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, 22 insertions, 23 deletions

diff --git a/src/widgets/kernel/qwidget.cpp b/src/widgets/kernel/qwidget.cpp
index 56e558eca0..c7303dde66 100644
--- a/src/widgets/kernel/qwidget.cpp
+++ b/src/widgets/kernel/qwidget.cpp
@@ -990,10 +990,10 @@ void QWidgetPrivate::init(QWidget *parentWidget, Qt::WindowFlags f)
     if (allWidgets)
         allWidgets->insert(q);
 
-    int targetScreen = -1;
+    QScreen *targetScreen = nullptr;
     if (parentWidget && parentWidget->windowType() == Qt::Desktop) {
         const QDesktopScreenWidget *sw = qobject_cast<const QDesktopScreenWidget *>(parentWidget);
-        targetScreen = sw ? sw->screenNumber() : 0;
+        targetScreen = sw ? sw->screen() : nullptr;
         parentWidget = nullptr;
     }
 
@@ -1006,10 +1006,10 @@ void QWidgetPrivate::init(QWidget *parentWidget, Qt::WindowFlags f)
     }
 #endif
 
-    if (targetScreen >= 0) {
-        topData()->initialScreenIndex = targetScreen;
+    if (targetScreen) {
+        topData()->initialScreen = targetScreen;
         if (QWindow *window = q->windowHandle())
-            window->setScreen(QGuiApplication::screens().value(targetScreen, nullptr));
+            window->setScreen(targetScreen);
     }
 
     data.fstrut_dirty = true;
@@ -1293,13 +1293,13 @@ void QWidgetPrivate::create()
     else
         win->resize(q->size());
     if (win->isTopLevel()) {
-        int screenNumber = topData()->initialScreenIndex;
-        topData()->initialScreenIndex = -1;
-        if (screenNumber < 0) {
-            screenNumber = q->windowType() != Qt::Desktop
-                ? QDesktopWidgetPrivate::screenNumber(q) : 0;
+        QScreen *targetScreen = topData()->initialScreen;
+        topData()->initialScreen = nullptr;
+        if (!targetScreen) {
+            targetScreen = q->windowType() != Qt::Desktop
+                ? q->screen() : nullptr;
         }
-        win->setScreen(QGuiApplication::screens().value(screenNumber, nullptr));
+        win->setScreen(targetScreen);
     }
 
     QSurfaceFormat format = win->requestedFormat();
@@ -1602,7 +1602,7 @@ void QWidgetPrivate::createTLExtra()
         x->sizeAdjusted = false;
         x->embedded = 0;
         x->window = nullptr;
-        x->initialScreenIndex = -1;
+        x->initialScreen = nullptr;
 
 #ifdef QWIDGET_EXTRA_DEBUG
         static int count = 0;
@@ -2507,8 +2507,8 @@ QScreen *QWidget::screen() const
         return associatedScreen;
     if (auto topLevel = window()) {
         if (auto topData = qt_widget_private(topLevel)->topData()) {
-            if (auto initialScreen = QGuiApplicationPrivate::screen_list.value(topData->initialScreenIndex))
-                return initialScreen;
+            if (topData->initialScreen)
+                return topData->initialScreen;
         }
         if (auto screenByPos = QGuiApplication::screenAt(topLevel->geometry().center()))
             return screenByPos;
@@ -3472,8 +3472,7 @@ QPoint QWidget::pos() const
     issues with windows.
 
     \note Do not use this function to find the width of a screen on
-    a \l{QDesktopWidget}{multiple screen desktop}. Read
-    \l{QDesktopWidget#Screen Geometry}{this note} for details.
+    a multi-screen desktop. See QScreen for details.
 
     By default, this property contains a value that depends on the user's
     platform and screen geometry.
@@ -10487,13 +10486,13 @@ void QWidgetPrivate::setParent_sys(QWidget *newparent, Qt::WindowFlags f)
     Qt::WindowFlags oldFlags = data.window_flags;
     bool wasCreated = q->testAttribute(Qt::WA_WState_Created);
 
-    int targetScreen = -1;
+    QScreen *targetScreen = nullptr;
     // Handle a request to move the widget to a particular screen
     if (newparent && newparent->windowType() == Qt::Desktop) {
         // make sure the widget is created on the same screen as the
         // programmer specified desktop widget
         const QDesktopScreenWidget *sw = qobject_cast<const QDesktopScreenWidget *>(newparent);
-        targetScreen = sw ? sw->screenNumber() : 0;
+        targetScreen = sw ? sw->screen() : nullptr;
         newparent = nullptr;
     }
 
@@ -10523,9 +10522,9 @@ void QWidgetPrivate::setParent_sys(QWidget *newparent, Qt::WindowFlags f)
 
     if (!newparent) {
         f |= Qt::Window;
-        if (targetScreen == -1) {
+        if (!targetScreen) {
             if (parent)
-                targetScreen = QDesktopWidgetPrivate::screenNumber(q->parentWidget()->window());
+                targetScreen = q->parentWidget()->window()->screen();
         }
     }
 
@@ -10568,12 +10567,12 @@ void QWidgetPrivate::setParent_sys(QWidget *newparent, Qt::WindowFlags f)
     q->setAttribute(Qt::WA_WState_ExplicitShowHide, explicitlyHidden);
 
     // move the window to the selected screen
-    if (!newparent && targetScreen != -1) {
+    if (!newparent && targetScreen) {
         // only if it is already created
         if (q->testAttribute(Qt::WA_WState_Created))
-            q->windowHandle()->setScreen(QGuiApplication::screens().value(targetScreen, 0));
+            q->windowHandle()->setScreen(targetScreen);
         else
-            topData()->initialScreenIndex = targetScreen;
+            topData()->initialScreen = targetScreen;
     }
 }