QWheelEvent Class

The QWheelEvent class contains parameters that describe a wheel event. More...

Header: #include <QWheelEvent>
CMake: find_package(Qt6 COMPONENTS Gui REQUIRED)
target_link_libraries(mytarget PRIVATE Qt6::Gui)
qmake: QT += gui
Inherits: QSinglePointEvent

Public Functions

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())
QPoint angleDelta() const
bool inverted() const
Qt::ScrollPhase phase() const
QPoint pixelDelta() const

Reimplemented Public Functions

virtual bool isBeginEvent() const override
virtual bool isEndEvent() const override
virtual bool isUpdateEvent() const override

Detailed Description

Wheel events are sent to the widget under the mouse cursor, but if that widget does not handle the event they are sent to the focus widget. Wheel events are generated for both mouse wheels and trackpad scroll gestures. There are two ways to read the wheel event delta: angleDelta() returns the deltas in wheel degrees. These values are always provided. pixelDelta() returns the deltas in screen pixels, and is available on platforms that have high-resolution trackpads, such as macOS. If that is the case, device()->type() will return QInputDevice::DeviceType::Touchpad.

The functions position() and globalPosition() return the mouse cursor's location at the time of the event.

A wheel event contains a special accept flag that indicates whether the receiver wants the event. You should call ignore() if you do not handle the wheel event; this ensures that it will be sent to the parent widget.

The QWidget::setEnabled() function can be used to enable or disable mouse and keyboard events for a widget.

The event handler QWidget::wheelEvent() receives wheel events.

See also QMouseEvent and QWidget::grabMouse().

Member Function Documentation

[since 5.12] 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.

This function was introduced in Qt 5.12.

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();
}

Note: On platforms that support scrolling phases, the delta may be null when:

  • 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).

Note: Getter function for property angleDelta.

See also pixelDelta().

[since 5.7] 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.)

Note: Many platforms provide no such information. On such platforms inverted always returns false.

Note: Getter function for property inverted.

This function was introduced in Qt 5.7.

[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.

[since 5.2] Qt::ScrollPhase QWheelEvent::phase() const

Returns the scrolling phase of this wheel event.

Note: The Qt::ScrollBegin and Qt::ScrollEnd phases are currently supported only on macOS.

Note: Getter function for property phase.

This function was introduced in Qt 5.2.

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();
}

Note: On platforms that support scrolling phases, the delta may be null when:

  • 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).

Note: On X11 this value is driver-specific and unreliable, use angleDelta() instead.

Note: Getter function for property pixelDelta.

© 2022 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.