QWheelEvent Class

Member Function Documentation

QWheelEvent::QWheelEvent(const QPointF &pos, const QPointF &globalPos, QPoint pixelDelta, QPoint angleDelta, Qt::MouseButtons buttons, Qt::KeyboardModifiers modifiers, Qt::ScrollPhase phase, bool inverted, Qt::MouseEventSource source = Qt::MouseEventNotSynthesized, const QPointingDevice *device = QPointingDevice::primaryPointingDevice())

Constructs a wheel event object.

The pos provides the location of the mouse cursor within the window. The position in global coordinates is specified by globalPos.

pixelDelta contains the scrolling distance in pixels on screen, while angleDelta contains the wheel rotation angle. pixelDelta is optional and can be null.

The mouse and keyboard states at the time of the event are specified by buttons and modifiers.

The scrolling phase of the event is specified by phase, and the source indicates whether this is a genuine or artificial (synthesized) event.

If the system is configured to invert the delta values delivered with the event (such as natural scrolling of the touchpad on macOS), inverted should be true. Otherwise, inverted is false

The device from which the wheel event originated is specified by device.

See also position(), globalPosition(), angleDelta(), pixelDelta(), phase(), inverted(), and device().

QPoint QWheelEvent::angleDelta() const

Returns the relative amount that the wheel was rotated, in eighths of a degree. A positive value indicates that the wheel was rotated forwards away from the user; a negative value indicates that the wheel was rotated backwards toward the user. angleDelta().y() provides the angle through which the common vertical mouse wheel was rotated since the previous event. angleDelta().x() provides the angle through which the horizontal mouse wheel was rotated, if the mouse has a horizontal wheel; otherwise it stays at zero. Some mice allow the user to tilt the wheel to perform horizontal scrolling, and some touchpads support a horizontal scrolling gesture; that will also appear in angleDelta().x().

Most mouse types work in steps of 15 degrees, in which case the delta value is a multiple of 120; i.e., 120 units * 1/8 = 15 degrees.

However, some mice have finer-resolution wheels and send delta values that are less than 120 units (less than 15 degrees). To support this possibility, you can either cumulatively add the delta values from events until the value of 120 is reached, then scroll the widget, or you can partially scroll the widget in response to each wheel event. But to provide a more native feel, you should prefer pixelDelta() on platforms where it's available.

Example:

void MyWidget::wheelEvent(QWheelEvent *event)
{
    QPoint numPixels = event->pixelDelta();
    QPoint numDegrees = event->angleDelta() / 8;

    if (!numPixels.isNull()) {
        scrollWithPixels(numPixels);
    } else if (!numDegrees.isNull()) {
        QPoint numSteps = numDegrees / 15;
        scrollWithDegrees(numSteps);
    }

    event->accept();
}

• scrolling is about to begin, but the distance did not yet change (Qt::ScrollBegin),
• or scrolling has ended and the distance did not change anymore (Qt::ScrollEnd).

See also pixelDelta().

bool QWheelEvent::inverted() const

Returns whether the delta values delivered with the event are inverted.

Normally, a vertical wheel will produce a QWheelEvent with positive delta values if the top of the wheel is rotating away from the hand operating it. Similarly, a horizontal wheel movement will produce a QWheelEvent with positive delta values if the top of the wheel is moved to the left.

However, on some platforms this is configurable, so that the same operations described above will produce negative delta values (but with the same magnitude). With the inverted property a wheel event consumer can choose to always follow the direction of the wheel, regardless of the system settings, but only for specific widgets. (One such use case could be that the user is rotating the wheel in the same direction as a visual Tumbler rotates. Another usecase is to make a slider handle follow the direction of movement of fingers on a touchpad regardless of system configuration.)

[override virtual] bool QWheelEvent::isBeginEvent() const

Reimplements: QSinglePointEvent::isBeginEvent() const.

Returns true if this event's phase() is Qt::ScrollBegin.

[override virtual] bool QWheelEvent::isEndEvent() const

Reimplements: QSinglePointEvent::isEndEvent() const.

Returns true if this event's phase() is Qt::ScrollEnd.

[override virtual] bool QWheelEvent::isUpdateEvent() const

Reimplements: QSinglePointEvent::isUpdateEvent() const.

Returns true if this event's phase() is Qt::ScrollUpdate or Qt::ScrollMomentum.

Qt::ScrollPhase QWheelEvent::phase() const

Returns the scrolling phase of this wheel event.

QPoint QWheelEvent::pixelDelta() const

Returns the scrolling distance in pixels on screen. This value is provided on platforms that support high-resolution pixel-based delta values, such as macOS. The value should be used directly to scroll content on screen.

Example:

void MyWidget::wheelEvent(QWheelEvent *event)
{
    QPoint numPixels = event->pixelDelta();
    QPoint numDegrees = event->angleDelta() / 8;

    if (!numPixels.isNull()) {
        scrollWithPixels(numPixels);
    } else if (!numDegrees.isNull()) {
        QPoint numSteps = numDegrees / 15;
        scrollWithDegrees(numSteps);
    }

    event->accept();
}

• scrolling is about to begin, but the distance did not yet change (Qt::ScrollBegin),
• or scrolling has ended and the distance did not change anymore (Qt::ScrollEnd).