QProgressBar in new window don't work

Question

I'm creating the new window with QProgressBar when i click on button of MainWindow but when new window is creating, QProgressBar don't appear while filling cycle is working. After then QProgressBar appear and it is filled.

Constructor:

ProgressWin::ProgressWin():QWidget()
{
    this->resize(273,98);
    this->move(670, 430);
    bar1 = new QProgressBar(this);
    bar1->setGeometry(20, 31, 251, 31);
    bar1->setMinimum(0);
    bar1->setMaximum(10000);
    this->show();
    unsigned long long secr, PQ;
    unsigned long long rv;
    unsigned long long decr;
    for(int v = 0; v <= 100000; v++) {
            bar1->setValue(v);
    }
}

Code of button that call new window:

void RsaMainWindow::ButtClickCrypt()
{
    FileName1 = ui->LineCrypt->text();
    if(FileName1.isEmpty()) {
        QMessageBox::information(0, "Information", "File for Crypt wasn't chosen");
        return;
    }
    NewWin = new ProgressWin;
}

Class for new window:

class ProgressWin : public QWidget
{
    QProgressBar *bar1;
public:
    ProgressWin();
};

Class for MainWindow:

[namespace Ui {
class RsaMainWindow;
}

class RsaMainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit RsaMainWindow(QWidget *parent = 0);
    ~RsaMainWindow();

private slots:
    void ButtClickViewCryp();
    void ButtClickViewDecr();
    void ButtClickViewKeys();
    void ButtClickCrypt();
    void ButtClickDecr();

private:
    Ui::RsaMainWindow *ui;
    QString FileName1;
    QString FileName2;
    QString FileName3;
    ProgressWin *NewWin;

};][1]

Why would you expect the progress bar to fill up visibly if you are filling it up in a for-loop that doesn't return control to the GUI to make its updates? — UnholySheep, Sep 15 '16 at 11:26
You should familiarize yourself with GUIs a bit more and read about the event handling loop etc. — Hayt, Sep 15 '16 at 11:36

Adrian Maire · Answer 1 · 2016-09-15T12:09:16.727

User interface usually work on the event-loop principle:

While (not closing the app)
    Wait for some event
    update app according event
endWhile

If you implement your heavy task in the GUI thread, when the user click on "Perform a heavy task", the code managing this click is called, and after it finish, a following event will trigger the drawing of the window. That mean your heavy task will freeze the user interface during the task.

To perform a heavy task correctly, you need to:

Create a background thread that perform the task. Each iteration, it update some shared-memory (or equivalent) status of the task. Some UI libraries, like QT allows to send queued messages, which help for those cases.
On the Main thread, on update of the status, set the progress bar to the new value and return.

QProgressBar in new window don't work

1 Answers1