How can I use QtConcurrent::Run and QThread? - c++

Question

My code is about images. It can be open image, changing quality, resize, showing image size... For resize and change quaity, I use slider and when I change sliders values, image is read from buffer again and again. Because of this, freezing is happening in my program. So, to solve the problem I want to use QtConcurrent::run and QThread or QFuture. Actually I have no idea how can I use them and I would like to your help to solve my problem.
Here is my code. The functions are that to cause freezing:
void MainWindow::reprocess_image(int scale, int quality) {
rescale_image(scale);
requality_image(quality);
show_pixmap();
}
void MainWindow::rescale_image(int scale) {
int w = m_image->width();
int h = m_image->height();
int new_w = (w * scale)/100;
int new_h = (h * scale)/100;
ui->lbl_width->setText(QString::number(new_w));
ui->lbl_height->setText(QString::number(new_h));
m_pixmap = QPixmap::fromImage(
m_image->scaled(new_w, new_h, Qt::KeepAspectRatio, Qt::FastTransformation));
ui->lbl_scale->setText(QString::number(scale));
}
void MainWindow::requality_image(int quality) {
QByteArray ba;
QBuffer buffer(&ba);
buffer.open(QIODevice::WriteOnly);
m_pixmap.save(&buffer, "WEBP", quality);
auto l_size_b = buffer.size();
double l_size_kb = buffer.size() / 1024.00;
ui->lbl_size->setText(QString::number(l_size_kb));
QImage image;
image.loadFromData(ba);
m_pixmap = QPixmap::fromImage(image);
ui->lbl_quality->setText(QString::number(quality));
double comp_p = 100.0 * l_size_b / m_orig_size;
if(comp_p>100) {
ui->lbl_compression->setText(QString::number(comp_p));
QLabel* m_label = ui->lbl_size;
m_label->setStyleSheet("QLabel { background-color : red; color : black; }");
}
else if(comp_p<=100) {
ui->lbl_compression->setText(QString::number(comp_p));
QLabel* m_label = ui->lbl_size;
m_label->setStyleSheet("QLabel { background-color : rgba(0,0,0,0%); color : black; }");
}
}
void MainWindow::on_sld_quality_valueChanged(int value) {
reprocess_image(ui->sld_scale->value(), value);
}
void MainWindow::on_sld_scale_valueChanged(int scale) {
reprocess_image(scale, ui->sld_quality->value());
}
And this is my mainwindow.h:
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QGraphicsPixmapItem>
QT_FORWARD_DECLARE_CLASS(QGraphicsScene)
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow {
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
protected:
virtual void showEvent(QShowEvent *e) override;
private slots:
void on_openButton_clicked();
void on_sld_quality_valueChanged(int value);
void on_sld_scale_valueChanged(int value);
void on_saveButton_clicked();
private:
void reprocess_image(int scale, int quality);
void rescale_image(int);
void requality_image(int);
void show_pixmap();
void change_size();
Ui::MainWindow *ui;
QPixmap m_pixmap;
QImage *m_image;
qint64 m_orig_size;
QGraphicsScene *m_scene;
};
#endif // MAINWINDOW_H
How can I integrate QtConcurrent::run(), QThread and QFuture to my code?

Answer 1

The whole point of QtConcurrent::run is that you're not managing your own threads. So there's nothing to integrate. To get notified about when code submitted to QtConcurrent::run has finished you can use QFutureWatcher or emit a signal from the callable.