Phase 2 of removing QDesktopWidget

Remove QDestopWidget public header, simplify the implementation that
maintains a Qt::Desktop type QWidget for each QScreen, and turn
QWidget's initial target screen into a QScreen pointer.

QApplication::desktop() now takes an optional QScreen pointer, and
returns a QWidget pointer, so that applications and widgets can get
access to the root widget for a specific screen without having to
resort to private APIs.

QDesktopWidgetPrivate implementations to look up a screen for an index,
widget, or point are now all inline functions that thinly wrap
QGuiApplication::screens/screenAt calls. We should consider adding those
as convenience APIs to QScreen instead.

Note that QWidget::screen is assumed to return a valid pointer; there is
code that handles the case that it returns nullptr (but also code that
trusts that it never is nullptr), so this needs to be defined, verified
with tests, and asserted. We can then simplify the code further.

Change-Id: Ifc89be65a0dce265b6729feaf54121c35137cb94
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

1 files changed, 15 insertions, 7 deletions

diff --git a/src/widgets/kernel/qapplication.cpp b/src/widgets/kernel/qapplication.cpp
index 0e96e8bc69..1af94efdf9 100644
--- a/src/widgets/kernel/qapplication.cpp
+++ b/src/widgets/kernel/qapplication.cpp
@@ -42,7 +42,7 @@
 #include "qapplication.h"
 #include "qclipboard.h"
 #include "qcursor.h"
-#include "qdesktopwidget.h"
+#include "qdesktopwidget_p.h"
 #include "qdir.h"
 #include "qevent.h"
 #include "qfile.h"
@@ -2521,22 +2521,30 @@ void QApplicationPrivate::sendSyntheticEnterLeave(QWidget *widget)
 /*!
     \internal
 
-    Returns the desktop widget (also called the root window).
+    Returns the desktop widget (also called the root window) for \a screen.
+
+    If \a screen is nullptr, then the widget that represents the entire virtual
+    desktop is returned, and its geometry will be the union of all screens.
+
+    Use the desktop widget for a specific screen as the parent of a new toplevel
+    widget to position the widget on a specific screen.
 
     The desktop may be composed of multiple screens, so it would be incorrect,
     for example, to attempt to \e center some widget in the desktop's geometry.
-    QDesktopWidget has various functions for obtaining useful geometries upon
-    the desktop, such as QDesktopWidget::screenGeometry() and
-    QDesktopWidget::availableGeometry().
+    Use QScreen::geometry() and QScreen::availableGeometry() to get the dimensions
+    of a specific screen instead.
 */
-QDesktopWidget *QApplication::desktop()
+QWidget *QApplication::desktop(QScreen *screen)
 {
     CHECK_QAPP_INSTANCE(nullptr)
     if (!qt_desktopWidget || // not created yet
          !(qt_desktopWidget->windowType() == Qt::Desktop)) { // reparented away
         qt_desktopWidget = new QDesktopWidget();
     }
-    return qt_desktopWidget;
+    if (!screen)
+        return qt_desktopWidget;
+    QDesktopWidgetPrivate *dwp = static_cast<QDesktopWidgetPrivate*>(qt_widget_private(qt_desktopWidget));
+    return dwp->widgetForScreen(screen);
 }
 
 /*