path: root/src/declarative/util/qmlfollow.cpp

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#include <limits.h>
#include <QtCore/qdebug.h>
#include "private/qobject_p.h"
#include "qmlfollow.h"
#include "private/qmlanimation_p.h"


QT_BEGIN_NAMESPACE
QML_DEFINE_TYPE(QmlFollow,Follow);

class QmlFollowPrivate : public QObjectPrivate
{
    Q_DECLARE_PUBLIC(QmlFollow)
public:
    QmlFollowPrivate()
        : sourceValue(0), maxVelocity(0), lastTime(0)
        , mass(1.0), spring(0.), damping(0.), velocity(0), epsilon(0.005), enabled(true), mode(Track), clock(this) {}

    QmlMetaProperty property;
    qreal currentValue;
    qreal sourceValue;
    qreal maxVelocity;
    qreal velocityms;
    int lastTime;
    qreal mass;
    qreal spring;
    qreal damping;
    qreal velocity;
    qreal epsilon;
    bool enabled;

    enum Mode {
        Track,
        Velocity,
        Spring
    };
    Mode mode;

    void tick(int);
    void updateMode();
    void start();
    void stop();

    QTickAnimationProxy<QmlFollowPrivate, &QmlFollowPrivate::tick> clock;
};

void QmlFollowPrivate::tick(int time)
{
    Q_Q(QmlFollow);

    int elapsed = time - lastTime;
    if (!elapsed)
        return;
    if (mode == Spring) {
        if (elapsed < 10) // capped at 100fps.
            return;
        // Real men solve the spring DEs using RK4.
        // We'll do something much simpler which gives a result that looks fine.
        int count = (elapsed+5) / 10;
        for (int i = 0; i < count; ++i) {
            qreal diff = sourceValue - currentValue;
            velocity = velocity + spring * diff - damping * velocity;
            // The following line supports mass.  Not sure its worth the extra divisions.
            // velocity = velocity + spring / mass * diff - damping / mass * velocity;
            if (maxVelocity > 0.) {
                // limit velocity
                if (velocity > maxVelocity)
                    velocity = maxVelocity;
                else if (velocity < -maxVelocity)
                    velocity = -maxVelocity;
            }
            currentValue += velocity * 10.0 / 1000.0;
        }
        if (qAbs(velocity) < epsilon && qAbs(sourceValue - currentValue) < epsilon) {
            velocity = 0.0;
            currentValue = sourceValue;
            clock.stop();
        }
        lastTime = time - (elapsed - count * 10);
    } else {
        qreal moveBy = elapsed * velocityms;
        qreal diff = sourceValue - currentValue;
        if (diff > 0) {
            currentValue += moveBy;
            if (currentValue > sourceValue) {
                currentValue = sourceValue;
                clock.stop();
            }
        } else {
            currentValue -= moveBy;
            if (currentValue < sourceValue) {
                currentValue = sourceValue;
                clock.stop();
            }
        }
        lastTime = time;
    }
    emit q->valueChanged(currentValue);
    property.write(currentValue);
}

void QmlFollowPrivate::updateMode()
{
    if (spring == 0. && maxVelocity == 0.)
        mode = Track;
    else if (spring > 0.)
        mode = Spring;
    else
        mode = Velocity;
}

void QmlFollowPrivate::start()
{
    if (!enabled)
        return;

    if (mode == QmlFollowPrivate::Track) {
        currentValue = sourceValue;
        property.write(currentValue);
    } else if (sourceValue != currentValue && clock.state() != QAbstractAnimation::Running) {
        lastTime = 0;
        clock.start(); // infinity??
    }
}

void QmlFollowPrivate::stop()
{
    clock.stop();
}

/*!
    \qmlclass Follow QmlFollow
    \brief The Follow element allows a property to track a value.

    In example below, Rect2 will follow Rect1 moving with a velocity of up to 200:
    \code
    Rect {
        id: Rect1
        width: 20; height: 20
        color: "#00ff00"
        y: 200  //initial value
        y: SequentialAnimation {
            running: true
            repeat: true
            NumericAnimation {
                to: 200
                easing: "easeOutBounce(amplitude:100)"
                duration: 2000
            }
            PauseAnimation { duration: 1000 }
        }
    }
    Rect {
        id: Rect2
        x: Rect1.width
        width: 20; height: 20
        color: "#ff0000"
        y: Follow { source: Rect1.y; velocity: 200 }
    }
    \endcode
*/

QmlFollow::QmlFollow(QObject *parent)
: QmlPropertyValueSource(*(new QmlFollowPrivate),parent)
{
}

QmlFollow::~QmlFollow()
{
}

void QmlFollow::setTarget(const QmlMetaProperty &property)
{
    Q_D(QmlFollow);
    d->property = property;
    d->currentValue = property.read().toDouble();
}

qreal QmlFollow::sourceValue() const
{
    Q_D(const QmlFollow);
    return d->sourceValue;
}

/*!
    \qmlproperty qreal Follow::source
    This property holds the source value which will be tracked.

    Bind to a property in order to track its changes.
*/

void QmlFollow::setSourceValue(qreal value)
{
    Q_D(QmlFollow);
    d->sourceValue = value;
    d->start();
}

/*!
    \qmlproperty qreal Follow::velocity
    This property holds the maximum velocity allowed when tracking the source.
*/

qreal QmlFollow::velocity() const
{
    Q_D(const QmlFollow);
    return d->maxVelocity;
}

void QmlFollow::setVelocity(qreal velocity)
{
    Q_D(QmlFollow);
    d->maxVelocity = velocity;
    d->velocityms = velocity / 1000.0;
    d->updateMode();
}

/*!
    \qmlproperty qreal Follow::spring
    This property holds the spring constant

    The spring constant describes how strongly the target is pulled towards the
    source.  Setting spring to 0 turns off spring tracking.  Useful values 0 - 5.0

    When a spring constant is set and the velocity property is greater than 0,
    velocity limits the maximum speed.
*/
qreal QmlFollow::spring() const
{
    Q_D(const QmlFollow);
    return d->spring;
}

void QmlFollow::setSpring(qreal spring)
{
    Q_D(QmlFollow);
    d->spring = spring;
    d->updateMode();
}

/*!
    \qmlproperty qreal Follow::damping
    This property holds the spring damping constant

    The damping constant describes how quickly a sprung follower comes to rest.
    Useful range is 0 - 1.0
*/
qreal QmlFollow::damping() const
{
    Q_D(const QmlFollow);
    return d->damping;
}

void QmlFollow::setDamping(qreal damping)
{
    Q_D(QmlFollow);
    if (damping > 1.)
        damping = 1.;

    d->damping = damping;
}


/*!
    \qmlproperty qreal Follow::epsilon
    This property holds the spring epsilon

    The epsilon is the rate and amount of change in the value which is close enough
    to 0 to be considered equal to zero. This will depend on the usage of the value.
    For pixel positions, 0.25 would suffice. For scale, 0.005 will suffice.

    The default is 0.005. Small performance improvements can result in tuning this
    value.
*/
qreal QmlFollow::epsilon() const
{
    Q_D(const QmlFollow);
    return d->epsilon;
}

void QmlFollow::setEpsilon(qreal epsilon)
{
    Q_D(QmlFollow);
    d->epsilon = epsilon;
}

/*!
    \qmlproperty qreal Follow::followValue
    The current value.
*/

/*!
    \qmlproperty bool Follow::enabled
    This property holds whether the target will track the source.
*/
bool QmlFollow::enabled() const
{
    Q_D(const QmlFollow);
    return d->enabled;
}

void QmlFollow::setEnabled(bool enabled)
{
    Q_D(QmlFollow);
    d->enabled = enabled;
    if (enabled)
        d->start();
    else
        d->stop();
}

qreal QmlFollow::value() const
{
    Q_D(const QmlFollow);
    return d->currentValue;
}

QT_END_NAMESPACE