QGraphicsView Class

See also QGraphicsScene, QGraphicsItem, QGraphicsSceneEvent

flags QGraphicsView::CacheMode

This enum describes the flags that you can set for a QGraphicsView's cache mode.

The CacheMode type is a typedef for QFlags<CacheModeFlag>. It stores an OR combination of CacheModeFlag values.

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This enum describes the default action for the view when pressing and dragging the mouse over the viewport.

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flags QGraphicsView::OptimizationFlags

This enum describes flags that you can enable to improve rendering performance in QGraphicsView. By default, none of these flags are set. Note that setting a flag usually imposes a side effect, and this effect can vary between paint devices and platforms.

The OptimizationFlags type is a typedef for QFlags<OptimizationFlag>. It stores an OR combination of OptimizationFlag values.

This enums describe the possible anchors that QGraphicsView can use when the user resizes the view or when the view is transformed.

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This enum describes how QGraphicsView updates its viewport when the scene contents change or are exposed.

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This property holds the alignment of the scene in the view when the whole scene is visible.

If the whole scene is visible in the view, (i.e., there are no visible scroll bars,) the view's alignment will decide where the scene will be rendered in the view. For example, if the alignment is Qt::AlignCenter, which is default, the scene will be centered in the view, and if the alignment is (Qt::AlignLeft | Qt::AlignTop), the scene will be rendered in the top-left corner of the view.

Access functions:

• alignment() const

• void setAlignment( alignment)

This property holds the background brush of the scene.

This property sets the background brush for the scene in this view. It is used to override the scene's own background, and defines the behavior of drawBackground(). To provide custom background drawing for this view, you can reimplement drawBackground() instead.

By default, this property contains a brush with the Qt::NoBrush pattern.

Access functions:

• backgroundBrush() const

• void setBackgroundBrush(const &brush)

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This property holds which parts of the view are cached

QGraphicsView can cache pre-rendered content in a QPixmap, which is then drawn onto the viewport. The purpose of such caching is to speed up the total rendering time for areas that are slow to render. Texture, gradient and alpha blended backgrounds, for example, can be notibly slow to render; especially with a transformed view. The CacheBackground flag enables caching of the view's background. For example:

QGraphicsView view;
view.setBackgroundBrush(QImage(":/images/backgroundtile.png"));
view.setCacheMode(QGraphicsView::CacheBackground);

The cache is invalidated every time the view is transformed. However, when scrolling, only partial invalidation is required.

By default, nothing is cached.

Access functions:

• cacheMode() const

• void setCacheMode( mode)

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This property holds the behavior for dragging the mouse over the scene while the left mouse button is pressed.

This property defines what should happen when the user clicks on the scene background and drags the mouse (e.g., scrolling the viewport contents using a pointing hand cursor, or selecting multiple items with a rubber band). The default value, NoDrag, does nothing.

This behavior only affects mouse clicks that are not handled by any item. You can define a custom behavior by creating a subclass of QGraphicsView and reimplementing mouseMoveEvent().

Access functions:

• dragMode() const

• void setDragMode( mode)

This property holds the foreground brush of the scene.

This property sets the foreground brush for the scene in this view. It is used to override the scene's own foreground, and defines the behavior of drawForeground(). To provide custom foreground drawing for this view, you can reimplement drawForeground() instead.

By default, this property contains a brush with the Qt::NoBrush pattern.

Access functions:

• foregroundBrush() const

• void setForegroundBrush(const &brush)

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This property holds whether the view allows scene interaction.

If enabled, this view is set to allow scene interaction. Otherwise, this view will not allow interaction, and any mouse or key events are ignored (i.e., it will act as a read-only view).

By default, this property is true.

Access functions:

• bool isInteractive() const

• void setInteractive(bool allowed)

flags that can be used to tune QGraphicsView's performance.

QGraphicsView uses clipping, extra bounding rect adjustments, and certain other aids to improve rendering quality and performance for the common case graphics scene. However, depending on the target platform, the scene, and the viewport in use, some of these operations can degrade performance.

The effect varies from flag to flag; see the OptimizationFlags documentation for details.

By default, no optimization flags are enabled.

Access functions:

• optimizationFlags() const

• void setOptimizationFlags( flags)

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This property holds the default render hints for the view

These hints are used to initialize QPainter before each visible item is drawn. QPainter uses render hints to toggle rendering features such as antialiasing and smooth pixmap transformation.

QPainter::TextAntialiasing is enabled by default.

Example:

QGraphicsScene scene;
scene.addRect(QRectF(-10, -10, 20, 20));

QGraphicsView view(&scene);
view.setRenderHints(QPainter::Antialiasing | QPainter::SmoothPixmapTransform);
view.show();

Access functions:

• renderHints() const

• void setRenderHints( hints)

how the view should position the scene when the view is resized.

QGraphicsView uses this property to decide how to position the scene in the viewport when the viewport widget's size changes. The default behavior, NoAnchor, leaves the scene's position unchanged during a resize; the top-left corner of the view will appear to be anchored while resizing.

Note that the effect of this property is noticeable when only a part of the scene is visible (i.e., when there are scroll bars). Otherwise, if the whole scene fits in the view, QGraphicsScene uses the view alignment to position the scene in the view.

Access functions:

• resizeAnchor() const

• void setResizeAnchor( anchor)

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This property holds the behavior for selecting items with a rubber band selection rectangle.

This property defines how items are selected when using the RubberBandDrag drag mode.

The default value is Qt::IntersectsItemShape; all items whose shape intersects with or is contained by the rubber band are selected.

Access functions:

• rubberBandSelectionMode() const

• void setRubberBandSelectionMode( mode)

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This property holds the area of the scene visualized by this view.

The scene rectangle defines the extent of the scene, and in the view's case, this means the area of the scene that you can navigate using the scroll bars.

If unset, or if a null QRectF is set, this property has the same value as QGraphicsScene::sceneRect, and it changes with QGraphicsScene::sceneRect. Otherwise, the view's scene rect is unaffected by the scene.

Note that, although the scene supports a virtually unlimited size, the range of the scroll bars will never exceed the range of an integer (INT_MIN, INT_MAX). When the scene is larger than the scroll bars' values, you can choose to use translate() to navigate the scene instead.

By default, this property contains a rectangle at the origin with zero width and height.

Access functions:

• sceneRect() const

• void setSceneRect(const &rect)

• void setSceneRect(qreal x, qreal y, qreal w, qreal h)

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how the view should position the scene during transformations.

QGraphicsView uses this property to decide how to position the scene in the viewport when the transformation matrix changes, and the coordinate system of the view is transformed. The default behavior, AnchorViewCenter, ensures that the scene point at the center of the view remains unchanged during transformations (e.g., when rotating, the scene will appear to rotate around the center of the view).

Note that the effect of this property is noticeable when only a part of the scene is visible (i.e., when there are scroll bars). Otherwise, if the whole scene fits in the view, QGraphicsScene uses the view alignment to position the scene in the view.

Access functions:

• transformationAnchor() const

• void setTransformationAnchor( anchor)

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how the viewport should update its contents.

QGraphicsView uses this property to decide how to update areas of the scene that have been reexposed or changed. Usually you do not need to modify this property, but there are some cases where doing so can improve rendering performance. See the ViewportUpdateMode documentation for specific details.

The default value is MinimalViewportUpdate, where QGraphicsView will update as small an area of the viewport as possible when the contents change.

Access functions:

• viewportUpdateMode() const

• void setViewportUpdateMode( mode)

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Constructs a QGraphicsView. parent is passed to QWidget's constructor.

Constructs a QGraphicsView and sets the visualized scene to scene. parent is passed to QWidget's constructor.

Destructs the QGraphicsView object.

Scrolls the contents of the viewport to ensure that the scene coordinate pos, is centered in the view.

Because pos is a floating point coordinate, and the scroll bars operate on integer coordinates, the centering is only an approximation.

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This is an overloaded function.

This function is provided for convenience. It's equivalent to calling centerOn(QPointF(x, y)).

This is an overloaded function.

Scrolls the contents of the viewport to ensure that item is centered in the view.

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Reimplements: QAbstractScrollArea::contextMenuEvent(QContextMenuEvent *e).

Reimplements: QAbstractScrollArea::dragEnterEvent(QDragEnterEvent *event).

Reimplements: QAbstractScrollArea::dragLeaveEvent(QDragLeaveEvent *event).

Reimplements: QAbstractScrollArea::dragMoveEvent(QDragMoveEvent *event).

Draws the background of the scene using painter, before any items and the foreground are drawn. Reimplement this function to provide a custom background for this view.

If all you want is to define a color, texture or gradient for the background, you can call setBackgroundBrush() instead.

All painting is done in scene coordinates. rect is the exposed rectangle.

The default implementation fills rect using the view's backgroundBrush. If no such brush is defined (the default), the scene's drawBackground() function is called instead.

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Draws the foreground of the scene using painter, after the background and all items are drawn. Reimplement this function to provide a custom foreground for this view.

If all you want is to define a color, texture or gradient for the foreground, you can call setForegroundBrush() instead.

All painting is done in scene coordinates. rect is the exposed rectangle.

The default implementation fills rect using the view's foregroundBrush. If no such brush is defined (the default), the scene's drawForeground() function is called instead.

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Reimplements: QAbstractScrollArea::dropEvent(QDropEvent *event).

Scrolls the contents of the viewport so that the scene rectangle rect is visible, with margins specified in pixels by xmargin and ymargin. If the specified rect cannot be reached, the contents are scrolled to the nearest valid position. The default value for both margins is 50 pixels.

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This is an overloaded function.

This function is provided for convenience. It's equivalent to calling ensureVisible(QRectF(x, y, w, h), xmargin, ymargin).

This is an overloaded function.

Scrolls the contents of the viewport so that the center of item item is visible, with margins specified in pixels by xmargin and ymargin. If the specified point cannot be reached, the contents are scrolled to the nearest valid position. The default value for both margins is 50 pixels.

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Reimplements: QAbstractScrollArea::event(QEvent *event).

Scales the view matrix and scrolls the scroll bars to ensure that the scene rectangle rect fits inside the viewport. rect must be inside the scene rect; otherwise, fitInView() cannot guarantee that the whole rect is visible.

This function keeps the view's rotation, translation, or shear. The view is scaled according to aspectRatioMode. rect will be centered in the view if it does not fit tightly.

It's common to call fitInView() from inside a reimplementation of resizeEvent(), to ensure that the whole scene, or parts of the scene, scales automatically to fit the new size of the viewport as the view is resized. Note though, that calling fitInView() from inside resizeEvent() can lead to unwanted resize recursion, if the new transformation toggles the automatic state of the scrollbars. You can toggle the scrollbar policies to always on or always off to prevent this (see horizontalScrollBarPolicy() and verticalScrollBarPolicy()).

If rect is empty, or if the viewport is too small, this function will do nothing.

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This is an overloaded function.

This convenience function is equivalent to calling fitInView(QRectF(x, y, w, h), aspectRatioMode).

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This is an overloaded function.

Ensures that item fits tightly inside the view, scaling the view according to aspectRatioMode.

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Reimplements: QWidget::focusInEvent(QFocusEvent *event).

Reimplements: QWidget::focusNextPrevChild(bool next).

Reimplements: QWidget::focusOutEvent(QFocusEvent *event).

Reimplements: QWidget::inputMethodEvent(QInputMethodEvent *event).

Reimplements: QWidget::inputMethodQuery(Qt::InputMethodQuery query) const.

Invalidates and schedules a redraw of layers inside rect. rect is in scene coordinates. Any cached content for layers inside rect is unconditionally invalidated and redrawn.

You can call this function to notify QGraphicsView of changes to the background or the foreground of the scene. It is commonly used for scenes with tile-based backgrounds to notify changes when QGraphicsView has enabled background caching.

Note that QGraphicsView currently supports background caching only (see QGraphicsView::CacheBackground). This function is equivalent to calling update() if any layer but QGraphicsScene::BackgroundLayer is passed.

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Returns true if the view is transformed (i.e., a non-identity transform has been assigned, or the scrollbars are adjusted).

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Returns the item at position pos, which is in viewport coordinates. If there are several items at this position, this function returns the topmost item.

Example:

void CustomView::mousePressEvent(QMouseEvent *event)
{
    if (QGraphicsItem *item = itemAt(event->pos())) {
        qDebug() << "You clicked on item" << item;
    } else {
        qDebug("You didn't click on an item.");
    }
}

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This is an overloaded function.

This function is provided for convenience. It's equivalent to calling itemAt(QPoint(x, y)).

Returns a list of all the items in the associated scene, in descending stacking order (i.e., the first item in the returned list is the uppermost item).

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Returns a list of all the items at the position pos in the view. The items are listed in descending stacking order (i.e., the first item in the list is the uppermost item, and the last item is the lowermost item). pos is in viewport coordinates.

This function is most commonly called from within mouse event handlers in a subclass in QGraphicsView. pos is in untransformed viewport coordinates, just like QMouseEvent::pos().

void CustomView::mousePressEvent(QMouseEvent *event)
{
    qDebug() << "There are" << items(event->pos()).size()
             << "items at position" << mapToScene(event->pos());
}

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This function is provided for convenience. It's equivalent to calling items(QPoint(x, y)).

This is an overloaded function.

Returns a list of all the items that, depending on mode, are either contained by or intersect with rect. rect is in viewport coordinates.

The default value for mode is Qt::IntersectsItemShape; all items whose exact shape intersects with or is contained by rect are returned.

The items are sorted in descending stacking order (i.e., the first item in the returned list is the uppermost item).

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This convenience function is equivalent to calling items(QRectF(x, y, w, h), mode).

This is an overloaded function.

Returns a list of all the items that, depending on mode, are either contained by or intersect with polygon. polygon is in viewport coordinates.

The default value for mode is Qt::IntersectsItemShape; all items whose exact shape intersects with or is contained by polygon are returned.

The items are sorted by descending stacking order (i.e., the first item in the returned list is the uppermost item).

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This is an overloaded function.

Returns a list of all the items that, depending on mode, are either contained by or intersect with path. path is in viewport coordinates.

The default value for mode is Qt::IntersectsItemShape; all items whose exact shape intersects with or is contained by path are returned.

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Reimplements: QAbstractScrollArea::keyPressEvent(QKeyEvent *e).

Reimplements: QWidget::keyReleaseEvent(QKeyEvent *event).

Returns the scene coordinate point to viewport coordinates.

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Returns the scene rectangle rect to a viewport coordinate polygon.

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Returns the scene coordinate polygon polygon to a viewport coordinate polygon.

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Returns the scene coordinate painter path path to a viewport coordinate painter path.

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This function is provided for convenience. It's equivalent to calling mapFromScene(QPointF(x, y)).

This function is provided for convenience. It's equivalent to calling mapFromScene(QRectF(x, y, w, h)).

Returns the viewport coordinate point mapped to scene coordinates.

Note: It can be useful to map the whole rectangle covered by the pixel at point instead of the point itself. To do this, you can call mapToScene(QRect(point, QSize(2, 2))).

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Returns the viewport rectangle rect mapped to a scene coordinate polygon.

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Returns the viewport polygon polygon mapped to a scene coordinate polygon.

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Returns the viewport painter path path mapped to a scene coordinate painter path.

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This function is provided for convenience. It's equivalent to calling mapToScene(QPoint(x, y)).

This function is provided for convenience. It's equivalent to calling mapToScene(QRect(x, y, w, h)).

Reimplements: QAbstractScrollArea::mouseDoubleClickEvent(QMouseEvent *e).

Reimplements: QAbstractScrollArea::mouseMoveEvent(QMouseEvent *e).

Reimplements: QAbstractScrollArea::mousePressEvent(QMouseEvent *e).

Reimplements: QAbstractScrollArea::mouseReleaseEvent(QMouseEvent *e).

Reimplements: QAbstractScrollArea::paintEvent(QPaintEvent *event).

Renders the source rect, which is in view coordinates, from the scene into target, which is in paint device coordinates, using painter. This function is useful for capturing the contents of the view onto a paint device, such as a QImage (e.g., to take a screenshot), or for printing to QPrinter. For example:

QGraphicsScene scene;
scene.addItem(...
...

QGraphicsView view(&scene);
view.show();
...

QPrinter printer(QPrinter::HighResolution);
printer.setPageSize(QPrinter::A4);
QPainter painter(&printer);

// print, fitting the viewport contents into a full page
view.render(&painter);

// print the upper half of the viewport into the lower.
// half of the page.
QRect viewport = view.viewport()->rect();
view.render(&painter,
            QRectF(0, printer.height() / 2,
                   printer.width(), printer.height() / 2),
            viewport.adjusted(0, 0, 0, -viewport.height() / 2));

If source is a null rect, this function will use viewport()->rect() to determine what to draw. If target is a null rect, the full dimensions of painter's paint device (e.g., for a QPrinter, the page size) will be used.

The source rect contents will be transformed according to aspectRatioMode to fit into the target rect. By default, the aspect ratio is kept, and source is scaled to fit in target.

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Resets any cached content. Calling this function will clear QGraphicsView's cache. If the current cache mode is CacheNone, this function does nothing.

This function is called automatically for you when the backgroundBrush or QGraphicsScene::backgroundBrush properties change; you only need to call this function if you have reimplemented QGraphicsScene::drawBackground() or QGraphicsView::drawBackground() to draw a custom background, and need to trigger a full redraw.

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Resets the view transformation to the identity matrix.

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Reimplements: QAbstractScrollArea::resizeEvent(QResizeEvent *event).

Rotates the current view transformation angle degrees clockwise.

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This signal is emitted when the rubber band rect is changed. The viewport Rect is specified by rubberBandRect. The drag start position and drag end position are provided in scene points with fromScenePoint and toScenePoint.

When rubberband selection ends this signal will be emitted with null vales.

This function was introduced in Qt 5.1.

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This functions returns the current rubber band area (in viewport coordinates) if the user is currently doing an itemselection with rubber band. When the user is not using the rubber band this functions returns (a null) QRectF().

Notice that part of this QRect can be outise the visual viewport. It can e.g contain negative values.

This function was introduced in Qt 5.1.

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Scales the current view transformation by (sx, sy).

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Returns a pointer to the scene that is currently visualized in the view. If no scene is currently visualized, nullptr is returned.

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Reimplements: QAbstractScrollArea::scrollContentsBy(int dx, int dy).

Enables flag if enabled is true; otherwise disables flag.

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If enabled is true, the render hint hint is enabled; otherwise it is disabled.

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Sets the current scene to scene. If scene is already being viewed, this function does nothing.

When a scene is set on a view, the QGraphicsScene::changed() signal is automatically connected to this view's updateScene() slot, and the view's scroll bars are adjusted to fit the size of the scene.

The view does not take ownership of scene.

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Sets the view's current transformation matrix to matrix.

If combine is true, then matrix is combined with the current matrix; otherwise, matrix replaces the current matrix. combine is false by default.

The transformation matrix transforms the scene into view coordinates. Using the default transformation, provided by the identity matrix, one pixel in the view represents one unit in the scene (e.g., a 10x10 rectangular item is drawn using 10x10 pixels in the view). If a 2x2 scaling matrix is applied, the scene will be drawn in 1:2 (e.g., a 10x10 rectangular item is then drawn using 20x20 pixels in the view).

Example:

QGraphicsScene scene;
scene.addText("GraphicsView rotated clockwise");

QGraphicsView view(&scene);
view.rotate(90); // the text is rendered with a 90 degree clockwise rotation
view.show();

To simplify interaction with items using a transformed view, QGraphicsView provides mapTo... and mapFrom... functions that can translate between scene and view coordinates. For example, you can call mapToScene() to map a view coordinate to a floating point scene coordinate, or mapFromScene() to map from floating point scene coordinates to view coordinates.

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Reimplements: QAbstractScrollArea::setupViewport(QWidget *viewport).

This slot is called by QAbstractScrollArea after setViewport() has been called. Reimplement this function in a subclass of QGraphicsView to initialize the new viewport widget before it is used.

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Shears the current view transformation by (sh, sv).

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Reimplements: QWidget::showEvent(QShowEvent *event).

Reimplements: QAbstractScrollArea::sizeHint() const.

Returns the current transformation matrix for the view. If no current transformation is set, the identity matrix is returned.

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Translates the current view transformation by (dx, dy).

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Schedules an update of the scene rectangles rects.

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Notifies QGraphicsView that the scene's scene rect has changed. rect is the new scene rect. If the view already has an explicitly set scene rect, this function does nothing.

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Reimplements: QAbstractScrollArea::viewportEvent(QEvent *event).

Returns a matrix that maps scene coordinates to viewport coordinates.

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Reimplements: QAbstractScrollArea::wheelEvent(QWheelEvent *e).

[virtual protected] void QGraphicsView::drawItems(QPainter *painter, int numItems, QGraphicsItem *[] items, const QStyleOptionGraphicsItem[] options)▲