flickable.cpp Example File

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 #include "flickable.h"

 #include <QtCore>
 #include <QtWidgets>

 class FlickableTicker: QObject
 {
 public:
     FlickableTicker(Flickable *scroller) {
         m_scroller = scroller;
     }

     void start(int interval) {
         if (!m_timer.isActive())
             m_timer.start(interval, this);
     }

     void stop() {
         m_timer.stop();
     }

 protected:
     void timerEvent(QTimerEvent *event) {
         Q_UNUSED(event);
         m_scroller->tick();
     }

 private:
     Flickable *m_scroller;
     QBasicTimer m_timer;
 };

 class FlickablePrivate
 {
 public:
     typedef enum {
         Steady,
         Pressed,
         ManualScroll,
         AutoScroll,
         Stop
     } State;

     State state;
     int threshold;
     QPoint pressPos;
     QPoint offset;
     QPoint delta;
     QPoint speed;
     FlickableTicker *ticker;
     QTime timeStamp;
     QWidget *target;
     QList<QEvent*> ignoreList;
 };

 Flickable::Flickable()
 {
     d = new FlickablePrivate;
     d->state = FlickablePrivate::Steady;
     d->threshold = 10;
     d->ticker = new FlickableTicker(this);
     d->timeStamp = QTime::currentTime();
     d->target = 0;
 }

 Flickable::~Flickable()
 {
     delete d;
 }

 void Flickable::setThreshold(int th)
 {
     if (th >= 0)
         d->threshold = th;
 }

 int Flickable::threshold() const
 {
     return d->threshold;
 }

 void Flickable::setAcceptMouseClick(QWidget *target)
 {
     d->target = target;
 }

 static QPoint deaccelerate(const QPoint &speed, int a = 1, int max = 64)
 {
     int x = qBound(-max, speed.x(), max);
     int y = qBound(-max, speed.y(), max);
     x = (x == 0) ? x : (x > 0) ? qMax(0, x - a) : qMin(0, x + a);
     y = (y == 0) ? y : (y > 0) ? qMax(0, y - a) : qMin(0, y + a);
     return QPoint(x, y);
 }

 void Flickable::handleMousePress(QMouseEvent *event)
 {
     event->ignore();

     if (event->button() != Qt::LeftButton)
         return;

     if (d->ignoreList.removeAll(event))
         return;

     switch (d->state) {

     case FlickablePrivate::Steady:
         event->accept();
         d->state = FlickablePrivate::Pressed;
         d->pressPos = event->pos();
         break;

     case FlickablePrivate::AutoScroll:
         event->accept();
         d->state = FlickablePrivate::Stop;
         d->speed = QPoint(0, 0);
         d->pressPos = event->pos();
         d->offset = scrollOffset();
         d->ticker->stop();
         break;

     default:
         break;
     }
 }

 void Flickable::handleMouseRelease(QMouseEvent *event)
 {
     event->ignore();

     if (event->button() != Qt::LeftButton)
         return;

     if (d->ignoreList.removeAll(event))
         return;

     QPoint delta;

     switch (d->state) {

     case FlickablePrivate::Pressed:
         event->accept();
         d->state = FlickablePrivate::Steady;
         if (d->target) {
             QMouseEvent *event1 = new QMouseEvent(QEvent::MouseButtonPress,
                                                   d->pressPos, Qt::LeftButton,
                                                   Qt::LeftButton, Qt::NoModifier);
             QMouseEvent *event2 = new QMouseEvent(*event);
             d->ignoreList << event1;
             d->ignoreList << event2;
             QApplication::postEvent(d->target, event1);
             QApplication::postEvent(d->target, event2);
         }
         break;

     case FlickablePrivate::ManualScroll:
         event->accept();
         delta = event->pos() - d->pressPos;
         if (d->timeStamp.elapsed() > 100) {
             d->timeStamp = QTime::currentTime();
             d->speed = delta - d->delta;
             d->delta = delta;
         }
         d->offset = scrollOffset();
         d->pressPos = event->pos();
         if (d->speed == QPoint(0, 0)) {
             d->state = FlickablePrivate::Steady;
         } else {
             d->speed /= 4;
             d->state = FlickablePrivate::AutoScroll;
             d->ticker->start(20);
         }
         break;

     case FlickablePrivate::Stop:
         event->accept();
         d->state = FlickablePrivate::Steady;
         d->offset = scrollOffset();
         break;

     default:
         break;
     }
 }

 void Flickable::handleMouseMove(QMouseEvent *event)
 {
     event->ignore();

     if (!(event->buttons() & Qt::LeftButton))
         return;

     if (d->ignoreList.removeAll(event))
         return;

     QPoint delta;

     switch (d->state) {

     case FlickablePrivate::Pressed:
     case FlickablePrivate::Stop:
         delta = event->pos() - d->pressPos;
         if (delta.x() > d->threshold || delta.x() < -d->threshold ||
                 delta.y() > d->threshold || delta.y() < -d->threshold) {
             d->timeStamp = QTime::currentTime();
             d->state = FlickablePrivate::ManualScroll;
             d->delta = QPoint(0, 0);
             d->pressPos = event->pos();
             event->accept();
         }
         break;

     case FlickablePrivate::ManualScroll:
         event->accept();
         delta = event->pos() - d->pressPos;
         setScrollOffset(d->offset - delta);
         if (d->timeStamp.elapsed() > 100) {
             d->timeStamp = QTime::currentTime();
             d->speed = delta - d->delta;
             d->delta = delta;
         }
         break;

     default:
         break;
     }
 }

 void Flickable::tick()
 {
     if (d->state == FlickablePrivate:: AutoScroll) {
         d->speed = deaccelerate(d->speed);
         setScrollOffset(d->offset - d->speed);
         d->offset = scrollOffset();
         if (d->speed == QPoint(0, 0)) {
             d->state = FlickablePrivate::Steady;
             d->ticker->stop();
         }
     } else {
         d->ticker->stop();
     }
 }