Qt widget element doesn't want to hide - qt

Question

In my Qt application after pressing button I want to hide that button and start quite long process. While this process is running PushButton shouldn't be visible but it seems to be waiting for process being executed and after that hide button. It looks like QWidget is refreshing after end of PushButton slot function. Here's my simplified code:
void MainWindow::on_pushButton_clicked()
{
ui->progressBar->setVisible(true);
ui->pushButton->setVisible(false);
while(x<1000000) x++; //e.g of my long time function
}
When that function (on_pushButton_clicked() -> generated by mouse->go to slot) ends up my "view" is updated and button dissappear.
Is there any function to refresh my widget or maybe I forgot about sth?
Thanks in advance

Answer 1

Changes to the gui aren't shown until the program has a chance to redraw itself which won't happen until you return.
you'll need to defer the execution of the code somehow:
void MainWindow::on_pushButton_clicked()
{
ui->progressBar->setVisible(true);
ui->pushButton->setVisible(false);
QMetaObject::invokeMethod(this, &MainWindow::longFunction, Qt::QueuedConnection);
}
void MainWindow::longFunction()
{
while(x<1000000) x++; //e.g of my long time function
}
This returns to the event loop and then runs the longFunction but it will still block on and the progress bar won't show any updates until it is done.
To fix that you will either need to move the execution to a new thread or split the function up in shorter parts and invoke them in sequence with QMetaObject::invokeMethod and a QueuedConnection.

Answer 2

In order for the button to change state, it needs to return to process events in the event loop.
You could call QApplication::processEvents before the while loop in order to fix this, though it would be better to return to the event loop naturally, before you start the long-time function, by invoking the function as a QueuedConnection.
Alternatively, the better method would be to run the function in a separate thread, which will enable your GUI to remain active during the processing of the 'long function'
Start by creating an object to encapsulate the function that will do the work:-
class Worker : public QObject {
Q_OBJECT
public:
Worker();
~Worker();
public slots:
void process(); // This is where your long function will process
signals:
void finished();
void error(QString err);
};
void Worker::process()
{
while(x<1000000) x++; //e.g of my long time function
emit finished();
}
Create a new thread and start it when the button is clicked
void MainWindow::on_pushButton_clicked()
{
// change button visibility
ui->progressBar->setVisible(true);
ui->pushButton->setVisible(false);
// create the new thread and start the long function
QThread* thread = new QThread;
Worker* worker = new Worker();
worker->moveToThread(thread);
connect(worker, SIGNAL(error(QString)), this, SLOT(errorString(QString)));
connect(thread, SIGNAL(started()), worker, SLOT(process()));
//ensure the objects are cleared up when the work is done
connect(worker, SIGNAL(finished()), thread, SLOT(quit()));
connect(worker, SIGNAL(finished()), worker, SLOT(deleteLater()));
connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
//start the thread and the long function processing
thread->start();
}