Qt C++ Drag QHeaderView between tables

I want to copy the selected column of a QTableWidget to another one.

So I tried to make selected columns draggable by adding this code:

void makeDraggable(QTableWidget *table)
{
    table->setDragEnabled(true);
    table->setAcceptDrops(true);
    table->setSelectionBehavior(QAbstractItemView::SelectColumns);
}

Result I got:

But I want to drag a whole column (horizontal and vertical headers) by clicking on headers only, not on cells, and copy its data to another table including the header text.

Solution

Dragging between different tables inside one application can be done with reimplementing custom QHeaderView and QTableWidget. In my example I generate text with indecies of table and column for drag event. Custom header:

#include <QHeaderView>

class ITableManager;

class DraggableHeaderView : public QHeaderView
{
    Q_OBJECT
public:
    explicit DraggableHeaderView(Qt::Orientation orientation, QWidget *parent = 0);

    int tag() const;
    void setTag(const int tag);
    void setTableManager(ITableManager* manager);

protected:
    void mouseMoveEvent(QMouseEvent *e);

    void dragEnterEvent(QDragEnterEvent *event);
    void dragMoveEvent(QDragMoveEvent *event);
    void dropEvent(QDropEvent *event);

signals:

public slots:

private:
    int m_tag;  //internal index of table
    ITableManager *m_tableManager;  //manager will convert table index into pointer
};

Custom header cpp

#include <QMouseEvent>
#include <QDrag>
#include <QMimeData>
#include <QDebug>
#include <QTableWidget>

#include <ITableManager.h>

DraggableHeaderView::DraggableHeaderView(Qt::Orientation orientation, QWidget *parent) :
    QHeaderView(orientation, parent)
{
    m_tag = 0;
    m_tableManager = 0;
    setAcceptDrops(true);
}

void DraggableHeaderView::mouseMoveEvent(QMouseEvent *e)
{
    if (e->buttons() & Qt::LeftButton)
    {
        int index = logicalIndexAt(e->pos());
        QDrag *drag = new QDrag(this);
        QMimeData *mimeData = new QMimeData;
        //custom drag text with indecies inside
        QString mimeTxt = "MoveHeader;Table:" + QString::number(m_tag) +
                ";Index:" + QString::number(index);
        mimeData->setText(mimeTxt);
        drag->setMimeData(mimeData);
        Qt::DropAction dropAction = drag->exec();
    }
}

int DraggableHeaderView::tag() const
{
    return m_tag;
}

void DraggableHeaderView::setTag(const int tag)
{
    m_tag = tag;
}

void DraggableHeaderView::dragEnterEvent(QDragEnterEvent *event)
{
    if (!m_tableManager)
    {
        event->ignore();
        return;
    }

    QString dragText = event->mimeData()->text();
    int index = dragText.indexOf("MoveHeader;");
    if (index == 0)
    {
        event->accept();
    }
    else
    {
        event->ignore();
    }
}

void DraggableHeaderView::dropEvent(QDropEvent *event)
{
    if (!m_tableManager)
    {
        event->ignore();
        return;
    }

    QStringList dragText = event->mimeData()->text().split(';');
    if (dragText.count() < 3 || dragText.at(0) != "MoveHeader")
    {
        event->ignore();
        return;
    }

    int tableIndex = dragText.at(1).mid(6).toInt();//6 - length 'Table:'
    QTableWidget* tableSrc = m_tableManager->getTableFromIndex(tableIndex);
    if (!tableSrc)
    {
        event->ignore();
        return;
    }

    //dst table as parent for header view
    QTableWidget *tableDst = qobject_cast<QTableWidget*> (this->parentWidget());
    if (!tableDst)
    {
        event->ignore();
        return;
    }

    //move column: modify for your needs
    //now moves only items text
    int columnIndex = logicalIndexAt(event->pos());
    int srcColumnIndex = dragText.at(2).mid(6).toInt(); //6 - length of 'Index:'
    tableDst->insertColumn(columnIndex);
    for (int iRow = 0; iRow < tableDst->rowCount() && iRow < tableSrc->rowCount(); ++iRow)
    {
        if (tableSrc->item(iRow, srcColumnIndex))
        {
            tableDst->setItem(iRow, columnIndex,
                              new QTableWidgetItem(tableSrc->item(iRow, srcColumnIndex)->text()));
        }
        else
        {
            tableDst->setItem(iRow, columnIndex, new QTableWidgetItem());
        }
    }
    tableSrc->removeColumn(srcColumnIndex);
}

void DraggableHeaderView::setTableManager(ITableManager *manager)
{
    m_tableManager = manager;
}

Now create custom QTableWidget with DraggableHeaderView inside

class CustomTableWidget : public QTableWidget
{
    Q_OBJECT
public:
    explicit CustomTableWidget(QWidget *parent = 0);

    void setTag(const int tag);
    void setTableManager(ITableManager* manager);
};

CustomTableWidget::CustomTableWidget(QWidget *parent) :
    QTableWidget(parent)
{
    DraggableHeaderView *headerView = new DraggableHeaderView(Qt::Horizontal, this);

    setHorizontalHeader(headerView);

    setAcceptDrops(true);
}

void CustomTableWidget::setTag(const int tag)
{
    DraggableHeaderView *header = qobject_cast<DraggableHeaderView*> (horizontalHeader());
    if (header)
    {
        header->setTag(tag);
    }
}

void CustomTableWidget::setTableManager(ITableManager *manager)
{
    DraggableHeaderView *header = qobject_cast<DraggableHeaderView*> (horizontalHeader());
    if (header)
    {
        header->setTableManager(manager);
    }
}

For converting table index to pointer I use ITableManager

class ITableManager
{
public:
    virtual QTableWidget* getTableFromIndex(const int index) = 0;
};

And implement it in QMainWindow

class MainWindow : public QMainWindow, ITableManager
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

    QTableWidget* getTableFromIndex(const int index);
}

QTableWidget * MainWindow::getTableFromIndex(const int index)
{
    switch (index)
    {
    case 1:
        return ui->tableWidget;
    case 2:
        return ui->tableWidget_2;
    default:
        return nullptr;
    }
}

Dont forget setup tags (indecies) and table manager for tables (in main window constructor)

ui->tableWidget->setTag(1);
ui->tableWidget_2->setTag(2);
ui->tableWidget->setTableManager(this);
ui->tableWidget_2->setTableManager(this);

EDIT: If you want change custom pixmap for dragging just set QDrag::setPixmap

void DraggableHeaderView::mouseMoveEvent(QMouseEvent *e)
{
    if (e->buttons() & Qt::LeftButton)
    {
        int index = logicalIndexAt(e->pos());
        QDrag *drag = new QDrag(this);
        QMimeData *mimeData = new QMimeData;
        QString mimeTxt = "MoveHeader;Table:" + QString::number(m_tag) +
                ";Index:" + QString::number(index);
        mimeData->setText(mimeTxt);
        drag->setMimeData(mimeData);
        drag->setPixmap(pixmapForDrag(index));
        Qt::DropAction dropAction = drag->exec();
    }
}

And method for taking pixmap of column can be like this

QPixmap DraggableHeaderView::pixmapForDrag(const int columnIndex) const
{
    QTableWidget *table = qobject_cast<QTableWidget*> (this->parentWidget());
    if (!table)
    {
        return QPixmap();
    }

    //image for first 5 row
    int height = table->horizontalHeader()->height();
    for (int iRow = 0; iRow < 5 && iRow < table->rowCount(); ++iRow)
    {
        height += table->rowHeight(iRow);
    }

    //clip maximum size
    if (height > 200)
    {
        height = 200;
    }

    QRect rect(table->columnViewportPosition(columnIndex) + table->verticalHeader()->width(),
                 table->rowViewportPosition(0),
                 table->columnWidth(columnIndex),
                 height);
    QPixmap pixmap(rect.size());
    table->render(&pixmap, QPoint(), QRegion(rect));
    return pixmap;
}