How to animate two values using QTimeLine - c++

Question

I have a QGraphicsItem for which I want to animate a size change. Because of this, I need to vary both the height and width of the object over time. I have used the QTimeLine in the past for single-variable animations and I would like to use it here for two-variable if possible. However, I don't see a QTimeLine::setFrameRange() that works for two variables.
How can I accomplish this? Is there a better Qt class for this?

Answer 1

Since animations have to be attached to objects, your graphics item will be a QGraphicsObject, so you can expose the relevant properties to the Qt property system.
Since you're animating what amounts to a QSizeF, the simplest way would be to expose this as a single property and use a QPropertyAnimation.
In general, if you have to animate multiple properties in parallel, set them up as individual QPropertyAnimations. Then run those in parallel using a QParallelAnimationGroup.
It's of course possible to use a QTimeLine, but then you have to interpolate between the endpoints manually, basing on the frame number, for example. Why bother.
Below is a complete example that shows how to animate three properties at once: pos, size and rotation.
main.cpp
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QPropertyAnimation>
#include <QParallelAnimationGroup>
#include <QSequentialAnimationGroup>
#include <QGraphicsObject>
#include <QPainter>
#include <QApplication>
class Item : public QGraphicsObject {
Q_OBJECT
qreal m_pen;
QSizeF m_size;
Q_PROPERTY(QSizeF size READ size WRITE setSize NOTIFY newSize)
Q_SIGNAL void newSize();
qreal width() const { return m_size.width() + m_pen; }
qreal height() const { return m_size.height() + m_pen; }
public:
explicit Item(QGraphicsItem *parent = 0) :
QGraphicsObject(parent), m_pen(5.0), m_size(50.0, 50.0) {}
QSizeF size() const { return m_size; }
void setSize(const QSizeF & size) {
if (m_size != size) {
m_size = size;
update();
emit newSize();
}
}
QRectF boundingRect() const {
return QRectF(-width()/2, -height()/2, width(), height());
}
void paint(QPainter *p, const QStyleOptionGraphicsItem *, QWidget *) {
p->setPen(QPen(Qt::black, m_pen));
p->drawEllipse(QPointF(0,0), width()/2, height()/2);
p->setPen(QPen(Qt::red, m_pen));
p->drawLine(0, 0, 0, height()/2);
}
};
void animate(QObject * obj)
{
QParallelAnimationGroup * group = new QParallelAnimationGroup(obj);
QPropertyAnimation * pos = new QPropertyAnimation(obj, "pos", obj);
QPropertyAnimation * size = new QPropertyAnimation(obj, "size", obj);
QPropertyAnimation * rot= new QPropertyAnimation(obj, "rotation", obj);
pos->setDuration(3000);
pos->setLoopCount(-1);
pos->setEasingCurve(QEasingCurve::InOutCubic);
pos->setStartValue(QPointF(-50, -50));
pos->setEndValue(QPointF(50, 50));
size->setDuration(1500);
size->setLoopCount(-1);
size->setEasingCurve(QEasingCurve::InOutElastic);
size->setStartValue(QSizeF(100, 100));
size->setEndValue(QSizeF(100, 30));
rot->setDuration(1000);
rot->setLoopCount(-1);
rot->setStartValue(0.0);
rot->setEndValue(360.0);
group->addAnimation(pos);
group->addAnimation(size);
group->addAnimation(rot);
group->start();
}
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QGraphicsScene s;
QGraphicsView v(&s);
Item * item = new Item;
s.addItem(item);
v.setRenderHint(QPainter::Antialiasing);
v.setSceneRect(-125, -125, 300, 300);
v.show();
animate(item);
return a.exec();
}
#include "main.moc"