Qt qslider is not smooth

One possible solution to delay your processing signal

is to connect it to the slot using Qt :: QueuedConnection.

connect(ui->zoomSlider, SIGNAL(valueChanged(int)), ui->map, SLOT(SetZoom(int)), Qt::QueuedConnection);

      

        
        
        
      

    

When an Qt::QueuedConnection

emitted valueChanged

signal event will not be processed during generation, as is the case with directly

linked signals. The event will be added to the event loop queue. This is how it Qt::QueuedConnection

is implemented internally in Qt.

Specially for Nejat

to test this approach, you can use the following code:

mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

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

private slots:
    void signalReceived();
    void signalReceivedQueued();

    void buttonPressed();

signals:

    void directConnectedSignal();
    void queuedConnectedSignal();

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

      

        
        
        
      

    

mainwindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QDebug>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    connect(this, SIGNAL(directConnectedSignal()), SLOT(signalReceived()), Qt::DirectConnection);
    connect(this, SIGNAL(queuedConnectedSignal()), SLOT(signalReceivedQueued()), Qt::QueuedConnection);

    connect(ui->pushButton, SIGNAL(pressed()), SLOT(buttonPressed()));
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::signalReceived()
{
    qDebug() << "signalReceived";
}

void MainWindow::signalReceivedQueued()
{
    qDebug() << "signalReceivedQueued";
}

void MainWindow::buttonPressed()
{
    emit queuedConnectedSignal();
    emit directConnectedSignal();
}

      

        
        
        
      

    

If you run the above code snippet you get the following output on the button:

signalReceived 
signalReceivedQueued 

      

        
        
        
      

    

The signal queue is emitted first, but received last. And this can be used in your case to prioritize the processing of emitted signals.

Most of all, however, using a queue in a queue won't help you because the user emits a slider event too much and the UI will freeze anyway. So I can suggest the following:

• Determine why the user interface is freezing, what part of the code is freezing it.
• Try not to freeze asynchronous calls or by moving logic to a separate thread or using QtConcurrent
  
  
• If you really have no control over how the map is scaled on your web page, try ignoring all QSlider-generated events and reacting to the last 500
  
  ms interval generated, for example .