Qt: distinguish between drag from same or other window

Question

I am using a QTableView, on which I set my own custom model, inheriting from QAbstractTableModel, using a call to QTableView::setModel().

The table view supports drag and drop: my model class reimplements mimeData() and dropMimeData(). Items can be dragged and dropped, even between 2 instances of the application.

What I'd like to achieve is: dropMimeData() should detect whether the dropped element comes from the same window, or from another window of another running copy of the application. And it should behave differently in each case.

Using the argument Qt::DropAction action of dropMimeData() seemed the most natural thing, but that does not work: it is always Qt::CopyAction or Qt::MoveAction, irrespective of the originating window, and only depending on the defaultDropAction of the QTableView.

Is this possible?

What if you encode that information into the mime data and decode it when you drop the item? — vahancho, Dec 11 '13 at 22:31
That would need a system-wide unique identifier of the originating window. Is there such thing? — user377486, Dec 12 '13 at 08:24
I would think simpler - just invent your identifier(s) and write them into the mime data as a byte array or such. — vahancho, Dec 12 '13 at 08:46

score 1 · Answer 1 · answered Aug 04 '15 at 18:58

Suppose a QMainWindow instance MainWindow has two QTableView instances as childeren:
ui->tableView1 and ui->tableView2

If the function is not already declared, add the following to mainwindow.h:

protected:
bool eventFilter(QObject *target, QEvent *event);

add the following to the constructor in the implementation file

ui->tableView1->viewport()->installEventFilter(this); 
ui->tableView2->viewport()->installEventFilter(this);

Add the following function in the implementation file mainwindow.cpp
(if the function already exists add extra if blocks)

bool MainWindow::eventFilter(QObject * target, QEvent *event)
{
    if (event->type() == QEvent::Drop) {
        QDropEvent *dropEvent = (QDropEvent*) event;
        // pointer target is the target and 
        // pointer dropEvent->source() is the source
        if (target == ui->tableView1->viewport()) {
            // ....
        } 
        if (target == ui->tableView2->viewport()) {
           // ...
        } 
    }
    return false; //leave further processing to widget
}

You can also intercept a QDragMoveEvent. By analyzing the pointers target and event->source() you can find out where it goes to and where it came from.

score 0 · Accepted Answer · answered Sep 06 '15 at 08:57

I ended up subclassing QTableView, which can be done in QT Designer by right-clicking on the table in the form and choosing Promote to...- In the subclass I followed adlag's suggestion and used dropEvent->source() to detect whether the source was the same widget or another one.

Qt: distinguish between drag from same or other window

2 Answers2