QTreeView + sort model + QStandardItemModel, the most efficient way to update the model?

JKSH

I'm definitely NOT going to subclass a QAbstractItemModel in any near future. That was the first thing I did after moving to tree view from tree widget, and it was a horrible experience.

If you don't need a tree, subclassing QAbstractTableModel is much nicer to use.

Do I understand correctly that my current implementation is the best possible for QStandardItemModel? The only improvement that comes to mind is leave everything the same, but replace model.clear() with model.setColumnCount().

Yes. setColumnCount() lets you keep the existing items (you can just modify their contents, instead of deleting everything and then reconstructing them all again).

Note that if the new column count is bigger than before, I think you need to explicitly construct new QStandardItems to put into the columns (I can't remember; I haven't used QStandardItemModel in a long long time)

Finally, if you only want to add new items, use addColumn()/appendColumn().

Violet Giraffe

Thanks. I didn't use table view because I thought it has less functionality even when there are no children items. Was I wrong?

JKSH

@Violet-Giraffe said:

Thanks. I didn't use table view because I thought it has less functionality even when there are no children items. Was I wrong?

If you don't have a tree model, then there is no benefit in using a tree view.

But anyway, I was talking about the model, not the view.

A QAbstractTableModel is a subclass of QAbstractItemModel, so it inherits all functionality. However, QAbstractTableModel is easier to use, because it already implements lots of things needed by QAbstractItemModel, so you don't have to implement them yourself.

antis

Hey guys,

I wrote a little application demonstrating the usage of QAbstractTableModel with generic table-like data. It can be found here and can easily handle 500.000+ items. As long as there's enough memory, there's no real maximum border.

Unless building a customized sort system, QAbstractItemModel::sort() changes the original item order - which usually comes with a lot of problems. Thus, I decided to use also a QSortFilterProxyModel approach. This can be improved as well, but is fast enough for the demo.

It is noteworthy, that item creation is about at least twice as fast, as with the tweaked QStandardItemModel approach. Memory footprint should also be much smaller.

@Violet-Giraffe Hope this helps to improve your understanding of Qt-MV programming.

Violet Giraffe

Thanks a lot, this looks interesting! Definitely a very, very useful example.
I expect a whole bunch of problems from switching tree view to table view and from QAbstractItemModel to QAbstractTableModel, but I'll give it a try!

Violet Giraffe

I've just tried QTableView and immediately recalled why I didn't use it in the first place. It just looks bad:

http://i.imgur.com/DQusulq.png

Two problems with it:

The top and bottom margins of every item are too huge, at least 5 times bigger than in a QTreeView;
despite the selection mode being set to "Select rows", the current item cursor (dotted frame) is only drawn around a specific column, not the whole row. This also isn't a problem with QTreeView. Here's a tree view screen shot for comparison: http://i.imgur.com/ICp2gNA.png

And I'd say the general visual style of the tree view (selection etc.) is better and looks more native to Windows. But the table does indeed work a bit faster.

antis

To 1: Take a look here or better try the following:

tableView->verticalHeader()->setSectionResizeMode(QHeaderView::ResizeToContents);
tableView->setStyleSheet( QStringLiteral("QAbstractItemView::item{ margin: 0; padding: 0; }") );

To 2: What's wrong with that? Same applies to QTreeView. This depends on the selection model used. You can simply set behaviour:

tableView->selectionModel().setSelectionBehaviour( QAbstractItemView::SelectRows );

Btw.: I don't want to confuse you, but do you know there's a QDirModel prepared for you? This seems pretty much like what you're looking for. Found a nice tutorial.

Violet Giraffe

Very interesting, I'll try your suggestions, shortly, thanks.
On 2: I have set selection behavior to "Select rows" for the view, but not the model. It worked for tree view, but not for table view, so it confused me.

I have heard of the QDirModel. I'm making a file manager, so I need a flexible and powerful solution. Which means I have to write it all myself. Besides, I need to separate the model (UI level code) from the actual data (core level).

Violet Giraffe

Wait, QItemSelectionModel doesn't have a setSelectionBehaviour member! So I can't do

tableView->selectionModel().setSelectionBehaviour( QAbstractItemView::SelectRows );

Violet Giraffe

Another observation: doing

verticalHeader()->setSectionResizeMode(QHeaderView::ResizeToContents);
setStyleSheet("QAbstractItemView::item{ margin: 0; padding: 0; }");

Totally kills performance. Looks like the view does a LOT of relayouting, or maybe just calculating font metrics for each item. Something like that, judging from the call stack. Either way, the UI thread is 100% busy and the event loop is barely executing.

antis

Wait, QItemSelectionModel doesn't have a setSelectionBehaviour member! So I can't do

Right, but QAbstractItemView has:

tableView->setSelectionBehaviour( QAbstractItemView::SelectRows );

antis

@Violet-Giraffe Just try without setting resize mode.

Further, the css rule might not be right. Sorry, you need to figure that out by yourself.

Violet Giraffe

@antis said:

Right, but QAbstractItemView has:
tableView->setSelectionBehaviour( QAbstractItemView::SelectRows );

That it does! And like I said, I have set this option. the table view does select a whole row, but it does not span the cursor across the whole row. While the tree view does both no problem.

antis

That it does! And like I said, I have set this option. the table view does select a whole row, but it does not span the cursor across the whole row. While the tree view does both no problem.

Got me. The dotted rectangle's shows the editing focus. Logically, the focus lies always on the focused table cell. In a QTreeView on the other hand, it lies on the the tree item. You can paint the focus rect over the whole table row by overriding "QTableView::paint".

antis

First, the table view is what you want for your use case. So please don't switch the view.

Ok, I did some further research:
My default file manager (and the file dialogs itself) display the focus around the item in the first column. This makes total sense, as this is the only editable column (the file name). The flag is defined in QAbstractTableModel::flags, which you need to override:

Qt::ItemFlags MyTableModel::flags(const QModelIndex & index) const {
  if (index.column() > 0) {
    // remove focusable flag for all columns > 0
    return QAbstractTableModel::flags(index) & ~Qt::ItemIsEditable;
  }

  return QAbstractTableModel::flags(index);
}

EDIT: Corrected the code snippet.

Violet Giraffe

Thanks. This is counter-intuitive, but as long as I can fix it reliably for all platforms with 3 lines of code - why not. I've already reverted the change from QtreeView to QTableView under the pressure of the issues, but it won't take too long to try again.

Filling QTableView is about twice as fast as QtreeView, so that's a good incentive, but the QtreeView definitely looks better on Windows (more native). At least so far. For example, the selection style is much different.