Allow QObjects to opt in to receiving ParentAboutToChange/ParentChange

QWidget already handles this, but it might be useful for non-Widget
object hierarchies as well, such as in Qt Quick.

The flag is opt in, and as QWidget already handles these events by
itself (without checking any flags), we assert that we don't end up
in this code path, instead of enabling it for QWidget. The latter
would mean refactoring the QWidget code, with possible regressions.

Docs and header comments have been updated to reflect that this
event is not widget specific. (This is an issue with other events
as well, that are documented to say "widget", since they came
from a time when there was only QWidget, but nowadays apply to
e.g. QWindow as well. That's something for another fix though).

Change-Id: Ib71962131d6011c17dcce8c01bd8adcdaa58d798
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

src/corelib/kernel/qcoreevent.cpp

@@ -159,8 +159,10 @@ Q_TRACE_POINT(qtcore, QEvent_dtor, QEvent *event, QEvent::Type type);
     \value OrientationChange                The screens orientation has changes (QScreenOrientationChangeEvent).
     \value Paint                            Screen update necessary (QPaintEvent).
     \value PaletteChange                    Palette of the widget changed.
-    \value ParentAboutToChange              The widget parent is about to change.
-    \value ParentChange                     The widget parent has changed.
+    \value ParentAboutToChange              The object parent is about to change.
+                                            Only sent to some object types, such as QWidget.
+    \value ParentChange                     The object parent has changed.
+                                            Only sent to some object types, such as QWidget.
     \value PlatformPanel                    A platform specific panel has been requested.
     \value PlatformSurface                  A native platform surface has been created or is about to be destroyed (QPlatformSurfaceEvent).
     \omitvalue Pointer

src/corelib/kernel/qcoreevent.h

@@ -77,7 +77,7 @@ public:
         Hide = 18,                              // widget is hidden
         Close = 19,                             // request to close widget
         Quit = 20,                              // request to quit application
-        ParentChange = 21,                      // widget has been reparented
+        ParentChange = 21,                      // object has been reparented
         ParentAboutToChange = 131,              // sent just before the parent change is done
         ThreadChange = 22,                      // object has changed threads
         WindowActivate = 24,                    // window was activated

src/corelib/kernel/qobject.cpp

@@ -179,6 +179,7 @@ QObjectPrivate::QObjectPrivate(int version)
     isQuickItem = false;
     willBeWidget = false;
     wasWidget = false;
+    receiveParentEvents = false;                // If object wants ParentAboutToChange and ParentChange
 }
 
 QObjectPrivate::~QObjectPrivate()
@@ -2249,7 +2250,15 @@ void QObjectPrivate::setParent_helper(QObject *o)
             }
         }
     }
+
+    if (receiveParentEvents) {
+        Q_ASSERT(!isWidget); // Handled in QWidget
+        QEvent e(QEvent::ParentAboutToChange);
+        QCoreApplication::sendEvent(q, &e);
+    }
+
     parent = o;
+
     if (parent) {
         // object hierarchies are constrained to a single thread
         if (threadData.loadRelaxed() != parent->d_func()->threadData.loadRelaxed()) {
@@ -2265,6 +2274,12 @@ void QObjectPrivate::setParent_helper(QObject *o)
             }
         }
     }
+
+    if (receiveParentEvents) {
+        Q_ASSERT(!isWidget); // Handled in QWidget
+        QEvent e(QEvent::ParentChange);
+        QCoreApplication::sendEvent(q, &e);
+    }
 }
 
 /*!

src/corelib/kernel/qobject.h

@@ -72,7 +72,8 @@ public:
     uint isQuickItem : 1;
     uint willBeWidget : 1; // for handling widget-specific bits in QObject's ctor
     uint wasWidget : 1; // for properly cleaning up in QObject's dtor
-    uint unused : 21;
+    uint receiveParentEvents: 1;
+    uint unused : 20;
     QAtomicInt postedEvents;
     QDynamicMetaObjectData *metaObject;
     QBindingStorage bindingStorage;

tests/auto/corelib/kernel/qobject/tst_qobject.cpp

@@ -80,6 +80,7 @@ private slots:
     void signalBlocking();
     void blockingQueuedConnection();
     void childEvents();
+    void parentEvents();
     void installEventFilter();
     void deleteSelfInSlot();
     void disconnectSelfInSlotAndDeleteAfterEmit();
@@ -3200,6 +3201,78 @@ void tst_QObject::childEvents()
     }
 }
 
+void tst_QObject::parentEvents()
+{
+#ifdef QT_BUILD_INTERNAL
+    EventSpy::EventList expected;
+
+    {
+        // Parent events not enabled
+        QObject parent;
+        QObject child;
+
+        EventSpy spy;
+        child.installEventFilter(&spy);
+
+        QCoreApplication::postEvent(&child, new QEvent(QEvent::Type(QEvent::User + 1)));
+
+        child.setParent(&parent);
+
+        QCoreApplication::postEvent(&child, new QEvent(QEvent::Type(QEvent::User + 2)));
+
+        expected =
+            EventSpy::EventList();
+        QCOMPARE(spy.eventList(), expected);
+        spy.clear();
+
+        QCoreApplication::processEvents();
+
+        expected =
+            EventSpy::EventList()
+            << qMakePair(&child, QEvent::Type(QEvent::User + 1))
+            << qMakePair(&child, QEvent::Type(QEvent::User + 2));
+        QCOMPARE(spy.eventList(), expected);
+    }
+
+    {
+        // Parent events enabled
+        QObject parent;
+        QObject child;
+        auto *childPrivate = QObjectPrivate::get(&child);
+        childPrivate->receiveParentEvents = true;
+
+        EventSpy spy;
+        child.installEventFilter(&spy);
+
+        QCoreApplication::postEvent(&child, new QEvent(QEvent::Type(QEvent::User + 1)));
+
+        child.setParent(&parent);
+        child.setParent(nullptr);
+
+        QCoreApplication::postEvent(&child, new QEvent(QEvent::Type(QEvent::User + 2)));
+
+        expected =
+            EventSpy::EventList()
+            << qMakePair(&child, QEvent::ParentAboutToChange)
+            << qMakePair(&child, QEvent::ParentChange)
+            << qMakePair(&child, QEvent::ParentAboutToChange)
+            << qMakePair(&child, QEvent::ParentChange);
+        QCOMPARE(spy.eventList(), expected);
+        spy.clear();
+
+        QCoreApplication::processEvents();
+
+        expected =
+            EventSpy::EventList()
+            << qMakePair(&child, QEvent::Type(QEvent::User + 1))
+            << qMakePair(&child, QEvent::Type(QEvent::User + 2));
+        QCOMPARE(spy.eventList(), expected);
+    }
+#else
+    QSKIP("Needs QT_BUILD_INTERNAL");
+#endif
+}
+
 void tst_QObject::installEventFilter()
 {
     QEvent event(QEvent::User);