- Qt 5.15
- Qt Widgets
- C++ Classes
- QScroller
- Qt 5.15.18 Reference Documentation

# QScroller Class

The QScroller class enables kinetic scrolling for any scrolling widget or graphics item. More...

Header: | #include <QScroller>
qmake: | QT += widgets
Since: | Qt 5.0

This class was introduced in Qt 5.0.

- List of all members, including inherited members

## Public Types

enum | Input { InputPress, InputMove, InputRelease }
enum | ScrollerGestureType { TouchGesture, LeftMouseButtonGesture, MiddleMouseButtonGesture, RightMouseButtonGesture }
enum | State { Inactive, Pressed, Dragging, Scrolling }

## Properties

- scrollerProperties : QScrollerProperties
- state : const State

## Public Functions

QPointF | finalPosition() const
bool | handleInput(Input input, const QPointF &position, qint64 timestamp = 0)
QPointF | pixelPerMeter() const
QScrollerProperties | scrollerProperties() const
void | setSnapPositionsX(const QList<qreal> &positions)
void | setSnapPositionsX(qreal first, qreal interval)
void | setSnapPositionsY(const QList<qreal> &positions)
void | setSnapPositionsY(qreal first, qreal interval)
State | state() const
void | stop()
QObject * | target() const
QPointF | velocity() const

## Public Slots

void | ensureVisible(const QRectF &rect, qreal xmargin, qreal ymargin, int scrollTime)
void | ensureVisible(const QRectF &rect, qreal xmargin, qreal ymargin)
void | resendPrepareEvent()
void | scrollTo(const QPointF &pos, int scrollTime)
void | scrollTo(const QPointF &pos)
void | setScrollerProperties(const QScrollerProperties &prop)

## Signals

void | scrollerPropertiesChanged(const QScrollerProperties &newProperties)
void | stateChanged(QScroller::State newState)

## Static Public Members

QList<QScroller *> | activeScrollers()
Qt::GestureType | grabGesture(QObject *target, ScrollerGestureType scrollGestureType = TouchGesture)
Qt::GestureType | grabbedGesture(QObject *target)
bool | hasScroller(QObject *target)
QScroller * | scroller(QObject *target)
const QScroller * | scroller(const QObject *target)
void | ungrabGesture(QObject *target)

## Detailed Description

With kinetic scrolling, the user can push the widget in a given direction and it will continue to scroll in this direction until it is stopped either by the user or by friction. Aspects of inertia, friction and other physical concepts can be changed in order to fine-tune an intuitive user experience.

The QScroller object is the object that stores the current position and scrolling speed and takes care of updates. QScroller can be triggered by a flick gesture

         QWidget *w = ...;
         QScroller::grabGesture(w, QScroller::LeftMouseButtonGesture);

or directly like this:

         QWidget *w = ...;
         QScroller *scroller = QScroller::scroller(w);
         scroller->scrollTo(QPointF(100, 100));

The scrolled QObjects receive a QScrollPrepareEvent whenever the scroller needs to update its geometry information and a QScrollEvent whenever the content of the object should actually be scrolled.

The scroller uses the global QAbstractAnimation timer to generate its QScrollEvents. This can be changed with QScrollerProperties::FrameRate on a per-QScroller basis.

The Dir View Example shows one way to use a QScroller with a QTreeView. An example in the `scroller` examples directory also demonstrates QScroller.

Even though this kinetic scroller has a large number of settings available via QScrollerProperties, we recommend that you leave them all at their default, platform optimized values. Before changing them you can experiment with the `plot` example in the `scroller` examples directory.

See also QScrollEvent, QScrollPrepareEvent, and QScrollerProperties.

## Member Type Documentation

### enum QScroller::Input

This enum contains an input device agnostic view of input events that are relevant for QScroller.

Constant | Value | Description
--- | --- | ---
`QScroller::InputPress` | `1` | The user pressed the input device (e.g. QEvent::MouseButtonPress, QEvent::GraphicsSceneMousePress, QEvent::TouchBegin)
`QScroller::InputMove` | `2` | The user moved the input device (e.g. QEvent::MouseMove, QEvent::GraphicsSceneMouseMove, QEvent::TouchUpdate)
`QScroller::InputRelease` | `3` | The user released the input device (e.g. QEvent::MouseButtonRelease, QEvent::GraphicsSceneMouseRelease, QEvent::TouchEnd)

### enum QScroller::ScrollerGestureType

This enum contains the different gesture types that are supported by the QScroller gesture recognizer.

Constant | Value | Description
--- | --- | ---
`QScroller::TouchGesture` | `0` | The gesture recognizer will only trigger on touch events. Specifically it will react on single touch points when using a touch screen and dual touch points when using a touchpad.
`QScroller::LeftMouseButtonGesture` | `1` | The gesture recognizer will only trigger on left mouse button events.
`QScroller::MiddleMouseButtonGesture` | `3` | The gesture recognizer will only trigger on middle mouse button events.
`QScroller::RightMouseButtonGesture` | `2` | The gesture recognizer will only trigger on right mouse button events.

### enum QScroller::State

This enum contains the different QScroller states.

Constant | Value | Description
--- | --- | ---
`QScroller::Inactive` | `0` | The scroller is not scrolling and nothing is pressed.
`QScroller::Pressed` | `1` | A touch event was received or the mouse button was pressed but the scroll area is currently not dragged.
`QScroller::Dragging` | `2` | The scroll area is currently following the touch point or mouse.
`QScroller::Scrolling` | `3` | The scroll area is moving on it's own.

## Property Documentation

### scrollerProperties : QScrollerProperties

This property holds the scroller properties of this scroller. The properties are used by the QScroller to determine its scrolling behavior.

Access functions:

QScrollerProperties | scrollerProperties() const
void | setScrollerProperties(const QScrollerProperties &prop)

Notifier signal:

void | scrollerPropertiesChanged(const QScrollerProperties &newProperties)

### state : const State

This property holds the state of the scroller

Access functions:

State | state() const

Notifier signal:

void | stateChanged(QScroller::State newState)

See also QScroller::State.

## Member Function Documentation

### `[slot]` void QScroller::ensureVisible(const QRectF &rect, qreal xmargin, qreal ymargin, int scrollTime)

This is an overloaded function.

This version will reach its destination position in scrollTime milliseconds.

### `[slot]` void QScroller::ensureVisible(const QRectF &rect, qreal xmargin, qreal ymargin)

Starts scrolling so that the rectangle rect is visible inside the viewport with additional margins specified in pixels by xmargin and ymargin around the rect.

In cases where it is not possible to fit the rect plus margins inside the viewport the contents are scrolled so that as much as possible is visible from rect.

The scrolling speed is calculated so that the given position is reached after a platform-defined time span.

This function performs the actual scrolling by calling scrollTo().

See also scrollTo().

### `[slot]` void QScroller::resendPrepareEvent()

This function resends the QScrollPrepareEvent. Calling resendPrepareEvent triggers a QScrollPrepareEvent from the scroller. This allows the receiver to re-set content position and content size while scrolling. Calling this function while in the Inactive state is useless as the prepare event is sent again before scrolling starts.

### `[slot]` void QScroller::scrollTo(const QPointF &pos, int scrollTime)

This is an overloaded function.

This version will reach its destination position in scrollTime milliseconds.

### `[slot]` void QScroller::scrollTo(const QPointF &pos)

Starts scrolling the widget so that point pos is at the top-left position in the viewport.

The behaviour when scrolling outside the valid scroll area is undefined. In this case the scroller might or might not overshoot.

The scrolling speed will be calculated so that the given position will be reached after a platform-defined time span.

pos is given in viewport coordinates.

See also ensureVisible().

### `[signal]` void QScroller::scrollerPropertiesChanged(const QScrollerProperties &newProperties)

QScroller emits this signal whenever its scroller properties change. newProperties are the new scroller properties.

Note: Notifier signal for property scrollerProperties.

See also scrollerProperties.

### `[signal]` void QScroller::stateChanged(QScroller::State newState)

QScroller emits this signal whenever the state changes. newState is the new State.

Note: Notifier signal for property state.

See also state.

### `[static]` QList<QScroller *> QScroller::activeScrollers()

Returns an application wide list of currently active QScroller objects. Active QScroller objects are in a state() that is not QScroller::Inactive. This function is useful when writing your own gesture recognizer.

### QPointF QScroller::finalPosition() const

Returns the estimated final position for the current scroll movement. Returns the current position if the scroller state is not Scrolling. The result is undefined when the scroller state is Inactive.

The target position is in pixel.

See also pixelPerMeter() and scrollTo().

### `[static]` Qt::GestureType QScroller::grabGesture(QObject *target, ScrollerGestureType scrollGestureType = TouchGesture)

Registers a custom scroll gesture recognizer, grabs it for the target and returns the resulting gesture type. If scrollGestureType is set to TouchGesture the gesture triggers on touch events. If it is set to one of LeftMouseButtonGesture, RightMouseButtonGesture or MiddleMouseButtonGesture it triggers on mouse events of the corresponding button.

Only one scroll gesture can be active on a single object at the same time. If you call this function twice on the same object, it will ungrab the existing gesture before grabbing the new one.

Note: To avoid unwanted side-effects, mouse events are consumed while the gesture is triggered. Since the initial mouse press event is not consumed, the gesture sends a fake mouse release event at the global position `(INT_MIN, INT_MIN)`. This ensures that internal states of the widget that received the original mouse press are consistent.

See also ungrabGesture() and grabbedGesture().

### `[static]` Qt::GestureType QScroller::grabbedGesture(QObject *target)

Returns the gesture type currently grabbed for the target or 0 if no gesture is grabbed.

See also grabGesture() and ungrabGesture().

### bool QScroller::handleInput(Input input, const QPointF &position, qint64 timestamp = 0)

This function is used by gesture recognizers to inform the scroller about a new input event. The scroller changes its internal state() according to the input event and its attached scroller properties. The scroller doesn't distinguish between the kind of input device the event came from. Therefore the event needs to be split into the input type, a position and a milli-second timestamp. The position needs to be in the target's coordinate system.

The return value is `true` if the event should be consumed by the calling filter or `false` if the event should be forwarded to the control.

Note: Using grabGesture() should be sufficient for most use cases.

### `[static]` bool QScroller::hasScroller(QObject *target)

Returns `true` if a QScroller object was already created for target; `false` otherwise.

See also scroller().

### QPointF QScroller::pixelPerMeter() const

Returns the pixel per meter metric for the scrolled widget.

The value is reported for both the x and y axis separately by using a QPointF.

Note: Please note that this value should be physically correct. The actual DPI settings that Qt returns for the display may be reported wrongly on purpose by the underlying windowing system, for example on macOS.

### `[static]` QScroller *QScroller::scroller(QObject *target)

Returns the scroller for the given target. As long as the object exists this function will always return the same QScroller instance. If no QScroller exists for the target, one will implicitly be created. At no point more than one QScroller will be active on an object.

See also hasScroller() and target().

### `[static]` const QScroller *QScroller::scroller(const QObject *target)

This is an overloaded function.

This is the const version of scroller().

### void QScroller::setSnapPositionsX(const QList<qreal> &positions)

Set the snap positions for the horizontal axis to a list of positions. This overwrites all previously set snap positions and also a previously set snapping interval. Snapping can be deactivated by setting an empty list of positions.

### void QScroller::setSnapPositionsX(qreal first, qreal interval)

Set the snap positions for the horizontal axis to regular spaced intervals. The first snap position is at first. The next at first + interval. This can be used to implement a list header. This overwrites all previously set snap positions and also a previously set snapping interval. Snapping can be deactivated by setting an interval of 0.0

### void QScroller::setSnapPositionsY(const QList<qreal> &positions)

Set the snap positions for the vertical axis to a list of positions. This overwrites all previously set snap positions and also a previously set snapping interval. Snapping can be deactivated by setting an empty list of positions.

### void QScroller::setSnapPositionsY(qreal first, qreal interval)

Set the snap positions for the vertical axis to regular spaced intervals. The first snap position is at first. The next at first + interval. This overwrites all previously set snap positions and also a previously set snapping interval. Snapping can be deactivated by setting an interval of 0.0

### void QScroller::stop()

Stops the scroller and resets its state back to Inactive.

### QObject *QScroller::target() const

Returns the target object of this scroller.

See also hasScroller() and scroller().

### `[static]` void QScroller::ungrabGesture(QObject *target)

Ungrabs the gesture for the target. Does nothing if no gesture is grabbed.

See also grabGesture() and grabbedGesture().

### QPointF QScroller::velocity() const

Returns the current scrolling velocity in meter per second when the state is Scrolling or Dragging. Returns a zero velocity otherwise.

The velocity is reported for both the x and y axis separately by using a QPointF.

See also pixelPerMeter().