Custom Table Model

EaccB

At the moment, I'm using QTableView with a custom model which holds a pointer to the students vector held in StudentManager. What do you think?

Another problem I have:

When I delete a student object from inside the StudentManager, how can I refresh the Table View to reflect the change in data?

Christian Ehrlicher

@EaccB said in Custom Table Model:

What do you think?

That's exactly how I would implement it.

how can I refresh the Table View to reflect the change in data?

Your model has to emit a dataChanged() signal.

EaccB

@Christian-Ehrlicher said in Custom Table Model:

That's exactly how I would implement it.

So just to be clear, the model is effectively a wrapper around a data structure held elsewhere?

How would I emit the dataChanged() signal? Do there need to be any parameters passed?
Also, as I'm changing the data from within the StudentManager object, how can I let the model know the data has been updated?

JonB

@EaccB

the model is effectively a wrapper around a data structure held elsewhere

In this case, yes. In other cases the model holds the data structure.

Start by reading through https://doc.qt.io/qt-5/qabstractitemmodel.html#dataChanged.

EaccB

@JonB

Here's what I've done, which seems to work:

void MainWindow::on_DeleteButton_clicked()
{
    QItemSelectionModel *selected = ui->tableView->selectionModel();
    QModelIndexList rowList = selected->selectedRows();


    for (int i = 0; i < rowList.count(); i++)
    {

        ui->label->setText(students.at(rowList.at(i).row()).name);
        students.removeAt(rowList.at(i).row());
        StudentModel->update();

    }
}

void StudentModel::update(){

   QModelIndex i = this->index(0,0,QModelIndex());
   QModelIndex p = this->index(0,1,QModelIndex());
   emit dataChanged(i,p);
}

What I don't understand is what the value in index() relate to. Admittedly I chose them randomly until it worked. Also, is it ok to get the index using "this->"?

Christian Ehrlicher

@EaccB said in Custom Table Model:

What I don't understand is what the value in index() relate to

You should take a look here

EaccB

@Christian-Ehrlicher

I see. So doing something like this would update every cell in the table?

QModelIndex i = this->index(0,0,QModelIndex());
QModelIndex p = this->index(students.size()-1,1,QModelIndex());

Christian Ehrlicher

Yes, but for this you should use begin/endResetModel() and you should only use it when you really need it. Make sure to only call dataChanged() for stuff you need.

EaccB

So are you saying that ideally I should only emit dataChanged() on the rows affected?

SGaist

Hi,

@EaccB said in Custom Table Model:

So are you saying that ideally I should only emit dataChanged() on the rows affected?

Yes, this will avoid unneeded processing and updating of the GUI for things that did not change.

EaccB

@SGaist

Ok, here's what I've done:

void TestModel::update(int row){

   QModelIndex i = this->index(row,0,QModelIndex());
   QModelIndex p = this->index(row,1,QModelIndex());

   emit dataChanged(i,p);
}

This way, only the affected row is updated.

SGaist

dataChange takes a range so here you always emit that the it starts from the first up to the row you pass as parameter. Is it really always the case ?

EaccB

@SGaist

I edited my answer.

EaccB

@SGaist

When emitting dataChanged(i,p) for the deleted row only, how is it that the following rows move up? Wouldn't that require a dataChanged(i,p) for the whole table?

Christian Ehrlicher

@EaccB said in Custom Table Model:

When emitting dataChanged(i,p) for the deleted row only,

You did not read my link. You must not call dataChanged() when removing a row.

EaccB

@Christian-Ehrlicher

Then what do I use to signal to the view that the model has been updated? I've removed dataChanged() and replaced it with layoutChanged() which seems to work fine.

To avoid any confusion, the table itself is read-only and should only reflect the contents of the students vector inside StudentManager. Any changes to the data (i.e. add student, edit student, remove student) are done directly on the students vector.

Christian Ehrlicher

@EaccB said in Custom Table Model:

Then what do I use to signal to the view that the model has been updated?

dataChanged() as explained above.

EaccB

@Christian-Ehrlicher

Ok, I'm really confused. You said that I shouldn't use dataChanged() when removing a row. So, what should I do when an element is removed from the vector? Or are you saying that I should emit dataChanged from the class which owns the vector and connect it to a slot in the Model?

Christian Ehrlicher

@EaccB said in Custom Table Model:

So, what should I do when an element is removed from the vector?

You really really should read the documentation. I already gave a the link two times...: https://doc.qt.io/qt-5/model-view-programming.html#inserting-and-removing-rows

JonB

@EaccB
To reiterate what @Christian-Ehrlicher is telling you. The signal code in a model has to depend on whether you are updating an existing row which is already there and will remain there, versus if you inserting or deleting a row.

dataChanged() is only for same rows, with values in columns changing.

For inserting/deleting rows you must use begin/endInsert/RemoveRows() instead.

Views, for example, need to know this and react differently according as the model reports its rows are being updated or are being deleted/inserted. You can imagine the action they take will differ between these cases.