How to ensure qtablewidget gets updated before another signal

[qt_keen_learner]

@SGaist I'm using Qt4.8, Qt Creator 3.0.1

1. Add Qt Designer Form class with the name QtForm

2. In the form ui edit, add a tableWidget and a pushbutton onto that Form

3. In the constructor of form class

ui->tableWidget->setSelectionMode(QAbstractItemView::SingleSelection); // only accept one row selection
ui->tableWidget->setSelectionBehavior(QAbstractItemView::SelectRows); // selection on row basis
ui->tableWidget->setHorizontalHeaderLabels(QStringList() << "cellData");
ui->tableWidget->horizontalHeader()->setResizeMode(QHeaderView::Stretch);

4. In the form ui editor, select the tableWidget, 'Go to slot...' -> cellChanged(int,int) to add  the slot to this signal, and then add debug code

qDebug() << "entered cellChanged"

5.  In the form ui edit, select the pushButton, add slot to the signal 'clicked()' with the debug code

qDebug() << "entered buttonClicked"

From the trace info for the problem scenario, you can see the clicked() signal arrives first.

[SGaist]

Are you calling exec on that dialog ?

On a side note, unless you're locked to 4.8, you should move at least to Qt 5.6 which is the LTS or the more recent version. Qt 4 as seen its last release with 4.8.7 and won't get any new patch unless it's a critical security issue.

[qt_keen_learner]

@SGaist
Thanks for advice.
Yes, I used 'exec' to create this model dialog.
I have to use 4.8 because much more code was developed using Qt4.8 convention and I'd like to save the troubles of porting the code to Qt5 and upwards, though it might be smooth.

I understand Qt4.8 is planned to be a stable version, so I'll adopt the workaround and see how it goes.

Thanks again.

[SGaist]

Qt 4.8 is not a stable version, it has reached end of life. The current "stable" as in "long term support" is Qt 5.6

And unless you're unlucky, the port from Qt 4 to Qt 5 is pretty painless, the old signals/slots syntax works exactly the same as before.

Back to your problem, what do you do exactly with that slot connected to the clicked signal ?

[VRonin]

4. In the form ui editor, select the tableWidget, 'Go to slot...' -> cellChanged(int,int) to add the slot to this signal, and then add debug code
5. In the form ui edit, select the pushButton, add slot to the signal 'clicked()' with the debug code

check the generated moc_[className].h and see if the connect for cellChanged comes before or after the one for clicked. (it should come before).

To have a quick and dirty fix add Qt::QueuedConnection as argument to the connect of the clicked signal

[qt_keen_learner]

@SGaist
what I did in the slot to the 'clicked' signal is to use the cell data, which is a variable (say., tableValue) updated in the slot function to 'cellChanged' .
Because the variable has yet been updated, the value of tableValue is not correct (it is still the value before update).

A very simple usage is

send(tableValue)

in the 'clicked' slot, to send the value out. Because tableValue has not been updated, the value sent out is incorrect.

[qt_keen_learner]

@VRonin
Thanks for help.
I don't see moc_<class>.h, but moc_<class>.cpp instead.

In this file, the slots are not 'connect'. They're dispatched by the id, something like

void QtForm::qt_static_metacall(QObject *_o, QMetaObject::Call _c, int _id, void **_a)
{
    if (_c == QMetaObject::InvokeMetaMethod) {
        Q_ASSERT(staticMetaObject.cast(_o));
        QtForm*_t = static_cast<QtForm*>(_o);
        switch (_id) {
        case 0: _t->on_tableWidget_cellChanged((*reinterpret_cast< int(*)>(_a[1])),(*reinterpret_cast< int(*)>(_a[2]))); break;
        case 1: _t->on_pushButton_clicked(); break;
        default: ;
                }
    }
}

So there is no way to add the quick-and-dirty fix with Qt::QueuedConnection

[VRonin]

sorry, my bad, it's not in moc but in ui_[classname] it's the file uic creates

[qt_keen_learner]

@VRonin
Thanks for the clarification.
I couldn't find any connect call in the ui_<class>.h, either.
Something looks like the function required:

QMetaObject::connectSlotsByName(QtFrame);

This is in the setupUi.

[mrjj]

@qt_keen_learner
Hi , If you use Creator to create/hook up signal and slot then no
connect is generated.
Its based on auto connect feature where
it looks for slots that matches

on_ObjectName_SignalName

So there is no connects in setupUI() :)

Update:
To do it yourself, simply rename slot to not follow syntax and
you can hook it up after setupUI and append t::QueuedConnection as wanted.

[SGaist]

Can you describe the workflow of that dialog ?

[qt_keen_learner]

@mrjj
Thanks for the update.
After learning QueuedConnection, I found it is related with multi-thread programing, but my case is single-thread, so I slightly suspect if this can work.

I also tried another workaround:
I implemented the slot to 'itemClicked', using a flag to call on_tableWidget_cellChanged. i.e.,
when the slot on_tableWidgetItem_cellClicked is called, a bool variable 'toUpdate' is set to true;
so then when the slot to button_clicked is called:

As shown in the code below, the problem is, if I explicitly call on_tableWidget_cellChanged(prevRow, prevCol), the value is the old value rather than the text I edited and about to take effect!
So, this workaround still does not work. There is no way for me to get the new text before the cellChanged signal is sent by the Qt system :-(

void QtForm::on_pushButton_clicked() {
    if (toUpdate)
        this->on_tableWidget_cellChanged(m_nPrevRow, m_nPrevCol);
    send(cellData);
}

void QtForm::on_tableWidget_cellClicked(int row, int col) {
    m_nPrevRow = row, m_nPrevCol = col;
    toUpdate = true;
}

void QtForm::on_tableWidget_cellChanged(int row, int col) {
    // update cellData
    toUpdate = false;
}