    void screenSizeChanged();
 
 protected:
     explicit AbstractBackend(QObject *parent = nullptr);

screens.cpp

@@ -184,4 +184,50 @@ int Screens::intersecting(const QRect &r) const
     return cnt;
 }
 
+BasicScreens::BasicScreens(AbstractBackend *backend, QObject *parent)
+    : Screens(parent)
+    , m_backend(backend)
+{
+}
+
+BasicScreens::~BasicScreens() = default;
+
+void BasicScreens::init()
+{
+    KWin::Screens::init();
+#ifndef KWIN_UNIT_TEST
+    connect(m_backend, &AbstractBackend::screenSizeChanged,
+            this, &BasicScreens::startChangedTimer);
+#endif
+    updateCount();
+    emit changed();
+}
+
+QRect BasicScreens::geometry(int screen) const
+{
+    if (screen == 0) {
+        return QRect(QPoint(0, 0), size(screen));
+    }
+    return QRect();
+}
+
+QSize BasicScreens::size(int screen) const
+{
+    if (screen == 0) {
+        return m_backend->screenSize();
+    }
+    return QSize();
+}
+
+void BasicScreens::updateCount()
+{
+    setCount(1);
+}
+
+int BasicScreens::number(const QPoint &pos) const
+{
+    Q_UNUSED(pos)
+    return 0;
+}
+
 } // namespace

screens.h

@@ -33,6 +33,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 namespace KWin
 {
 class AbstractClient;
+class AbstractBackend;
 
 class KWIN_EXPORT Screens : public QObject
 {
@@ -148,6 +149,26 @@ private:
     KWIN_SINGLETON(Screens)
 };
 
+/**
+ * @brief A base implementation for backends with just a (nested) window
+ **/
+class KWIN_EXPORT BasicScreens : public Screens
+{
+    Q_OBJECT
+public:
+    BasicScreens(AbstractBackend *backend, QObject *parent = nullptr);
+    virtual ~BasicScreens();
+
+    void init() override;
+    QRect geometry(int screen) const override;
+    int number(const QPoint &pos) const override;
+    QSize size(int screen) const override;
+    void updateCount() override;
+
+private:
+    AbstractBackend *m_backend;
+};
+
 inline
 void Screens::setConfig(KSharedConfig::Ptr config)
 {