Show live data into a table

makopo

Hi everyone,

I want to show data (some results of mathematical operations) into a table. This data should updated every second. And I am not sure if the signal and slot principle is the correct way, or if delegates (model/view programming) do a better job for this.
In my program I have a method that calculates a value and emit a signal with this value as parameter. The slot that receives casts the integer value into a string (with QVartiant) and this string is inserted a item into a table cell.
I use no database or csv-file. All values that are calculated should shown directly to the frontend.

Is this a good style doing this?

SGaist

Hi,

Using the model/view paradigm will do that automatically for you. Just set the value on the model and it will trigger the update of the table view.

JonB

@makopo
For best results, you should derive from QAbstractTableModel to create your own model. Then bind a QTableView to that to show the data. When you add/update data in your model that will raise signals for you which are attached to the table view, and that will update whenever it sees a change in the model.

If speed is not an issue, you can simplify by using a QStandardItemModel for your data instead of your own QAbstractTableModel, or even a QTableWidget to wrap the model + view into one. You might start out with these if you are a beginner, and maybe upgrade to a QAbstractTableModel/QTableView when you are ready for better performance/typing.

makopo

Great! I'd read the documentation of the model/view paradigm, but it ws not clear for me that this also works without databases.

Thank you @SGaist and @JonB !

makopo

Hi there,

is there a functionality to add a QTableView to a GridLayout? I create a QTableView successfully with the following lines but this (logically) creates a new window.

m_tableView->setModel(m_tableDataModel);
m_tableView->show();

JonB

@makopo
What is a GridLayout? If you mean QGridLayout, that is a QLayout. QTableView is a QWidget. You can always add a QWidget to a QLayout.

makopo

What a silly question of mine! I tried it few hours ang and adding QTableView to QGridLayout did not work. Now it works... Thank you

Thank You

@makopo If it is solved then mark it as solved. So others can give time on other unsolved question 💥😀

makopo

Unfortunatly the view did not work like expected. I create a data model (derived from QAbstractTableModel) and with this model I can generate a table view. But this view is static and values did not update.
I change, for example, the video mode of the stream and on the console this change is detected. In the table view the initial state is detected and did not change.
I make QTableView editable, but I think that this is not the right way for my requirement.

I implement the virtual data function like this:

QVariant DataTableModel::data(const QModelIndex& index, int role) const
{
    if (!index.isValid()) {
        return QVariant();
    }

    if (index.row() >= m_parameter.size() && index.row() >= m_value.size()) {
        return QVariant();
    }

    if (role == Qt::DisplayRole || role == Qt::EditRole) {
        if (index.column() == 0) {
            return m_parameter.at(index.row());
        }

        if (index.column() == 1) {
            return m_value.at(index.row());
        }
    }
    return QVariant();
}

In the main class I create two QVector types that stores the values for the table cells.

QVector<QString> parameters;
Qvector<QString> values;
parameters.append("Video mode");
values.append(QVariant(m_displayMode->GetDisplayMode()).toString());

m_tableDataModel->AddData(parameters, values);
m_tableView->setModel(m_tableDataModel);

As descipted I get the initial video mode, but after changing the mode the state is not updated.
Is there a restiction with the data types and the update process?

kshegunov

@makopo said in Show live data into a table:

Is there a restiction with the data types and the update process?

Of course, adding something to a vector notifies nobody. If you'd kept an int and changed it, the result'd be the same. To add to the model define your addRow or alike and thereafter follow the documentation. There's a battery of signals that the model must emit so the view gets notified of changes - beginInsertRows, endInsertRows and so on.

SGaist

On addition to @kshegunov you also need to re-implement setData so that you can properly notify when data changes.

makopo

I did not understand how to implement insertRows() correctly and it also looks like that the function is not called. Because the QAbstractTableModel did not work, I rebuild the Qt tutorial which uses the QAbstractListModel.

In insertRows() I call beginInsertRows(), than I run the loop and than I call endInsertRows().

bool DataTableModel::insertRows(int position, int rows, const QModelIndex& parent)
{
    beginInsertRows(QModelIndex(), position, position + rows - 1);
    for (int x = 1; x <= 10; ++x) {
        m_stringList.push_back(QString::number(x));
    }
    endInsertRows();
    qDebug() << "insertRows is called.";

    return true;
}

As result I get an empty List. On console I can also see that the function is not called. So my question is, how to handle insertRows() function? Did stringList.insert(position, ""); from the Model/View Programming tutorial only reservs an empty line? I thought that there is the entry-point where I have to insert the data.

JonB

@makopo
Your insertRows() is called with a position to start the insert from and a number of rows to insert there. But all you do is ignore these and append 10 rows. You need to respect and act on the parameters. (Same btw for deleteRows().) The newly inserted rows should be blank for this call.

makopo

@JonB
Thank you. I rewrote the loop as recommended in the Model/View Programming tutorial.

beginInsertRows(QModelIndex(), position, position + rows - 1);
for (int row = 0; row < rows; ++row) {
    m_stringList.insert(position, "");
}
endInsertRows();
qDebug() << "insertRows is called.";

As I can see on the console insertRows() is not called.

JonB

@makopo
? Why/when do you expect it to be called? You have to call it if you wish to insert some rows. (And again for deleteRows().)

makopo

@JonB

insertRows(int row, int count, const QModelIndex &parent = QModelIndex()) is virtual. I thought it is called automatically?

As I understand now m_stringList.insert(position, ""); only reservs an empty row and I have to add data to the table outside the model class?

JonB

@makopo
The virtual just means that if anything calls QAbstractTableModel::insertRows() --- even if it knows nothing about your derived class --- the implementation code you have written will be called.

The outside world will call insertRows() when it wants/needs to. The outside world will do that with no knowledge that you have implemented it via m_stringList.insert(). The outside world will call setData() for the desired column values on newly inserted row(s) after it has called insertRows().

makopo

@JonB
That was not clear for me. I did not call the headerData(...) and data(...) function. I only generate a instance of the model class, set the model to the view and get a table. Thought that setData(...) and insertRows(...) works on the same way.

So as I understand from your last post, I have to do something like this:

//QMainClass.cpp
m_tableDataModel->insertRows(0, 1, QModelIndex());
m_tableDataModel->setData(QModelIndex(), QParameterList(), QValueList(), 2); //valuelist is a vector and stores data that should be overwritten in the view
m_tableView->setModel(m_tableDataModel);

I'am sry. As a beginner the principle of table view is hard to understand.

JonB

@makopo
Those are the right calls. But you'll have to work a bit on all your parameters to setData(). If you have multiple columns (I don't know if you do) you'll have to call setData() for each one.

SGaist

A QTableView is just a widget showing your model as a table. Nothing more.

From the looks of it, you did not understand that the model is a wrapper on top of your data structure. From what you wrote it's a QStringList. So you have a model with a single column and as many rows as your string list.

The setData method shall be called to modify the data of one element of your data structure.

If you want to initialise your model with a ready made list, add a method for that.

Note that if your data structure is a QStringList you might as well use the QStringListModel class.