Understanding signals, slot order, and parent widgets

Question

So I have a situation where I can't get a couple slots to fire in the order that I would like them to.

The basic set-up is that I have a Mainwindow with a statusbar that needs to get updated based on a signal from a child widget (SearchWidget). When the "Go" button is clicked on the child widget, I would like it to update the status bar to say "Searching..." and then perform the actual database search. However, I can only get the updateStatusBar slot to trigger AFTER the search is complete and displayed in a tablewidget. I have tried re-arranging the connections to the appropriate order, I have tried a separate function that emits the signal for the statusbar and then the signal for the search, but nothing seems to work. The search always executes first and the statusbar doesn't change until after that is complete.

I'm a newbie a this, but I'm guessing maybe the issue has something to do with the parent-child relationship between the mainwindow and the widget? Perhaps slots within the same widget are prioritized in some way? See basic code below.

Mainwindow class:

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent)
{
    initMembers();
    initUI();

    connect(searchWidget, SIGNAL(searchingStatus(QString)), this, SLOT(updateStatusBar(QString)));

}

void MainWindow::initMembers()
{
    tabWidget = new QTabWidget(this);
    searchWidget = new SearchWidget(this);
    saleWidget = new SaleWidget(this);

    statusBar = new QStatusBar();

    setCentralWidget(tabWidget);
    setStatusBar(statusBar);
}

void MainWindow::initUI()
{
    tabWidget->addTab(searchWidget, "Search");
    tabWidget->addTab(saleWidget, "Sale Data");
}


void MainWindow::updateStatusBar(QString status)
{
    statusBar->showMessage(status);
}

SearchWidget class:

SearchWidget::SearchWidget(QWidget *parent) : QWidget(parent)
{

    connect(goButton, SIGNAL(clicked), this, SLOT(buildQuery()));

}

void SearchWidget::buildQuery()
{
    emit searchingStatus("Searching...");
   //builds sql query
}

Any enlightening info would be much appreciated!

Answer 1

I suspect that it's not an ordering issue, but the fact that the SQL query is blocking the main GUI event loop: try inserting a call to QApplication::processEvents() after your emit searchStatus(...) call. If I'm correct, you should see the status bar update before the database search completes.

However, because you're still blocking the event loop, your GUI will still freeze while the DB call executes, which isn't great. You can eliminate this by running the query on a different thread (one of the simplest ways is via http://doc.qt.io/qt-5/qtconcurrent.html#run), but beware you then have to worry about concurrency issues (e.g., now you can click the go button lots of times in a row...).