Has anyone done dynamic column spanning with a QTableWidget?

[RolandHughes]

I need to use QTableWidget to create what us old timers call a control break report. Yes, it has to be a table, not just a text thing or a list. There are other things we wish to be able to do and columns are a nice way of getting alignment across languages and fonts. The database has millions of rows and we will only display 30 or so at a time for obvious reasons. Since many of you reading this may be kids who never had a report writing or logic course I will try to draw it a bit here.


some-long-multi-cell-here-which-repeats-occasionally
xxxx xxxx xxxx xxxx xxxx xxxx xxxx
xxxx xxxx xxxx xxxx
xxxx xxxx xxxx xxxx
xxxx xxxx xxxx xxxx xxxx xxxx
xxxx xxxx xxxx xxxx
xxxx xxxx xxxx xxxx xxxx xxxx xxxx

I don't believe the empty cells will be a problem given a custom model. I'm trying to avoid having to do a custom item delegate just to get that dynamic cell spanning though. Any ideas?

[d_stranz]

If you're using a custom model, then you want QTableView, not QTableWidget, no?

Could you not reimplement QAbstractItemModel::span() to do this? (Duh, I should RTFM - "Note: Currently, span is not used." That's exactly what you need).

What you're asking for sort of breaks the model / view separation. I think you might be able to accomplish what you want by implementing a signal in a custom model that is emitted whenever the Qt::DisplayRole is requested for one of these header row indexes in the QAbstractTableModel::data() method. The widget that contains your QTableView could listen for this signal and set the column span appropriately.

Something like:

QVariant MyCustomModel::data( const QModelIndex & index, int role )
{
    QVariant v;
    if ( Qt::DisplayRole == role )
    {
       v = "whatever";
        if ( indexIsAHeaderRow( index ) )
           emit setColumnSpanForHeaderRow( index.row(), index.column() );
    }
     return v;
}

QVariant MyCustomModel::data( const QModelIndex & index, int role )
{
    QVariant v;
    if ( Qt::DisplayRole == role )
    {
       v = "whatever";
        if ( indexIsAHeaderRow( index ) )
           emit setColumnSpanForHeaderRow( index.row(), index.column() );
    }
     return v;
}

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[RolandHughes]

If you're using a custom model, then you want QTableView, not QTableWidget, no?

Could you not reimplement QAbstractItemModel::span() to do this? (Duh, I should RTFM - "Note: Currently, span is not used." That's exactly what you need).

What you're asking for sort of breaks the model / view separation. I think you might be able to accomplish what you want by implementing a signal in a custom model that is emitted whenever the Qt::DisplayRole is requested for one of these header row indexes in the QAbstractTableModel::data() method. The widget that contains your QTableView could listen for this signal and set the column span appropriately.

Something like:

QVariant MyCustomModel::data( const QModelIndex & index, int role )
{
    QVariant v;
    if ( Qt::DisplayRole == role )
    {
       v = "whatever";
        if ( indexIsAHeaderRow( index ) )
           emit setColumnSpanForHeaderRow( index.row(), index.column() );
    }
     return v;
}

QVariant MyCustomModel::data( const QModelIndex & index, int role )
{
    QVariant v;
    if ( Qt::DisplayRole == role )
    {
       v = "whatever";
        if ( indexIsAHeaderRow( index ) )
           emit setColumnSpanForHeaderRow( index.row(), index.column() );
    }
     return v;
}

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Thanks. I will give it a whirl. Yes, I meant View. Shouldn't type posts first thing Sunday morning I guess.

[wysota]

In my opinion the proper way to solve the problem is to implement a custom view based on QTableView. Could be that it is enough to just reimplement visualRect() so that it takes into consideration "dynamic spanning" needs of the model.

[d_stranz]

Hmm, might work. I suppose it depends on whether the table draws the grid lines before or after the cell contents, and if the grid is drawn first, on whether the painting clears the visual rect before painting it with its contents. This would be a good excuse to implement the span() method in the model.

Documentation of QTableView::visualRect() seems to be missing from the Qt 5.2 docs, even though as a pure virtual method in QAbstractItemView it has to be implemented in QTableView.

[forgottenduck]

I was able to accomplish this in 2 parts.
First, I implemented span() for my model, which would look something like this:

QSize MyModel::span(QModelIndex index) {
    if(isSpanningColumn(index)) {
            return QSize(spanAmount, 1); 
    }
    return QSize(1, 1);
}

QSize MyModel::span(QModelIndex index) {
    if(isSpanningColumn(index)) {
            return QSize(spanAmount, 1); 
    }
    return QSize(1, 1);
}

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Then I reimplemented the setModel() method in my view:

void MyTableView::setModel(QAbstractItemModel* model){
    QTableView::setModel(model);
    for(int col = 0; col < model->columnCount(); col++){
          for (int row = 0; row < model->rowCount(); row++){
                QSize span = model->span(model->index(row, col));
                if(span != QSize(1,1))    //Preform this check so you don't get a bunch of warnings about setting a span of 1 being ignored
                    setSpan(row, col, span.height(), span.width());  
         }
   }
}

void MyTableView::setModel(QAbstractItemModel* model){
    QTableView::setModel(model);
    for(int col = 0; col < model->columnCount(); col++){
          for (int row = 0; row < model->rowCount(); row++){
                QSize span = model->span(model->index(row, col));
                if(span != QSize(1,1))    //Preform this check so you don't get a bunch of warnings about setting a span of 1 being ignored
                    setSpan(row, col, span.height(), span.width());  
         }
   }
}

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I hope that helps

[RolandHughes]

Thanks to all who replied. I ended up using the solution provided by d_stranz today. Here is a side note though. I found this out completely by accident since I put a debug print in the slot.

The view/model in Qt 4.8.x (and probably 5.x as well) is a vicious CPU squanderer. When I was poking through the doc I wondered why there were no signals going from the model which could be caught by the view indicating cell(x,y) had changed. In part I guess this was due to my confusion about the infintesimally small lifespan of QModelIndex. After stumbling on this with my slot I put a debug print in ::data() of the model to verify.

The reason we have no signals for data changes is because the view goes into an infinite loop of fetching data from the model for all visible cells. This seems to be SO not Qt. The model should have signal(s) which get emitted when data has changed and the view using the model should catch those. If you pull up something static, such as an XML or spreadsheet file, or a periscope over a section of database tables, the displayed data will not change, but the view will sit there sucking the battery life out of every device.

Now I need to figure out how to mark this as solved.

Thanks again.

[wysota]

The reason we have no signals for data changes (...)

We don't? What is QAbstractItemModel::dataChanged() then?

(...) because the view goes into an infinite loop of fetching data from the model for all visible cells.

Infinite loop? Have you verified properly that what you say was true? I'm sure it is not for a regular table with a regular delegate and a regular model. If you get such behavior then you probably introduced the loop yourself.

[RolandHughes]

I emailed several others and they conducted their own tests with their own code and got the same results. We didn't share any code. Just be sure to use the tableview with the abstract table model when you conduct your test.

[wysota]

I emailed several others and they conducted their own tests with their own code and got the same results. We didn't share any code.

Well, apparently you all got wrong results.

#include <QtGui>
 
class Model : public QAbstractTableModel {
public:
  Model() {}
  int rowCount(const QModelIndex &parent = QModelIndex()) const {
    if(parent.isValid()) return 0;
    return 1;
  }
  int columnCount(const QModelIndex &parent = QModelIndex()) const { return 1; }
 
  QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const {
    Q_UNUSED(index);
    if(role != Qt::DisplayRole) return QVariant();
    qDebug() << Q_FUNC_INFO;
    return "A";
  }
};
 
int main(int argc, char **argv) {
  QApplication app(argc, argv);
  Model model;
  QTableView view;
  view.setModel(&model);
  view.show();
  return app.exec();
}

#include <QtGui>

class Model : public QAbstractTableModel {
public:
  Model() {}
  int rowCount(const QModelIndex &parent = QModelIndex()) const {
    if(parent.isValid()) return 0;
    return 1;
  }
  int columnCount(const QModelIndex &parent = QModelIndex()) const { return 1; }
  
  QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const {
    Q_UNUSED(index);
    if(role != Qt::DisplayRole) return QVariant();
    qDebug() << Q_FUNC_INFO;
    return "A";
  }
};

int main(int argc, char **argv) {
  QApplication app(argc, argv);
  Model model;
  QTableView view;
  view.setModel(&model);
  view.show();
  return app.exec();
}

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This test application clearly indicates there is no loop. If you want you can even disrupt the dataChanged() signal connection, modify the model and see that the view doesn't update until something else forces the view to ask explicitly for the item data.

I do agree though that the default table view and the default delegate read the model data more than one time. This is probably related to auto-sizing columns and rows but this is only my guess.