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How can I use one horizontal scrollbar to scroll several QGraphicsViews?

In order to use one scrollbar to scroll several views, you can put your views and a QScrollBar [doc.qt.nokia.com] inside a widget and connect the scrollbar’s value changed signal to the setValue() [doc.qt.nokia.com] slots of the hidden horizontal scrollbars. In addition the ranges need to be kept in sync, although you can easily change this if you want to make it a bit smarter. See the following example for a demonstration:

  1. #include <QtGui>
  2.  
  3. class Widget : public QWidget
  4. {
  5.     Q_OBJECT
  6. public:
  7.     Widget(QWidget *parent = 0) : QWidget(parent)
  8.     {
  9.         QGraphicsScene *scene = new QGraphicsScene();
  10.         scene->addEllipse(QRectF(-100, -100, 300, 200), QPen(Qt::blue), QBrush(Qt::cyan));
  11.  
  12.         QGraphicsView *view1 = new QGraphicsView(scene);
  13.         view1->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
  14.  
  15.         QGraphicsView *view2 = new QGraphicsView(scene);
  16.         view2->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
  17.  
  18.         scrollbar = new QScrollBar(Qt::Horizontal, this);
  19.         QVBoxLayout *vertical = new QVBoxLayout(this);
  20.         QWidget *subWidget = new QWidget(this);
  21.         QHBoxLayout *layout = new QHBoxLayout(subWidget);
  22.         layout->addWidget(view1);
  23.         layout->addWidget(view2);
  24.  
  25.         vertical->addWidget(subWidget);
  26.         vertical->addWidget(scrollbar);
  27.         scrollbar->setRange(view1->horizontalScrollBar()->minimum(), view1->horizontalScrollBar()->maximum());
  28.  
  29.         connect(scrollbar, SIGNAL(valueChanged(int)), view1->horizontalScrollBar(), SLOT(setValue(int)));
  30.         connect(scrollbar, SIGNAL(valueChanged(int)), view2->horizontalScrollBar(), SLOT(setValue(int)));
  31.         connect(view1->horizontalScrollBar(), SIGNAL(rangeChanged(int, int)), this, SLOT(updateRange(int, int)));
  32.     }
  33. public slots:
  34.     void updateRange(int min, int max)
  35.     {
  36.         scrollbar->setRange(min, max);
  37.     }
  38. private:
  39.     QScrollBar *scrollbar;
  40. };
  41.  
  42. #include "main.moc"
  43.  
  44. int main(int argc, char** argv)
  45. {
  46.     QApplication app(argc, argv);
  47.     Widget widget;
  48.     widget.show();
  49.     return app.exec();
  50. }
  51.  

4 comments

November 22, 2010

Picture of JesseHobbs JesseHobbs

Lab Rat

  • Qt Developer Days 2011

That works if the views are scrolled only using the scroll bar. Using the cursor keys
up/down/pgup/pgdown or <ALT>+mousewheel causes the views to become out of sync.
The QGraphicsViews need their scroll bars cross connected with valueChange/setValue functions in addition to connecting to the master scroll bar. The connect code shown above should be:

connect(scrollbar, SIGNAL), view1->horizontalScrollBar(), SLOT));
//connect(scrollbar, SIGNAL), view2->horizontalScrollBar(), SLOT));
connect(view1->horizontalScrollBar(), SIGNAL), this, SLOT));

connect(view1->horizontalScrollBar(),SIGNAL), view2->horizontalScrollBar(),SLOT));
connect(view2->horizontalScrollBar(),SIGNAL), view1->horizontalScrollBar(),SLOT));
connect(view1->horizontalScrollBar(), SIGNAL), scrollbar, SLOT));

The views are smart enough to prevent infinite signal recursion on setValue

November 22, 2010

Picture of JesseHobbs JesseHobbs

Lab Rat

  • Qt Developer Days 2011

Let’s try again. The formatter made the code unreadable.
The code should read:

connect ( scrollbar, SIGNAL ( valueChanged ( int ) ) , view1->horizontalScrollBar ( ), SLOT ( setValue ( int ) ) ) ;
connect ( view1->horizontalScrollBar ( ) , SIGNAL ( rangeChanged ( int, int ) ) , this, SLOT ( updateRange ( int, int ) ) ) ;

connect ( view1->horizontalScrollBar ( ) , SIGNAL ( valueChanged ( int ) ) , view2->horizontalScrollBar ( ) , SLOT ( setValue ( int ) ) ) ;
connect ( view2->horizontalScrollBar ( ) , SIGNAL ( valueChanged ( int ) ) , view1->horizontalScrollBar ( ) , SLOT ( setValue ( int ) ) ) ;
connect ( view1->horizontalScrollBar ( ) , SIGNAL ( valueChanged ( int ) ) , scrollbar, SLOT ( setValue ( int ) ) ) ;

December 4, 2010

Picture of xsacha xsacha

Lab Rat

  • Groupie L1
  • Qt DevNet Moderator

Formatting won’t work in here Jesse. We get what you’re saying though :)

I don’t know if it’s good to rely on infinite recursion detection. Doesn’t it cause a slow down?

December 13, 2010

Picture of JesseHobbs JesseHobbs

Lab Rat

  • Qt Developer Days 2011

I have not seen any slowdown. Scrolling response is instantaneous and there is no duplicate drawing occurring in either GraphicsView.

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