QWheelEvent Class

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

Header: #include <QWheelEvent>
qmake: QT += gui
Inherits: QInputEvent

Public Functions

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

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 delta in wheel degrees. This value is always provided. pixelDelta() returns the delta in screen pixels and is available on platforms that have high-resolution trackpads, such as macOS. If that is the case, source() will return Qt::MouseEventSynthesizedBySystem.

The functions pos() and globalPos() 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

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

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 distance. 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.

If the wheel event comes from a physical mouse wheel, source is set to Qt::MouseEventNotSynthesized. If it comes from a gesture detected by the operating system, or from a non-mouse hardware device, such that pixelDelta is directly related to finger movement, source is set to Qt::MouseEventSynthesizedBySystem. If it comes from Qt, source would be set to Qt::MouseEventSynthesizedByQt.

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

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

QPoint QWheelEvent::angleDelta() const

Returns the distance that the wheel is 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.

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.

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).

Qt::MouseButtons QWheelEvent::buttons() const

Returns the mouse state when the event occurred.

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.

This function was introduced in Qt 5.7.

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.

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

Qt::MouseEventSource QWheelEvent::source() const

Returns information about the wheel event source.

The source can be used to distinguish between events that come from a mouse with a physical wheel and events that are generated by some other means, such as a flick gesture on a touchpad.

Note: Many platforms provide no such information. On such platforms Qt::MouseEventNotSynthesized is returned always.

This function was introduced in Qt 5.5.

See also Qt::MouseEventSource.

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