Drag & drop with model/view (Qt4)

[Nickolay]

I have another question about drag&drop inside MVC framework.
I need simply move rows inside QtableView,
I go along proposed in docs way - reimplement: mimeType, mimeData, dropMimeData.

But i need such behaviour: when I start dragging row, I need remove this row from table,
then after droping put them in new place, or return to original position if droping was out of widget bounds.
But in what function I can call models func - removeRows? I cann't do this in mimeData because it const.
Where correct place to initialy(on dragging start) remove row?

Thank you!

[Nickolay]

Is there no one has done such before?

[wysota]

What you require is somewhat against traditional approach to drag and drop. What happens if you start a drag (hence remove the item) and then cancel the drag without dropping the item on the widget or if you drop the item on some external application? The drop is traditionally handled by the target and not the source. Of course you can have the behaviour you want by reimplementing mouse events and starting the drag manually (and prior to doing that you need to remove the item from the model) but then you have to handle all possible drop situations manually (and you'll have no information about WHO accepted the drop).

[Nickolay]

Why not traditional? I use chrome browser, when I start drag tab in horizontal direction, it removes from its initial position and sticks to cursor, then I can move it where I want, of course inside tabbar, seems my approach the same.

Athrough if I move tab outside tab bar, somewhere inside browser window, its like you say, like tab losses it parent. And then if I simply drop tab anywhere, new window will creates for this tab.

So what I required, if I want during move row, detach it from its initial position?

[wysota]

Why not traditional?

Because typically the operation is performed during the drop and not during the drag.

I use chrome browser, when I start drag tab in horizontal direction, it removes from its initial position and sticks to cursor, then I can move it where I want, of course inside tabbar, seems my approach the same.

Your approach is different. For instance you may have multiple views on the same model, removing the item from all the views handling that model is counter-intuitive.

Athrough if I move tab outside tab bar, somewhere inside browser window, its like you say, like tab losses it parent. And then if I simply drop tab anywhere, new window will creates for this tab.

That's not the case with your situation, is it? Furthermore look that in Chrome the drop is actually performed during the drag, the tab is not removed when you start dragging it but only when you move it past the border of the next tab (or you drag it outside the tab bar).

As I said, you can do what you want, but since this is not a typical approach to drag and drop, there is no built-in support for it. You may implement your own view and handle it the way you want.

[Nickolay]

That's not the case with your situation, is it? Furthermore look that in Chrome the drop is actually performed during the drag, the tab is not removed when you start dragging it but only when you move it past the border of the next tab (or you drag it outside the tab bar).

What I need to reimplement in therms of MVC? I need subclass QTableView and override mouse handlers?

[wysota]

You need to remplement mouse events and rendering-related methods (which in practice makes up most if not all of the view code).

[Nickolay]

In very simple case, in current default implementation, to implement row moving inside one table,
what is the best approach with functions removeRows and insertRows?
Could I use them directly one after another inside dropMimeData?
So I can pack in MIME number of line of dragged item and inside dropMimeData I retrieve number of drop line.

[wysota]

what is the best approach with functions removeRows and insertRows?
Could I use them directly one after another inside dropMimeData?

I would say it is better to use QAbstractItemModel::beginMoveRows() and QAbstractItemModel::endMoveRows() in this case. In a general case dropMimeData() would only insert rows and rows removal would be done in the code when the drag is started when a "move" operation is detected.

[Nickolay]

I'm not clearly understand what do you mean.
generally I didn't get what sense of QAbstractItemModel::beginMoveRows() and QAbstractItemModel::endMoveRows()

By docs I should use them from removeRows, like

bool TableModel::removeRows(int position, int rows, const QModelIndex& index)
{
Q_UNUSED(index);
beginRemoveRows(QModelIndex(), position, position + rows - 1);
endRemoveRows();
return true;
}

But dragging begins from const func mimeData, where I can't call non const method.

and rows removal would be done in the code when the drag is started when a "move" operation is detected.

Where that place you have mentioned?

[wysota]

I suggest you read a little about model-view framework in Qt. Everything should become clearer then. One hint -- drag and drop in Qt is handled in the model and not in the view.