本文記錄QChildEvent事件相關代碼分析。

注1：限于筆者研究水平，難免有表述不當，歡迎批評指正。

注2：博文會不定期更新，敬請關注。

一、QChildEvent的發送

分析QObject::setParent代碼，當修改父對象時，便會舊parent發送QEvent::ChildRemoved事件；向新parent發送QEvent::ChildAdded事件。

/*!Makes the object a child of \a parent.\sa parent(), children()
*/
void QObject::setParent(QObject *parent)
{Q_D(QObject);Q_ASSERT(!d->isWidget);d->setParent_helper(parent);
}

void QObjectPrivate::setParent_helper(QObject *o)
{Q_Q(QObject);Q_ASSERT_X(q != o, Q_FUNC_INFO, "Cannot parent a QObject to itself");
#ifdef QT_DEBUGconst auto checkForParentChildLoops = qScopeGuard([&](){int depth = 0;auto p = parent;while (p) {if (++depth == CheckForParentChildLoopsWarnDepth) {qWarning("QObject %p (class: '%s', object name: '%s') may have a loop in its parent-child chain; ""this is undefined behavior",q, q->metaObject()->className(), qPrintable(q->objectName()));}p = p->parent();}});
#endifif (o == parent)return;if (parent) {QObjectPrivate *parentD = parent->d_func();if (parentD->isDeletingChildren && wasDeleted&& parentD->currentChildBeingDeleted == q) {// don't do anything since QObjectPrivate::deleteChildren() already// cleared our entry in parentD->children.} else {const int index = parentD->children.indexOf(q);if (parentD->isDeletingChildren) {parentD->children[index] = 0;} else {parentD->children.removeAt(index);if (sendChildEvents && parentD->receiveChildEvents) {QChildEvent e(QEvent::ChildRemoved, q);QCoreApplication::sendEvent(parent, &e);}}}}parent = o;if (parent) {// object hierarchies are constrained to a single threadif (threadData != parent->d_func()->threadData) {qWarning("QObject::setParent: Cannot set parent, new parent is in a different thread");parent = 0;return;}parent->d_func()->children.append(q);if(sendChildEvents && parent->d_func()->receiveChildEvents) {if (!isWidget) {QChildEvent e(QEvent::ChildAdded, q);QCoreApplication::sendEvent(parent, &e);}}}if (!wasDeleted && !isDeletingChildren && declarativeData && QAbstractDeclarativeData::parentChanged)QAbstractDeclarativeData::parentChanged(declarativeData, q, o);
}

因為QObject構造函數可以設置parent，此時發送QChildEvent事件，但對象并不一定創建完畢。

Refs. from QObject::childEvent(QChildEvent *)

QEvent::ChildAdded?and?QEvent::ChildRemoved?events are sent to objects when children are added or removed. In both cases you can only rely on the child being a?QObject, or if?isWidgetType() returns?true, a?QWidget. (This is because, in the?ChildAdded?case, the child is not yet fully constructed, and in the?ChildRemoved?case it might have been destructed already).

二、應用案例

在SALOME中，SUIT_Desktop通過響應QChildEvent消息來講SUIT_ViewWindow添加到SUIT_Desktop中。

class SUIT_Desktop::ReparentEvent : public QEvent
{
public:ReparentEvent( Type t, QObject* obj ) : QEvent( t ), myObj( obj ) {};QObject* object() const { return myObj; }private:QPointer<QObject> myObj;
};

/*!Child event.
*/
void SUIT_Desktop::childEvent( QChildEvent* e )
{if ( e->type() == QEvent::ChildAdded && e->child()->isWidgetType() ) {// The following line is a workaround to avoid showing view window as a top-level window// before re-parenting it to workstack (issue #23467).// See SUIT_ViewWindow::setVisible() and SUIT_Desktop::customEvent().e->child()->setProperty("blockShow", true );QApplication::postEvent( this, new ReparentEvent( QEvent::Type( Reparent ), e->child() ) );}else {QtxMainWindow::childEvent( e );}
}void SUIT_Desktop::customEvent( QEvent* e )
{if ( (int)e->type() != Reparent )return;ReparentEvent* re = (ReparentEvent*)e;SUIT_ViewWindow* wid = ::qobject_cast<SUIT_ViewWindow*>( re->object() );if ( wid ){bool invis = wid->testAttribute( Qt::WA_WState_ExplicitShowHide ) &&wid->testAttribute( Qt::WA_WState_Hidden );// The following line is a workaround to avoid showing view window as a top-level window// before re-parenting it to workstack (issue #23467).// See SUIT_ViewWindow::setVisible() and SUIT_Desktop::childEvent().wid->setProperty("blockShow", false);addWindow( wid );wid->setVisible( !invis );}
}

Refs. from?QObject::customEvent(QEvent *event)?

This event handler can be reimplemented in a subclass to receive custom events. Custom events are user-defined events with a type value at least as large as the?QEvent::User?item of the?QEvent::Type?enum, and is typically a?QEvent?subclass. The event is passed in the?event?parameter.?

/*! Constructor.*/
SUIT_ViewWindow::SUIT_ViewWindow( SUIT_Desktop* theDesktop ): QMainWindow( theDesktop ), myManager( 0 ), myIsDropDown( true ), mySyncAction( 0 )
{myDesktop = theDesktop;setWindowIcon( myDesktop ? myDesktop->windowIcon() : QApplication::windowIcon() );setAttribute( Qt::WA_DeleteOnClose );myToolMgr = new QtxActionToolMgr( this );setProperty( "VectorsMode", false );
}

網絡

QChildEventhttps://doc.qt.io/qt-6/qchildevent.html

QObject::childEvent(QChildEvent* event)https://doc.qt.io/qt-6/qobject.html#childEvent

QObject::customEvent(QEvent *event)https://doc.qt.io/qt-6/qobject.html#customEvent

SALOME源碼分析：GUI模塊https://blog.csdn.net/qq_26221775/article/details/127759145?spm=1001.2014.3001.5502

SALOME源碼分析: GUI模塊主要組件https://blog.csdn.net/qq_26221775/article/details/148845050?spm=1001.2014.3001.5502