QStackedLayout: Fix crash when focus widget is destroyed in hide()

We also have to make sure that when moving back to a page
that has a focusWidget(), the focus should go to the focusWidget()

Task-number: QTBUG-18242

Change-Id: Ibfa7d6361c1a456480b2f1584a88ef4c4f405709
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/widgets/kernel/qstackedlayout.cpp

@@ -300,7 +300,9 @@ void QStackedLayout::setCurrentIndex(int index)
         parent->setUpdatesEnabled(false);
     }
 
-    QWidget *fw = parent ? parent->window()->focusWidget() : 0;
+    QPointer<QWidget> fw = parent ? parent->window()->focusWidget() : 0;
+    const bool focusWasOnOldPage = fw && (prev && prev->isAncestorOf(fw));
+
     if (prev) {
         prev->clearFocus();
         if (d->stackingMode == StackOne)
@@ -315,24 +317,25 @@ void QStackedLayout::setCurrentIndex(int index)
     // was somewhere on the outgoing widget.
 
     if (parent) {
-        if (fw && (prev && prev->isAncestorOf(fw))) { // focus was on old page
+        if (focusWasOnOldPage) {
             // look for the best focus widget we can find
             if (QWidget *nfw = next->focusWidget())
                 nfw->setFocus();
             else {
                 // second best: first child widget in the focus chain
-                QWidget *i = fw;
-                while ((i = i->nextInFocusChain()) != fw) {
-                    if (((i->focusPolicy() & Qt::TabFocus) == Qt::TabFocus)
-                        && !i->focusProxy() && i->isVisibleTo(next) && i->isEnabled()
-                        && next->isAncestorOf(i)) {
-                        i->setFocus();
-                        break;
+                if (QWidget *i = fw) {
+                    while ((i = i->nextInFocusChain()) != fw) {
+                        if (((i->focusPolicy() & Qt::TabFocus) == Qt::TabFocus)
+                            && !i->focusProxy() && i->isVisibleTo(next) && i->isEnabled()
+                            && next->isAncestorOf(i)) {
+                            i->setFocus();
+                            break;
+                        }
                     }
+                    // third best: incoming widget
+                    if (i == fw )
+                        next->setFocus();
                 }
-                // third best: incoming widget
-                if (i == fw )
-                    next->setFocus();
             }
         }
     }

tests/auto/widgets/widgets/qstackedwidget/tst_qstackedwidget.cpp

@@ -47,6 +47,7 @@
 #include <qstackedwidget.h>
 #include <qpushbutton.h>
 #include <QHBoxLayout>
+#include <qlineedit.h>
 
 class tst_QStackedWidget : public QObject
 {
@@ -59,6 +60,7 @@ public:
 private slots:
     void getSetCheck();
     void testMinimumSize();
+    void dynamicPages();
 };
 
 tst_QStackedWidget::tst_QStackedWidget()
@@ -117,5 +119,91 @@ void tst_QStackedWidget::getSetCheck()
     delete var2;
 }
 
+// QTBUG-18242, a widget that deletes its children in hideEvent().
+// This caused a crash in QStackedLayout::setCurrentIndex() since
+// the focus widget was destroyed while hiding the previous page.
+class TestPage : public QWidget
+{
+public:
+    TestPage (bool staticWidgets = false) : QWidget(0), m_staticWidgets(staticWidgets)
+    {
+        new QVBoxLayout (this);
+    }
+
+    ~TestPage() {
+        destroyWidgets();
+    }
+
+    void setN(int n)
+    {
+        m_n = n;
+        if (m_staticWidgets)
+            createWidgets();
+    }
+
+    virtual void showEvent (QShowEvent *)
+    {
+        if (!m_staticWidgets)
+            createWidgets();
+    }
+
+    virtual void hideEvent (QHideEvent *)
+    {
+        if (!m_staticWidgets)
+            destroyWidgets();
+    }
+
+private:
+    void createWidgets() {
+        for (int i = 0; i < m_n; ++i) {
+            QLineEdit *le = new QLineEdit(this);
+            le->setObjectName(QString::fromLatin1("lineEdit%1").arg(i));
+            layout ()->addWidget(le);
+            m_les << le;
+        }
+    }
+
+    void destroyWidgets()
+    {
+        qDeleteAll(m_les);
+        m_les.clear ();
+    }
+
+    int m_n;
+    const bool m_staticWidgets;
+    QList<QLineEdit*> m_les;
+};
+
+void tst_QStackedWidget::dynamicPages()
+{
+    QStackedWidget *sw = new QStackedWidget;
+
+    TestPage *w1 = new TestPage(true);
+    w1->setN(3);
+
+    TestPage *w2 = new TestPage;
+    w2->setN(3);
+
+    sw->addWidget(w1);
+    sw->addWidget(w2);
+
+    QLineEdit *le11 = w1->findChild<QLineEdit*>(QLatin1String("lineEdit1"));
+    le11->setFocus();   // set focus to second widget in the page
+    sw->resize(200, 200);
+    sw->show();
+    qApp->setActiveWindow(sw);
+    QTest::qWaitForWindowActive(sw);
+    QTRY_COMPARE(QApplication::focusWidget(), le11);
+
+    sw->setCurrentIndex(1);
+    QLineEdit *le22 = w2->findChild<QLineEdit*>(QLatin1String("lineEdit2"));
+    le22->setFocus();
+    QTRY_COMPARE(QApplication::focusWidget(), le22);
+    // Going back should move focus back to le11
+    sw->setCurrentIndex(0);
+    QTRY_COMPARE(QApplication::focusWidget(), le11);
+
+}
+
 QTEST_MAIN(tst_QStackedWidget)
 #include "tst_qstackedwidget.moc"