Add option to disable "session management by closing windows".

That feature is a poor man's session management for applications
that do not implement any specific session management features.
It badly interferes with proper session management support, so
applications must be able to disable it.

This enables fixing applications with
QGuiApplication::quitOnLastWindowClosed() true - the default -
dying too early, before they are enumerated for the list of
applications to restart on session restore, thus preventing them
from being restored. See
https://bugs.kde.org/show_bug.cgi?id=354724

[ChangeLog][QtGui] Qt asking to close windows on session exit as
a fallback session management mechanism has been made optional.
Disabling it fixes session management for applications that
implement full session management. See
QGuiApplication::isFallbackSessionManagementEnabled().

Task-number: QTBUG-49667
Change-Id: Ib22e58c9c64351dea8b7e2a74db91d26dd7ab7aa
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: David Faure <david.faure@kdab.com>

2 files changed, 18 insertions, 0 deletions

diff --git a/examples/widgets/mainwindows/application/mainwindow.cpp b/examples/widgets/mainwindows/application/mainwindow.cpp
index 86dfae166f..861b908189 100644
--- a/examples/widgets/mainwindows/application/mainwindow.cpp
+++ b/examples/widgets/mainwindows/application/mainwindow.cpp
@@ -59,6 +59,10 @@ MainWindow::MainWindow()
     connect(textEdit->document(), &QTextDocument::contentsChanged,
             this, &MainWindow::documentWasModified);
 
+    QGuiApplication::setFallbackSessionManagementEnabled(false);
+    connect(qApp, &QGuiApplication::commitDataRequest,
+            this, &MainWindow::commitData);
+
     setCurrentFile(QString());
     setUnifiedTitleAndToolBarOnMac(true);
 }
@@ -383,3 +387,15 @@ QString MainWindow::strippedName(const QString &fullFileName)
     return QFileInfo(fullFileName).fileName();
 }
 //! [49]
+
+void MainWindow::commitData(QSessionManager &manager)
+{
+    if (manager.allowsInteraction()) {
+        if (!maybeSave())
+            manager.cancel();
+    } else {
+        // Non-interactive: save without asking
+        if (textEdit->document()->isModified())
+            save();
+    }
+}
diff --git a/examples/widgets/mainwindows/application/mainwindow.h b/examples/widgets/mainwindows/application/mainwindow.h
index 08b4aa17f5..9712604125 100644
--- a/examples/widgets/mainwindows/application/mainwindow.h
+++ b/examples/widgets/mainwindows/application/mainwindow.h
@@ -47,6 +47,7 @@ QT_BEGIN_NAMESPACE
 class QAction;
 class QMenu;
 class QPlainTextEdit;
+class QSessionManager;
 QT_END_NAMESPACE
 
 //! [0]
@@ -69,6 +70,7 @@ private slots:
     bool saveAs();
     void about();
     void documentWasModified();
+    void commitData(QSessionManager &);
 
 private:
     void createActions();