Creating a Pixmap for drag and drop

[spirit]

did you set 'setAcceptDrops(true)'?

[Cruz]

I can drag and drop the frame into a QLineEdit and it will show whatever I set as text into the MimeData. So I assume this line edit has its accept drops set to true. The drag itself works fine. The problem is only related to what the drag looks like. In particular I don't seem to be able to draw on the pixmap. I don't thing the accept drops has anyhting to do with it.

[spirit]

this example works fine for me, but it is not complite, just demo for creating and moving grabbed image.

#include <QtGui>
#include "test.h"
 
Test::Test(QWidget *parent)
	: QMainWindow(parent)
{
	QWidget *widget = new QWidget;
	QGridLayout *grid = new QGridLayout(widget);
 
	QPushButton *pb = new QPushButton(tr("Button"));
	pb->installEventFilter(this);
	QComboBox *cb = new QComboBox;
	cb->installEventFilter(this);
	QCheckBox *chb = new QCheckBox(tr("Check box"));
	chb->installEventFilter(this);
	QRadioButton *rb = new QRadioButton(tr("Radio button"));
	rb->installEventFilter(this);
	QSpinBox *sb = new QSpinBox;
	sb->installEventFilter(this);
	QDoubleSpinBox *dsp = new QDoubleSpinBox;
	dsp->installEventFilter(this);
 
	grid->addWidget(pb, 0, 0);
	grid->addWidget(cb, 0, 1);
	grid->addWidget(chb, 0, 2);
	grid->addWidget(rb, 1, 0);
	grid->addWidget(sb, 1, 1);
	grid->addWidget(dsp, 1, 2);
 
	setCentralWidget(widget);
 
	setAcceptDrops(true);
}
 
void Test::mousePressEvent(QMouseEvent *event)
{
	processDrag(this, event);
}
 
void Test::processDrag(QWidget *widget, QMouseEvent *event)
{
	QPixmap pixmap = QPixmap::grabWidget(widget);
	QPainter painter(&pixmap);
	painter.setPen(Qt::red);
	painter.drawText(0, pixmap.rect().center().y(), "hello Qt");
 
    QByteArray itemData;
    QDataStream dataStream(&itemData, QIODevice::WriteOnly);
    dataStream << pixmap << QPoint(event->pos());
 
    QMimeData *mimeData = new QMimeData;
    mimeData->setData("application/x-dnditemdata", itemData);
 
    QDrag *drag = new QDrag(this);
    drag->setMimeData(mimeData);
    drag->setPixmap(pixmap);
    drag->setHotSpot(event->pos());		
 
	drag->exec();
}
 
bool Test::eventFilter(QObject *o, QEvent *e)
{
	if (e->type() == QEvent::MouseButtonPress) {
		QMouseEvent *me = static_cast<QMouseEvent *>(e);
		QWidget *widget = qobject_cast<QWidget *>(o);
		if (widget && me->button() == Qt::LeftButton) {
			processDrag(widget, me);
			return true;
		}
	}
 
	return QMainWindow::eventFilter(o, e);
}

#include <QtGui>
#include "test.h"

Test::Test(QWidget *parent)
	: QMainWindow(parent)
{
	QWidget *widget = new QWidget;
	QGridLayout *grid = new QGridLayout(widget);

	QPushButton *pb = new QPushButton(tr("Button"));
	pb->installEventFilter(this);
	QComboBox *cb = new QComboBox;
	cb->installEventFilter(this);
	QCheckBox *chb = new QCheckBox(tr("Check box"));
	chb->installEventFilter(this);
	QRadioButton *rb = new QRadioButton(tr("Radio button"));
	rb->installEventFilter(this);
	QSpinBox *sb = new QSpinBox;
	sb->installEventFilter(this);
	QDoubleSpinBox *dsp = new QDoubleSpinBox;
	dsp->installEventFilter(this);

	grid->addWidget(pb, 0, 0);
	grid->addWidget(cb, 0, 1);
	grid->addWidget(chb, 0, 2);
	grid->addWidget(rb, 1, 0);
	grid->addWidget(sb, 1, 1);
	grid->addWidget(dsp, 1, 2);

	setCentralWidget(widget);

	setAcceptDrops(true);
}

void Test::mousePressEvent(QMouseEvent *event)
{
	processDrag(this, event);
}

void Test::processDrag(QWidget *widget, QMouseEvent *event)
{
	QPixmap pixmap = QPixmap::grabWidget(widget);
	QPainter painter(&pixmap);
	painter.setPen(Qt::red);
	painter.drawText(0, pixmap.rect().center().y(), "hello Qt");
	
    QByteArray itemData;
    QDataStream dataStream(&itemData, QIODevice::WriteOnly);
    dataStream << pixmap << QPoint(event->pos());

    QMimeData *mimeData = new QMimeData;
    mimeData->setData("application/x-dnditemdata", itemData);

    QDrag *drag = new QDrag(this);
    drag->setMimeData(mimeData);
    drag->setPixmap(pixmap);
    drag->setHotSpot(event->pos());		

	drag->exec();
}

bool Test::eventFilter(QObject *o, QEvent *e)
{
	if (e->type() == QEvent::MouseButtonPress) {
		QMouseEvent *me = static_cast<QMouseEvent *>(e);
		QWidget *widget = qobject_cast<QWidget *>(o);
		if (widget && me->button() == Qt::LeftButton) {
			processDrag(widget, me);
			return true;
		}
	}
	
	return QMainWindow::eventFilter(o, e);
}

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[Cruz]

Wow! Thanks a lot for all the effort you are putting into this. I built a project from your example and it works for me too. Now I need to scrutinize the differences and see which bit is responsible.

[Cruz]

I found the bugger! It's the style sheet.

QWidget {
   background-color: beige;
}

QWidget {
   background-color: beige;
}

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With this I set the bg color of the application. If I take this out, the drag looks like it should. If this is back in, the drag is only a beige rect.

Why is the style sheet influencing the drag this way? The drag itself is not a widget is it?

I tried taking this out of my style sheet and explicitly setting the background of my main window (which is a QWidget and not a QMainWindow in my case) in Designer. It has the same bad effect on the drag.

So the last thing to figure out is how to set a background color for the application AND have nice looking drags.

[spirit]

try to use QPalette insted of using style sheets.

[Cruz]

Yes, the palette works. At the end using style sheets brought me more trouble than good. It's nice how you can change the appearance easily by editing a singe style sheet and not even have to recompile for it. But it doesn't look the same on Windows and on Linux and there is troubles like with the drag. There is still nothing like pixel exact coding.

[spirit]

try to update your Qt version, maybe that behavior which you described is a bug. what version do you use?

[Cruz]

4.4 I think it's the latest. I just installed it 5 days ago.

[onamatic]

Thanks for this interesting thread chaps; I just wanted to say that I had a similar experience with dragging pixmaps being messed up by stylesheets (I gave up on the fancy stuff in the end and just settled for the standard little drag box). If I remember correctly, I also had problems displaying tooltips for widgets which had stylesheets. There was a note somewhere in the documentation warning against mixing palette and stylesheet effects which is fair enough I suppose but stylesheets on their own cause unexpected results sometimes. I try to avoid them now.