Draggable line segment

viniltc

I wonder if I have multiple line segments to drag, having more drag status variables is the best way to implement?

for example:

    bool dragging1 = false; // for line 1
    bool dragging2 = false; // fpr line 2

and

void MainWindow::mousePressEvent(QMouseEvent *event)
{
    QPoint mp = event->pos(); 
    if (distance ( mp, p1) < 10 && ( mp.x() > p1.x() && mp.x() < p2.x() ) ) {
        dragging1 = true;
    }
    else if (distance ( mp, p3) < 10 && ( mp.x() > p3.x() && mp.x() < p4.x() ) ) {
        dragging2 = true; 
    }
}

void MainWindow::mouseMoveEvent(QMouseEvent *event)
{
    if ( dragging1 ) {
        changeP1value(event->y());

    }
    else if (dragging2) {
        changeP2value(event->y());
    }
}

Whats your suggestion?

mrjj

@viniltc
Hi
Well that is a ok way if you have 2.
But if you had many, i would go with a
QPoint *CurPoint1=nullptr;
QPoint *CurPoint2=nullptr;

and then in

void MainWindow::mousePressEvent(QMouseEvent *event)
{
    QPoint mp = event->pos(); 
    if (distance ( mp, p1) < 10 && ( mp.x() > p1.x() && mp.x() < p2.x() ) ) {
        dragging = true;
       CurPoint1 = &p1;
       CurPoint2 = &p2;
    }
    else if (distance ( mp, p3) < 10 && ( mp.x() > p3.x() && mp.x() < p4.x() ) ) {
        dragging = true; 
        CurPoint1 = &p3;
        CurPoint1 = &p4;
    }
}

and then use that in to update the points in ValueChanged
as not to have same code over and over for more points.

the check and && ( mp.x() > p3.x() && mp.x() < p4.x() ) )
and assignment could also be changed to use the pointers instead as not to replicate code since
its the same but just with other points.
A list could also be used to allow as many as you need.

Word of warning. make sure to test if CurPoint1/2 is null before using them as
if you start with dragging slider, the values are null as we first assign on mouse.

viniltc

@mrjj Thanks a lot for your feedback, it works well.
I also did this in the constructor to show the line in a predefined height:

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    p1.setY(100);
    p2.setY(100);
    p3.setY(100);
    p4.setY(100);

}

what is the best way to set a range to drag, for example from 100 the line should only draggable within (50,150).

mrjj

@viniltc

Hi
You need to keep the original y values (in its own variables in .h/class)
and then in mousepress store then values ( or value since they have same y both points)
so in
mouseMoveEvent you can check if its still within allowed drag range and if not then do not call
the ValueChanged.

It will then stay with in your range.

viniltc

@mrjj

I did this:

private:
    Ui::MainWindow *ui;
     bool dragging = false;
     QPoint p1 = QPoint(400,100);
     QPoint p2 = QPoint(450,100);
     QPoint p3 = QPoint(450,100);
     QPoint p4 = QPoint(500,100);

     QPoint *CurPoint1=nullptr;
     QPoint *CurPoint2=nullptr;

I'm a bit confused how to implement those conditons in mousePressEvent and mouseMoveEvent
If I do this in the mouseMoveEvent

 if (dragging)
    {

        if( CurPoint1->y()>50 && CurPoint1->y()<150)
        {

        changeP1value(event->y());

        }
    }

It just stop update if I go beyond the limit. What might be the issue?

mrjj

Hi
If it stops showing the line maybe

 if (dragging)
    {
        if( CurPoint1->y()>50 && CurPoint1->y()<150)
        {
        changeP1value(event->y());
        }        
       else       
         update();    
}

To redraw the point with the last points value.

viniltc

@mrjj

Actually it just stops when I hit 50 or 150 , it is not updating after that.

I doubt if it's a problem with mousePressEvent , now I do this:

void MainWindow::mousePressEvent(QMouseEvent *event)
{
    QPoint mp = event->pos(); 
    if (distance ( mp, p1) < 10 && ( mp.x() > p1.x() && mp.x() < p2.x() ) ) {
        dragging = true; 
        CurPoint1 = &p1;
        CurPoint2 = &p2;        
    }
    else if (distance ( mp, p3) < 10 && ( mp.x() > p3.x() && mp.x() < p4.x() ) ) {
        dragging = true; 
        CurPoint1 = &p3;
        CurPoint2 = &p4;
    }
}

// then when mouse move
void MainWindow::mouseMoveEvent(QMouseEvent *event)
{
   // If we are dragging, call your normal slider changed function to update your points.
    if (dragging)
    {

        if( CurPoint1->y()>50 && CurPoint1->y()<150)
        {
        changeP1value(event->y());
        }
        else
        update();
    }

}

void MainWindow::changeP1value(int value)
{

    CurPoint1->setY(value);
    CurPoint2->setY(value);
    update();
}

mrjj

@viniltc said in Draggable line segment:

it is not updating after that.

You mean it stops drawing the line or in what way ?

viniltc

@mrjj

I mean, if I drag beyond 50 or 150 (y values). The line just stops at those points and freezes, can't drag anymore.

mrjj

@viniltc
Yes, was that not the goal ??
Or do u mean it get stucked and cannot be dragged down again ?

viniltc

@mrjj
Yeah, I can't drag down again. Once I hit 50 or 150. The line just freezes there.

viniltc

@mrjj

This is what I'm doing now, but the situation is same, the line gets stuck and can't drag down:

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();
     void paintEvent(QPaintEvent *e);

public slots:

      void changeP1value(int);


private:
    Ui::MainWindow *ui;
     bool dragging = false; 
     QPoint p1 = QPoint(400,100);
     QPoint p2 = QPoint(450,100);
     QPoint p3 = QPoint(450,100);
     QPoint p4 = QPoint(500,100);

     QPoint *CurPoint1=nullptr;
     QPoint *CurPoint2=nullptr;

protected:
     virtual void mousePressEvent(QMouseEvent *event) override;
     virtual void mouseReleaseEvent(QMouseEvent *event) override;
     virtual void mouseMoveEvent(QMouseEvent *event) override;
};
#endif // MAINWINDOW_H

and in .cpp

int distance(QPoint x1, QPoint x2)
{
    return abs(x2.y() - x1.y());
}


void MainWindow::paintEvent(QPaintEvent *e)
{
    QPainter painter1(this);

    QPen pointpen(Qt::black);
    pointpen.setWidth(5);
    QPen linepen1(Qt::red);
    linepen1.setWidth(2);
    QPen linepen2(Qt::green);
    linepen2.setWidth(2);

    painter1.setPen(linepen1);
    painter1.drawLine(p1,p2);
    painter1.setPen(linepen2);
    painter1.drawLine(p3,p4);

    qDebug()<<"Values of p1.y and p2.y:"<<p1.y()<<"and"<<p2.y();
}


void MainWindow::mousePressEvent(QMouseEvent *event)
{
    QPoint mp = event->pos(); 

    if (distance ( mp, p1) < 20 && ( mp.x() > p1.x() && mp.x() < p2.x() ) ) {
        dragging = true;

        CurPoint1 = &p1;
        CurPoint2 = &p2;
    }
    else if (distance ( mp, p3) < 20 && ( mp.x() > p3.x() && mp.x() < p4.x() ) ) {
        dragging = true;
        CurPoint1 = &p3;
        CurPoint2 = &p4;
    }
}

void MainWindow::mouseReleaseEvent(QMouseEvent *event)
{
    dragging = false; 
}


void MainWindow::mouseMoveEvent(QMouseEvent *event)
{
 
    if(CurPoint1!=nullptr)
        qDebug()<<"Curr point is NOT null";

    if (dragging)
    {
      if(CurPoint1->y()>50 && CurPoint1->y()<150)
        {
        changeP1value(event->y());
        }

      else
      update();
    }

}

void MainWindow::changeP1value(int value)
{

    CurPoint1->setY(value);
    CurPoint2->setY(value);
    update();

}

Can you spot any spot any issues?

mrjj

Hi
There is no errors as such but since we can continue drag mouse after the endpoint,
the mouse event coordinates drifts and hence even dragging is still true some of the other checks is not valid anymore. ( like you are not on the line anymore)

I do not know any good solution to it currently.