setting needle size in qwtCompass

Hello,
This is my first post so I hope I give all the required info.
I use Qt 4.2.3 and Qwt 5.0.1

I'm trying to create a compass or a dial that will show wind direction.
I created a class that inherits QwtCompass and in it's constructor:

windNeedle = new QwtCompassWindArrow( Style1, red, black );
rose = new QwtSimpleCompassRose( 4, 1 );

I instanciate this class in a layout which is inside a QWidget. So the compass is only part of what is displayed on the screen.

I have 2 problems:

1. I would like to change the size and direction of the windNeedle. The function I found is drawStyle1Needle(...) or draw() which calls that function.
When I try to use that function in the constructor I get the error: QPainter::begin: Widget painting can only begin as a result of a paintEvent.
What is the right way to use those functions? How do I create the paintEvent and how and when do I use the draw() function?

2. I would also like to change the direction of the windNeedle (since it represents wind direction and it may change) - How do I get the value after the user moved the needle and how do I set the needle to a certain direction? - I guess the setting part would require redrawing of the compass which is my first problem.

Quote:

---

How do I get the value after the user moved the needle...

---

You have to look at the derived API of the base classes of QwtCompass:

1) QwtDoubleRange::value(), or
2) QwtAbstractSlider::valueChanged(double) signal.

0.0 means north, 270.0 is west.

Guess you want write something like this: "connect(compass, SIGNAL(valueChanged(double)), ...);"

Quote:

---

... and how do I set the needle to a certain direction.

---

"QwtAbstractSlider::setValue(double)", you don't have to repaint anything.

Quote:

---

I would like to change the size of the windNeedle

---

Code:

---

class YourCompass::public QwtCompass
{
    ...
 
    virtual void YourCompass::drawNeedle(
        QPainter *painter, const QPoint &pos,
            int radius, double direction, 
            QPalette::ColorGroup colorGroup) const
    {
        radius = ...; // decrease the radius here
        QwtCompass::drawNeedle(painter, pos, radius,
            direction, colorGroup);
    }
}

---

HTH,
Uwe

Thank you Uwe.

I can set and get the Needle value now, but I still have a problem with the needle size.

Can you please tell me what I'm missing?

I have a class that has an instance of the QwtCompass:

Code:

---

class Atmosphere: public QWidget
{
Atmosphere::Atmosphere
{
a=new myCompass();
connect(myCompass, SIGNAL(valueChanged(double)), ...);
}
...
}
 
class myCompass::public QwtCompass
{    
...   
myCompass::myCompass( parent ) : QwtCompass(parent)
{
  windNeedle = new QwtCompassWindArrow( Style1, red, black );
  rose = new QwtSimpleCompassRose( 4, 1 );
  update();
}  
 
// As in your replay, I've added:
virtual void YourCompass::drawNeedle(  QPainter *painter, const QPoint &pos,
            int radius, double direction,
             QPalette::ColorGroup colorGroup) const
...
}
 
}

---

I've added a printout in my drawNeedle function. It looks as if the function is never executed (not even when I move the needle or resize the window) . Do I need to call it manually?

Of course it has to look like this.

Code:

---

class myCompass: public QwtCompass
{
...
 
   virtual void drawNeedle( QPainter *painter, const QPoint &pos,
      int radius, double direction,
      QPalette::ColorGroup colorGroup) const
   {
      ...
   }
};

---

If this doesn't help start the debugger, set a breakpoint in QwtDial::drawNeedle and check if it is called from myCompass::drawNeedle.

Uwe

Thank you Uwe.

It works now. I didn't type it properly: const QPoint instead of const QPoint & :o