Dragging out to non-Qt desktop

[ShinSat]

Hello,
I’m working on dragging and dropping capability in my desktop app, which consists of multiple QWidget.
D&D works OK if it’s internal movement where the drop receiver is another QWidget.
My question is how can I achieve the same functionality between the desktop app and non Qt app such as Window desktop where no qt app exists.
How and where can I handle the dropEvent outside of Qt?

Many thanks in advance for your help.
Sat

[raven-worx]

@ShinSat
your question is very unclear.

How and where can I handle the dropEvent outside of Qt?

You shouldn't and you wont handle the drop outside your application. Similar to a copy and aste via clipboard you simply provide data to the operating system and let it do the multi-window drag-handling. The final target application which receives the drop handles the data on it's own.
The only result you get is when your QDrag::exec() returnes and check the returned drop result.

[mrjj]

Hi
To drop to desktop, you must follow windows rules for the mimedata
else it wont accept.

This sample drags file from Qt to desktop / explorer
It uses a native windows header. (#include <Shlobj.h>)

#include "mainwindow.h"
#include <QApplication>
#include <QtGui>
#include <Shlobj.h>

class Widget : public QWidget {
  void mousePressEvent(QMouseEvent* e) {
    QPoint hotSpot = e->pos();
    QList<QUrl> strList;
    FILEGROUPDESCRIPTOR fgd;
    FILEDESCRIPTOR fd[ 1 ];

    strList.append( QUrl( "http://www.qt.io/" ) );
    fgd.cItems = 1;

    fgd.fgd[ 0 ].dwFlags = FD_PROGRESSUI;
    wcscpy( fgd.fgd[ 0 ].cFileName, L"qtrules.url" );
    QMimeData* mimeData = new QMimeData;
    mimeData->setText("www.gaspar.net");
    mimeData->setUrls( strList );
    mimeData->setData("text/plain", QByteArray( "http://www.gaspar.net" ) );
    mimeData->setData("text/html", QByteArray( "<!--StartFragment--><a href=\"http://www.gaspar.net\">The Gaspar Website</a><!--EndFragment-->" ) );
    mimeData->setData("FileName", QByteArray( "Gaspar Website.url" ) );
    mimeData->setData("FileGroupDescriptorW", QByteArray::fromRawData( ( const char* )&fgd, sizeof ( FILEGROUPDESCRIPTOR ) ) );
    mimeData->setData("FileContents", QByteArray( "[InternetShortcut]\r\nURL=http://www.gaspar.net" ) );

    QPixmap pixmap(":/images/imagelists/16x16/webpage.png");
    render(&pixmap);

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

    Qt::DropAction dropAction = drag->exec(Qt::CopyAction | Qt::MoveAction, Qt::CopyAction);

    if (dropAction == Qt::MoveAction) {
      update();
    }
  }
};

int main(int argc, char* argv[]) {
  QApplication app(argc, argv);
  Widget window;
  window.show();
  return app.exec();
}

Credits goes to http://www.gaspar.net/oteqmimewindows.html

[JonB]

@mrjj
There should be a "Code Snippets" forum here, and this kind of post could be put there with a decent title.

[mrjj]

@JNBarchan
Yes, that would be great. I think we might be able to use the wiki for it since it already has samples.

[raven-worx]

@mrjj
Just a note: File drags to desktop can also be achieved using Qt classes only:

1. start a drag containing all the needed mime-types
2. create an empty QTemporaryFile with the target file-name and set it's file path to the mime-data (file-urls type)
3. fill the temporary file once it is requested in the QMimeData::retrieveData() method (see the delayed encoding example)
4. the OS copies the (temp-)file to the drop-position

[mrjj]

@raven-worx

Did you try delayed encoding sample ?

The sample will not work for me on win 7 or win 10.
Tried 2 pc. Desktop/explorer wont accept.

So i wonder if issue with my pcs or just dont work like that in newer Windows.

[raven-worx]

@mrjj
yes, but it's a while ago. I will give it a try.

[mrjj]

@raven-worx

Super. Maybe i just do it wrong.
I drag the export Button out from window. It show red stop sign.

[raven-worx]

@mrjj
the following (unoptimized code) works (Qt 5.9, Windows 7):

MimeData::MimeData()
    : QMimeData()
{
    QString filePath = QStandardPaths::writableLocation(QStandardPaths::TempLocation) % QDir::separator() % "Gaspar Website.url";
    m_File = new QFile( filePath, this );
}

MimeData::~MimeData()
{
    if( m_File && m_File->isOpen() )
        m_File->close();
}

QStringList MimeData::formats() const
{
    return QMimeData::formats() << QLatin1String("text/uri-list");
}

QVariant MimeData::retrieveData(const QString &mimeType, QVariant::Type type) const
{
    if( mimeType == QStringLiteral("text/uri-list") )
    {
        // since this method may be called multiple during a single drag it's not necessary to also write to the file multiple times -> needs optimization
        if( m_File->open(QFile::WriteOnly) )
        {
            m_File->write( QByteArray("[InternetShortcut]\r\nURL=http://www.gaspar.net") );
            m_File->close();
        }

        return QVariant::fromValue<QUrl>( QUrl::fromLocalFile(QFileInfo(*m_File).absoluteFilePath()) );
    }
    else
        return QMimeData::retrieveData(mimeType, type);
}

Although it seems there are some caveats:

1. it stopped working when i set the path of the temp file into a subfolder in the temp-dir ... o.O
2. the retrieveData() always requests the data with the type QVariant::QVariantList. But still it only works when i return a QUrl (not a QList) packed into the variant data.

Very strange.

[mrjj]

@raven-worx

Good work.
Ehh so it likes only root of tmp dir ?? odd
2: that is a bit odd but i guess it was only ment for 1 file then.

Would it be possible to get the test code ?
Want to see if win 10 likes it also :)

This is nicer than the platform bound i found :) \o/

[raven-worx]

@mrjj said in Dragging out to non-Qt desktop:

Would it be possible to get the test code ?

thats actually all the code (despite the obvious header file).

All i did additionally for testing was this:

void MainWindow::mousePressEvent(QMouseEvent *event)
{
    event->accept();

    MimeData* mimeData = new MimeData;

    QDrag drag( this );
        drag.setMimeData(mimeData);
    drag.exec(Qt::CopyAction);
}

the mime type is text/uri-list, so a list does make sense. It also makes sense to copy multiple files at once to a folder in the filesystem explorer.
I have no idea why it's not working. Would need to debug into Qt source code to make sure.