KISS

Keep It Simple Stupid

Old: Qt Symbian: kinetic scrolling helper

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It was long-long time ago. Two years back actually. I wrote an application for the Symbian-based smartphones using Qt 4. Another post on this is here.

What was very unusual was the lack of kinetic scrolling — it’s the thing everyone’s got used to on mobile devices: when you swipe a list, it continues to scroll in that direction for a while. The first solution I’d found was FlickCharm, and the second one showed up some time later: QtScroller. I can’t recollect now what pros and cons each had, but unfortunately neither worked perfectly.

So to be able to switch them easily and leave which one was better, I created a kinetic scrolling wrapper class:

(scroller.h) download
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// http://www.egeek.me

#ifndef SCROLLER_H
#define SCROLLER_H

#include <QWidget>

/*
  Wrapper that provides kinetic scrolling for widgets.
  Can use either FlickCharm or QtScroller.
*/

class ScrollerPrivate;

class Scroller : public QObject
{
public:
    /* Activates kinetic scrolling on the given widget.
      The vertical scrollbar is usually hidden, but can be shown
      when withVScrollBar is true.
      If passClickEventsOnly is true, only mouse click events
      will be propagated to the widget. It's the default behavior.
    */
    static void activateOn(QWidget *widget, bool withVScrollBar = false, bool passClickEventsOnly = true);

protected:
    bool eventFilter(QObject *obj, QEvent *evt);

private:
    Scroller();
    ~Scroller();

    Scroller(const Scroller &);
    Scroller &operator=(const Scroller &);

    ScrollerPrivate *const d_ptr;

    Q_DECLARE_PRIVATE(Scroller)
};

#endif // SCROLLER_H
(scroller.cpp) download
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// http://www.egeek.me

#include "scroller.h"
#include <QtDebug>
#include <QAbstractItemView>
#include <QAbstractScrollArea>
#include <QWebFrame>
#include <QWebView>
#include <QMouseEvent>
#include <QApplication>
#include <QHash>
#include <QSet>

// Undefine to use QtScroller
//#define USE_FLICKCHARM

#ifdef USE_FLICKCHARM
#include "flickcharm.h"
#else
#include "qtscroller.h"

/* Max number of pixels between mouse press and release
  events that triggers sending fake mouse events */
#define SINGLE_CLICK_MAX_DIST   15
#endif


class ScrollerPrivate
{
public:
    static Scroller *instance();

    /* Adjusts (decreases) deceleration factor of scroller to be
      able to scroll more. */
    static void adjustDecelerationFactor(QtScroller* scroller);

    /* Reduces the mouse press delay in case when not only
      mouse click events are passed. */
    static void reduceMousePressDelay(QtScroller *scroller);

    QHash<QWidget *, QPoint> startPoints;
    QSet<QEvent *> ignoredEvents;
};

Scroller *ScrollerPrivate::instance()
{
    static Scroller scroller;
    return &scroller;
}

void ScrollerPrivate::adjustDecelerationFactor(QtScroller *scroller)
{
    if (!scroller)
        return;

    QtScrollerProperties props = scroller->scrollerProperties();
    float df = props.scrollMetric(QtScrollerProperties::DecelerationFactor).toFloat();
    df /= 2;
    props.setScrollMetric(QtScrollerProperties::DecelerationFactor, QVariant(df));
    scroller->setScrollerProperties(props);
}

void ScrollerPrivate::reduceMousePressDelay(QtScroller *scroller)
{
    if (!scroller)
        return;

    QtScrollerProperties props = scroller->scrollerProperties();
    props.setScrollMetric(QtScrollerProperties::MousePressEventDelay, .1f);
    scroller->setScrollerProperties(props);
}


Scroller::Scroller() :
    d_ptr(new ScrollerPrivate())
{
}

Scroller::~Scroller()
{
    delete d_ptr;
}

void Scroller::activateOn(QWidget *widget, bool withVScrollBar, bool passClickEventsOnly)
{
#ifdef USE_FLICKCHARM
    FlickCharm::instance()->activateOn(widget);
#else
    QtScroller::ScrollerGestureType type = QtScroller::LeftMouseButtonGesture;

    QAbstractScrollArea *scrollArea = qobject_cast<QAbstractScrollArea*>(widget);
    if (scrollArea) {
        if (!withVScrollBar) {
            scrollArea->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
        }
        scrollArea->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);


        QAbstractItemView *itemView = qobject_cast<QAbstractItemView*>(scrollArea);
        if (itemView) {
            // fix scrolling of lists
            itemView->setHorizontalScrollMode(QAbstractItemView::ScrollPerPixel);
            itemView->setVerticalScrollMode(QAbstractItemView::ScrollPerPixel);
        }

        if (passClickEventsOnly) {
            scrollArea->viewport()->installEventFilter(ScrollerPrivate::instance());
            scrollArea->installEventFilter(ScrollerPrivate::instance());
        }

        QtScroller::grabGesture(scrollArea->viewport(), type);
        QtScroller::grabGesture(scrollArea, type);

        QtScroller *scroller = QtScroller::scroller(scrollArea->viewport());
        ScrollerPrivate::adjustDecelerationFactor(scroller);
        if (!passClickEventsOnly) {
            ScrollerPrivate::reduceMousePressDelay(scroller);
        }

        scroller = QtScroller::scroller(scrollArea);
        ScrollerPrivate::adjustDecelerationFactor(scroller);
        if (!passClickEventsOnly) {
            ScrollerPrivate::reduceMousePressDelay(scroller);
        }

        return;
    }

    QWebView *webView = qobject_cast<QWebView*>(widget);
    if (webView) {
        QWebFrame *frame = webView->page()->mainFrame();
        if (!withVScrollBar) {
            frame->setScrollBarPolicy(Qt::Vertical, Qt::ScrollBarAlwaysOff);
        }
        frame->setScrollBarPolicy(Qt::Horizontal, Qt::ScrollBarAlwaysOff);

        if (passClickEventsOnly) {
            webView->installEventFilter(ScrollerPrivate::instance());
        }

        QtScroller::grabGesture(webView, type);

        if (!passClickEventsOnly) {
            ScrollerPrivate::reduceMousePressDelay(QtScroller::scroller(webView));
        }

        return;
    }

    qWarning() << "Scroller only works on QAbstractScrollArea (and derived classes)";
    qWarning() << "or QWebView (and derived classes)";
#endif
}

bool Scroller::eventFilter(QObject *obj, QEvent *evt)
{
#ifndef USE_FLICKCHARM
    Q_D(Scroller);

    /* to avoid selection in webviews and lists before
      scrolling has started, we need to consume all mouse
      events, except for single clicks */

    QWidget *widget = qobject_cast<QWidget *>(obj);
    Q_ASSERT(widget);
    switch (evt->type())
    {
    case QMouseEvent::MouseMove:
        /* don't propagate mouse move events */
        return true;
    case QMouseEvent::MouseButtonPress:
        /* on mouse press save the pos to the hash
          in order to check if it makes up a single click */
        if (d->ignoredEvents.contains(evt))
        {
            d->ignoredEvents.remove(evt);
            return false;
        }
        else
            d->startPoints[widget] = static_cast<QMouseEvent *>(evt)->pos();
        return true;
    case QMouseEvent::MouseButtonRelease:
        if (d->ignoredEvents.contains(evt))
        {
            d->ignoredEvents.remove(evt);
            return false;
        }
        else if (d->startPoints.contains(widget))
        {
            QPoint startPos = d->startPoints[widget];
            d->startPoints.remove(widget);

            /* if the distance is less than a limit,
              we consider it a single click */
            QPoint pos = static_cast<QMouseEvent *>(evt)->pos();
            if ((pos - startPos).manhattanLength() < SINGLE_CLICK_MAX_DIST)
            {
                QMouseEvent *event1 = new QMouseEvent(QEvent::MouseButtonPress,
                                                      pos, Qt::LeftButton,
                                                      Qt::LeftButton, Qt::NoModifier);
                QMouseEvent *event2 = new QMouseEvent(QEvent::MouseButtonRelease,
                                                      pos, Qt::LeftButton,
                                                      Qt::LeftButton, Qt::NoModifier);

                /* put the artifical events in the ignoredEvents set
                  to let them pass this method */
                d->ignoredEvents << event1 << event2;
                /* for some reason, postEvent caused a lot of events,
                  almost recursively, here */
                QApplication::sendEvent(obj, event1);
                QApplication::sendEvent(obj, event2);

                delete event2;
                delete event1;
            }
        }
        return true;
    default:
        break;
    }
#endif

    return QObject::eventFilter(obj, evt);
}

Maybe someone will find it useful.

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