QDrag seems to block mouse events.

[Weener]

Hi,

Firstly - thanks - the amount of gold in this forum has allowed me to trawl for answers for about 6 months, and not have to post. I'm impressed!

Unfortunately I am pretty confident I actually have an issue here. Hopefully I am being stupid, for that would mean a simple fix is in order.

I've sub classes QTreeView, and enabled it to provide a QDrag object, on mouse move. The trouble is; once the drag is over, signals like 'entered' are no longer emitted; and this is my issue; I'd like them back.

Below is a minimal coding example that produces the issue;

The TreeView shows the current item highlighted under the cursor, then after the drag, this stops, and the "ItemEntered" slot is never called. (Items don't have to be dragged anywhere - the drag just has to start - and things break)

Any help would be most appreciated.

Thanks

#include <QApplication>
#include <QStandardItemModel>
#include <QTreeView>
#include <QMouseEvent>
 
//////////////////////////////////////////////////////
// This is so we can provide "drag from" to the main window.
class DragAndDropTreeView : public QTreeView
{
    Q_OBJECT
public:
	DragAndDropTreeView(QObject * parent) ;
 
public slots:
	void ItemEntered(const QModelIndex & index) ;
 
protected:
	void mousePressEvent(QMouseEvent *) ;
	void mouseMoveEvent(QMouseEvent *) ;
	void mouseReleaseEvent(QMouseEvent *) ;
 
	QPoint _clickPoint ;
	int _activeItem ;
	bool _mouseDown ;
	bool _displayingShadow ;
} ;
 
DragAndDropTreeView::DragAndDropTreeView(QObject * parent)
{
	_mouseDown = false ;
	_displayingShadow = false ;
	setMouseTracking(true);
	connect(this,SIGNAL(entered(const QModelIndex &)),this,SLOT(ItemEntered(const QModelIndex &))) ;
}
 
void DragAndDropTreeView::mousePressEvent(QMouseEvent * e)
{
    _clickPoint = e->pos() ;
	setCursor(Qt::ClosedHandCursor);
	QTreeView::mousePressEvent(e) ;
	// this is ugly, and probably inefficient.
	_activeItem = selectedIndexes()[0].sibling(selectionModel()->currentIndex().row(),0).data().toInt() ; ;
	_mouseDown = true ;
}
 
void DragAndDropTreeView::mouseMoveEvent(QMouseEvent * e)
{
	if (!_mouseDown) 
	{
		QTreeView::mouseMoveEvent(e) ;
		return ;
	}
	if (QLineF(e->pos(), _clickPoint).length() < QApplication::startDragDistance()) 
	{
		QTreeView::mouseMoveEvent(e) ;
		return;
    }
 
    QDrag *drag = new QDrag(this) ;
	if (drag == NULL) return ;
    QMimeData *mime = new QMimeData ;
	if (mime == NULL) return ;
    drag->setMimeData(mime) ;
 
	mime->setText(QString("Temp/%1").arg(_activeItem));
 
	// note to the unwary !! THIS BLOCKS!
	drag->exec() ;
	QTreeView::mouseMoveEvent(e) ;
}
 
void DragAndDropTreeView::mouseReleaseEvent(QMouseEvent * e)
{
	_mouseDown = false ;
	QTreeView::mouseReleaseEvent(e) ;
}
 
// this is called properly before any drag event - but not after.
void DragAndDropTreeView::ItemEntered(const QModelIndex & index) 
{
	_displayingShadow = true ;
}
 
 
int main(int argc, char * argv[])
{
    QApplication app(argc,argv);
 
	DragAndDropTreeView * DDTV = new DragAndDropTreeView(NULL) ;
 
	// set up the model to accept data.
	QStandardItemModel * _model = new QStandardItemModel(0, 1) ;
	// slap some stuff in.
    _model->setHeaderData(0, Qt::Horizontal, QObject::tr("Number"));
	for (int i = 0 ; i < 10 ; i++)
	{
		_model->insertRow(i);
		_model->setData(_model->index(i, 0), QString("%1").arg(i));
	}
	// show all the beauty of it
    DDTV->setRootIsDecorated(false) ;
    DDTV->setModel(_model) ;
    DDTV->setDragEnabled(true) ;
    DDTV->setSortingEnabled(true) ;
	DDTV->setSelectionMode(QAbstractItemView::SingleSelection) ;
	DDTV->show() ;
    return app.exec();
}

#include <QApplication>
#include <QStandardItemModel>
#include <QTreeView>
#include <QMouseEvent>

//////////////////////////////////////////////////////
// This is so we can provide "drag from" to the main window.
class DragAndDropTreeView : public QTreeView
{
    Q_OBJECT
public:
	DragAndDropTreeView(QObject * parent) ;

public slots:
	void ItemEntered(const QModelIndex & index) ;

protected:
	void mousePressEvent(QMouseEvent *) ;
	void mouseMoveEvent(QMouseEvent *) ;
	void mouseReleaseEvent(QMouseEvent *) ;

	QPoint _clickPoint ;
	int _activeItem ;
	bool _mouseDown ;
	bool _displayingShadow ;
} ;

DragAndDropTreeView::DragAndDropTreeView(QObject * parent)
{
	_mouseDown = false ;
	_displayingShadow = false ;
	setMouseTracking(true);
	connect(this,SIGNAL(entered(const QModelIndex &)),this,SLOT(ItemEntered(const QModelIndex &))) ;
}

void DragAndDropTreeView::mousePressEvent(QMouseEvent * e)
{
    _clickPoint = e->pos() ;
	setCursor(Qt::ClosedHandCursor);
	QTreeView::mousePressEvent(e) ;
	// this is ugly, and probably inefficient.
	_activeItem = selectedIndexes()[0].sibling(selectionModel()->currentIndex().row(),0).data().toInt() ; ;
	_mouseDown = true ;
}

void DragAndDropTreeView::mouseMoveEvent(QMouseEvent * e)
{
	if (!_mouseDown) 
	{
		QTreeView::mouseMoveEvent(e) ;
		return ;
	}
	if (QLineF(e->pos(), _clickPoint).length() < QApplication::startDragDistance()) 
	{
		QTreeView::mouseMoveEvent(e) ;
		return;
    }

    QDrag *drag = new QDrag(this) ;
	if (drag == NULL) return ;
    QMimeData *mime = new QMimeData ;
	if (mime == NULL) return ;
    drag->setMimeData(mime) ;

	mime->setText(QString("Temp/%1").arg(_activeItem));

	// note to the unwary !! THIS BLOCKS!
	drag->exec() ;
	QTreeView::mouseMoveEvent(e) ;
}

void DragAndDropTreeView::mouseReleaseEvent(QMouseEvent * e)
{
	_mouseDown = false ;
	QTreeView::mouseReleaseEvent(e) ;
}

// this is called properly before any drag event - but not after.
void DragAndDropTreeView::ItemEntered(const QModelIndex & index) 
{
	_displayingShadow = true ;
}


int main(int argc, char * argv[])
{
    QApplication app(argc,argv);

	DragAndDropTreeView * DDTV = new DragAndDropTreeView(NULL) ;

	// set up the model to accept data.
	QStandardItemModel * _model = new QStandardItemModel(0, 1) ;
	// slap some stuff in.
    _model->setHeaderData(0, Qt::Horizontal, QObject::tr("Number"));
	for (int i = 0 ; i < 10 ; i++)
	{
		_model->insertRow(i);
		_model->setData(_model->index(i, 0), QString("%1").arg(i));
	}
	// show all the beauty of it
    DDTV->setRootIsDecorated(false) ;
    DDTV->setModel(_model) ;
    DDTV->setDragEnabled(true) ;
    DDTV->setSortingEnabled(true) ;
	DDTV->setSelectionMode(QAbstractItemView::SingleSelection) ;
	DDTV->show() ;
    return app.exec();
}

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[Santosh Reddy]

Add this

void DragAndDropTreeView::mouseMoveEvent(QMouseEvent * e)
{
    //....
    _mouseDown = false;  //<<<<<<<<<< Add in the end
}
 
void DragAndDropTreeView::mouseReleaseEvent(QMouseEvent * e) //<<<<< Mouse release event will not be called when drag is released, as it will be captured by QDrag
{
    _mouseDown = false ;
    QTreeView::mouseReleaseEvent(e) ;
}

void DragAndDropTreeView::mouseMoveEvent(QMouseEvent * e)
{
    //....
    _mouseDown = false;  //<<<<<<<<<< Add in the end
}

void DragAndDropTreeView::mouseReleaseEvent(QMouseEvent * e) //<<<<< Mouse release event will not be called when drag is released, as it will be captured by QDrag
{
    _mouseDown = false ;
    QTreeView::mouseReleaseEvent(e) ;
}

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[Weener]

Oh - my. Thanks. Well - at least this fits into the simple fix category. Sorry to trouble you.