attach Qslider to a paint event

[viniltc]

Hi all,

I have a paint event implemented as follows:

void MainWindow::paintEvent(QPaintEvent *e)
{
    QPainter painter(this);
    QPen pointpen(Qt::black);
    pointpen.setWidth(5);
    QPen linepen(Qt::red);
    linepen.setWidth(2);


    QPoint p1;
    p1.setX(400);
    p1.setY(200);  //--> set QSlider value

    QPoint p2;
    p2.setX(500);
    p2.setY(100); //---> set QSlider value

    painter.setPen(linepen);


    painter.drawLine(p1,p2);

    painter.setPen(pointpen);
    painter.drawPoint(p1);
    painter.drawPoint(p2);
}

in this, I'm setting fixed y values to p1 and p2.
How do I set dynamic values to y from a QSlider?

Thanks in advance

[ODБOï]

@viniltc hi,
you can move p1 and p2 to member variables section, then just connect your sliders valueChanged signal to a method where you modify p1 and p2

[viniltc]

This post is deleted!

[viniltc]

@LeLev

Thanks for your feedback.

Here I can now vary p1.y and p2.y with QSlider, but as I'm calling:

painter.drawLine(p1,p2);

I expect realtime change of height of line as I change the Slider. But this is stuck at one value and does not change

void MainWindow::paintEvent(QPaintEvent *e)
{

    p1.setX(400);
    p2.setX(500);

    QPainter painter(this);
    QPen pointpen(Qt::black);
    pointpen.setWidth(5);
    QPen linepen(Qt::red);
    linepen.setWidth(2);
    painter.setPen(linepen);

    painter.drawLine(p1,p2); //----> can't see realtime chnage as I change Slider

    painter.setPen(pointpen);
    painter.drawPoint(p1);
    painter.drawPoint(p2);
}

void MainWindow::changeP1value(int value)
{
    p1.setY(value);
    p2.setY(value);
}

//connection in class constructor
connect(ui->verticalSlider, &QSlider::valueChanged, this, &MainWindow::changeP1value);

Can you spot an error?

[mrjj]

@viniltc said in attach Qslider to a paint event:
Hi

void MainWindow::changeP1value(int value)
{
p1.setY(value);
p2.setY(value);
update(); // make it repaint so it uses the new values
}

[viniltc]

@mrjj Thanks a lot, now it works fine! :)

[mrjj]

@viniltc
Hi
Well you did good job of following @LeLev advice so it was just a very minor detail
but its good to note that when you change member values,
you must call update() for it to repaint as it won't detect you change them.