Notify when the primary screen changes

Makes it possible to notify that the QGuiApplication::primaryScreen has
changed.
XCB backend adopts the new API, as it was accessing QGuiApplication private
API directly.

Change-Id: Icde05c44138265f865fa42d2cd6974c552fdc5e2
Task-number: QTBUG-38404
Task-number: QTBUG-40659
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@theqtcompany.com>

src/gui/kernel/qguiapplication.cpp

@@ -921,6 +921,18 @@ QList<QScreen *> QGuiApplication::screens()
     \since 5.4
 */
 
+/*!
+    \fn void QGuiApplication::primaryScreenChanged(QScreen *screen)
+
+    This signal is emitted whenever the primary \a screen changes. This way
+    applications can keep track of the primaryScreen and react if there is a
+    new primary screen.
+
+    \sa primaryScreen
+
+    \since 5.6
+*/
+
 
 /*!
     Returns the highest screen device pixel ratio found on

src/gui/kernel/qguiapplication.h

@@ -70,6 +70,7 @@ class Q_GUI_EXPORT QGuiApplication : public QCoreApplication
     Q_PROPERTY(Qt::LayoutDirection layoutDirection READ layoutDirection WRITE setLayoutDirection NOTIFY layoutDirectionChanged)
     Q_PROPERTY(QString platformName READ platformName STORED false)
     Q_PROPERTY(bool quitOnLastWindowClosed  READ quitOnLastWindowClosed WRITE setQuitOnLastWindowClosed)
+    Q_PROPERTY(QScreen *primaryScreen READ primaryScreen NOTIFY primaryScreenChanged STORED false)
 
 public:
 #ifdef Q_QDOC
@@ -158,6 +159,7 @@ Q_SIGNALS:
     void fontDatabaseChanged();
     void screenAdded(QScreen *screen);
     void screenRemoved(QScreen *screen);
+    void primaryScreenChanged(QScreen *screen);
     void lastWindowClosed();
     void focusObjectChanged(QObject *focusObject);
     void focusWindowChanged(QWindow *focusWindow);

src/gui/kernel/qplatformintegration.cpp

@@ -456,6 +456,24 @@ void QPlatformIntegration::screenAdded(QPlatformScreen *ps, bool isPrimary)
         QGuiApplicationPrivate::screen_list.append(screen);
     }
     emit qGuiApp->screenAdded(screen);
+
+    if (isPrimary)
+        emit qGuiApp->primaryScreenChanged(screen);
+}
+
+/*!
+  Just removes the screen, call destroyScreen instead.
+
+  \sa destroyScreen()
+*/
+
+void QPlatformIntegration::removeScreen(QScreen *screen)
+{
+    const bool wasPrimary = (!QGuiApplicationPrivate::screen_list.isEmpty() && QGuiApplicationPrivate::screen_list[0] == screen);
+    QGuiApplicationPrivate::screen_list.removeOne(screen);
+
+    if (wasPrimary && qGuiApp && !QGuiApplicationPrivate::screen_list.isEmpty())
+        emit qGuiApp->primaryScreenChanged(QGuiApplicationPrivate::screen_list[0]);
 }
 
 /*!
@@ -469,11 +487,30 @@ void QPlatformIntegration::screenAdded(QPlatformScreen *ps, bool isPrimary)
 void QPlatformIntegration::destroyScreen(QPlatformScreen *screen)
 {
     QScreen *qScreen = screen->screen();
-    QGuiApplicationPrivate::screen_list.removeOne(qScreen);
+    removeScreen(qScreen);
     delete qScreen;
     delete screen;
 }
 
+/*!
+  Should be called whenever the primary screen changes.
+
+  When the screen specified as primary changes, this method will notify
+  QGuiApplication and emit the QGuiApplication::primaryScreenChanged signal.
+ */
+
+void QPlatformIntegration::setPrimaryScreen(QPlatformScreen *newPrimary)
+{
+    QScreen* newPrimaryScreen = newPrimary->screen();
+    int idx = QGuiApplicationPrivate::screen_list.indexOf(newPrimaryScreen);
+    Q_ASSERT(idx >= 0);
+    if (idx == 0)
+        return;
+
+    QGuiApplicationPrivate::screen_list.swap(0, idx);
+    emit qGuiApp->primaryScreenChanged(newPrimaryScreen);
+}
+
 QStringList QPlatformIntegration::themeNames() const
 {
     return QStringList();

src/gui/kernel/qplatformintegration.h

@@ -172,9 +172,13 @@ public:
     virtual QOpenGLContext::OpenGLModuleType openGLModuleType();
 #endif
     virtual void setApplicationIcon(const QIcon &icon) const;
+
+    void removeScreen(QScreen *screen);
+
 protected:
     void screenAdded(QPlatformScreen *screen, bool isPrimary = false);
     void destroyScreen(QPlatformScreen *screen);
+    void setPrimaryScreen(QPlatformScreen *newPrimary);
 };
 
 QT_END_NAMESPACE

src/gui/kernel/qplatformscreen.cpp

@@ -56,7 +56,7 @@ QPlatformScreen::~QPlatformScreen()
     Q_D(QPlatformScreen);
     if (d->screen) {
         qWarning("Manually deleting a QPlatformScreen. Call QPlatformIntegration::destroyScreen instead.");
-        QGuiApplicationPrivate::screen_list.removeOne(d->screen);
+        QGuiApplicationPrivate::platformIntegration()->removeScreen(d->screen);
         delete d->screen;
     }
 }

src/plugins/platforms/xcb/qxcbconnection.cpp

@@ -305,13 +305,10 @@ void QXcbConnection::updateScreens(const xcb_randr_notify_event_t *event)
                     screen->updateRefreshRate(output.mode);
 
                 // If the screen became primary, reshuffle the order in QGuiApplicationPrivate
-                // TODO: add a proper mechanism for updating primary screen
                 if (!wasPrimary && screen->isPrimary()) {
-                    QScreen *realScreen = static_cast<QPlatformScreen*>(screen)->screen();
-                    QGuiApplicationPrivate::screen_list.removeOne(realScreen);
-                    QGuiApplicationPrivate::screen_list.prepend(realScreen);
-                    m_screens.removeOne(screen);
-                    m_screens.prepend(screen);
+                    const int idx = m_screens.indexOf(screen);
+                    m_screens.swap(0, idx);
+                    QXcbIntegration::instance()->setPrimaryScreen(screen);
                 }
                 qCDebug(lcQpaScreen) << "output has changed" << screen;
             }

src/widgets/kernel/qdesktopwidget.cpp

@@ -134,6 +134,7 @@ QDesktopWidget::QDesktopWidget()
     setObjectName(QLatin1String("desktop"));
     d->_q_updateScreens();
     connect(qApp, SIGNAL(screenAdded(QScreen*)), this, SLOT(_q_updateScreens()));
+    connect(qApp, SIGNAL(primaryScreenChanged(QScreen*)), this, SIGNAL(primaryScreenChanged()));
 }
 
 QDesktopWidget::~QDesktopWidget()

src/widgets/kernel/qdesktopwidget.h

@@ -77,6 +77,7 @@ Q_SIGNALS:
     void resized(int);
     void workAreaResized(int);
     void screenCountChanged(int);
+    void primaryScreenChanged();
 
 protected:
     void resizeEvent(QResizeEvent *e) Q_DECL_OVERRIDE;

src/widgets/kernel/qdesktopwidget.qdoc

@@ -271,3 +271,17 @@
 
     \sa screenCount
 */
+
+
+/*!
+    \fn void QDesktopWidget::primaryScreenChanged()
+
+    \since 5.6
+
+    \brief This signal is emitted whenever the primary screen changes.
+
+    \note This doesn't mean the QDesktopWidget::primaryScreen index will
+    necessarily be different, but now it will refer to the new primary screen.
+
+    \sa primaryScreen, screenGeometry()
+*/