QGraphicsLayout Class

The QGraphicsLayout class provides the base class for all layouts in Graphics View. More...

Header: #include <QGraphicsLayout>
qmake: QT += widgets
Since: Qt 4.4
Inherits: QGraphicsLayoutItem
Inherited By:

QGraphicsAnchorLayout, QGraphicsGridLayout, and QGraphicsLinearLayout

This class was introduced in Qt 4.4.

Public Functions

QGraphicsLayout(QGraphicsLayoutItem *parent = nullptr)
virtual ~QGraphicsLayout()
void activate()
virtual int count() const = 0
virtual void invalidate()
bool isActivated() const
virtual QGraphicsLayoutItem *itemAt(int i) const = 0
virtual void removeAt(int index) = 0
void setContentsMargins(qreal left, qreal top, qreal right, qreal bottom)
virtual void widgetEvent(QEvent *e)

Reimplemented Public Functions

virtual void getContentsMargins(qreal *left, qreal *top, qreal *right, qreal *bottom) const override
virtual void updateGeometry() override

Protected Functions

void addChildLayoutItem(QGraphicsLayoutItem *layoutItem)

Detailed Description

QGraphicsLayout is an abstract class that defines a virtual API for arranging QGraphicsWidget children and other QGraphicsLayoutItem objects for a QGraphicsWidget. QGraphicsWidget assigns responsibility to a QGraphicsLayout through QGraphicsWidget::setLayout(). As the widget is resized, the layout will automatically arrange the widget's children. QGraphicsLayout inherits QGraphicsLayoutItem, so, it can be managed by any layout, including its own subclasses.

Writing a Custom Layout

You can use QGraphicsLayout as a base to write your own custom layout (e.g., a flowlayout), but it is more common to use one of its subclasses instead - QGraphicsLinearLayout or QGraphicsGridLayout. When creating a custom layout, the following functions must be reimplemented as a bare minimum:

QGraphicsLayoutItem::setGeometry()Notifies you when the geometry of the layout is set. You can store the geometry in your own layout class in a reimplementation of this function.
QGraphicsLayoutItem::sizeHint()Returns the layout's size hints.
QGraphicsLayout::count()Returns the number of items in your layout.
QGraphicsLayout::itemAt()Returns a pointer to an item in your layout.
QGraphicsLayout::removeAt()Removes an item from your layout without destroying it.

For more details on how to implement each function, refer to the individual function documentation.

Each layout defines its own API for arranging widgets and layout items. For example, with a grid layout, you require a row and a column index with optional row and column spans, alignment, spacing, and more. A linear layout, however, requires a single row or column index to position its items. For a grid layout, the order of insertion does not affect the layout in any way, but for a linear layout, the order is essential. When writing your own layout subclass, you are free to choose the API that best suits your layout.

QGraphicsLayout provides the addChildLayoutItem() convenience function to add layout items to a custom layout. The function will automatically reparent graphics items, if required.

Activating the Layout

When the layout's geometry changes, QGraphicsLayout immediately rearranges all of its managed items by calling setGeometry() on each item. This rearrangement is called activating the layout.

QGraphicsLayout updates its own geometry to match the contentsRect() of the QGraphicsLayoutItem it is managing. Thus, it will automatically rearrange all its items when the widget is resized. QGraphicsLayout caches the sizes of all its managed items to avoid calling setGeometry() too often.

Note: A QGraphicsLayout will have the same geometry as the contentsRect() of the widget (not the layout) it is assigned to.

Activating the Layout Implicitly

The layout can be activated implicitly using one of two ways: by calling activate() or by calling invalidate(). Calling activate() activates the layout immediately. In contrast, calling invalidate() is delayed, as it posts a LayoutRequest event to the managed widget. Due to event compression, the activate() will only be called once after control has returned to the event loop. This is referred to as invalidating the layout. Invalidating the layout also invalidates any cached information. Also, the invalidate() function is a virtual function. So, you can invalidate your own cache in a subclass of QGraphicsLayout by reimplementing this function.

Event Handling

QGraphicsLayout listens to events for the widget it manages through the virtual widgetEvent() event handler. When the layout is assigned to a widget, all events delivered to the widget are first processed by widgetEvent(). This allows the layout to be aware of any relevant state changes on the widget such as visibility changes or layout direction changes.

Margin Handling

The margins of a QGraphicsLayout can be modified by reimplementing setContentsMargins() and getContentsMargins().

Member Function Documentation

QGraphicsLayout::QGraphicsLayout(QGraphicsLayoutItem *parent = nullptr)

Constructs a QGraphicsLayout object.

parent is passed to QGraphicsLayoutItem's constructor and the QGraphicsLayoutItem's isLayout argument is set to true.

If parent is a QGraphicsWidget the layout will be installed on that widget. (Note that installing a layout will delete the old one installed.)

[virtual] QGraphicsLayout::~QGraphicsLayout()

Destroys the QGraphicsLayout object.

void QGraphicsLayout::activate()

Activates the layout, causing all items in the layout to be immediately rearranged. This function is based on calling count() and itemAt(), and then calling setGeometry() on all items sequentially. When activated, the layout will adjust its geometry to its parent's contentsRect(). The parent will then invalidate any layout of its own.

If called in sequence or recursively, e.g., by one of the arranged items in response to being resized, this function will do nothing.

Note that the layout is free to use geometry caching to optimize this process. To forcefully invalidate any such cache, you can call invalidate() before calling activate().

See also invalidate().

[protected] void QGraphicsLayout::addChildLayoutItem(QGraphicsLayoutItem *layoutItem)

This function is a convenience function provided for custom layouts, and will go through all items in the layout and reparent their graphics items to the closest QGraphicsWidget ancestor of the layout.

If layoutItem is already in a different layout, it will be removed from that layout.

If custom layouts want special behaviour they can ignore to use this function, and implement their own behaviour.

This function was introduced in Qt 4.6.

See also graphicsItem().

[pure virtual] int QGraphicsLayout::count() const

This pure virtual function must be reimplemented in a subclass of QGraphicsLayout to return the number of items in the layout.

The subclass is free to decide how to store the items.

See also itemAt() and removeAt().

[override virtual] void QGraphicsLayout::getContentsMargins(qreal *left, qreal *top, qreal *right, qreal *bottom) const

Reimplements: QGraphicsLayoutItem::getContentsMargins(qreal *left, qreal *top, qreal *right, qreal *bottom) const.

[virtual] void QGraphicsLayout::invalidate()

Clears any cached geometry and size hint information in the layout, and posts a LayoutRequest event to the managed parent QGraphicsLayoutItem.

See also activate() and setGeometry().

bool QGraphicsLayout::isActivated() const

Returns true if the layout is currently being activated; otherwise, returns false. If the layout is being activated, this means that it is currently in the process of rearranging its items (i.e., the activate() function has been called, and has not yet returned).

See also activate() and invalidate().

[pure virtual] QGraphicsLayoutItem *QGraphicsLayout::itemAt(int i) const

This pure virtual function must be reimplemented in a subclass of QGraphicsLayout to return a pointer to the item at index i. The reimplementation can assume that i is valid (i.e., it respects the value of count()). Together with count(), it is provided as a means of iterating over all items in a layout.

The subclass is free to decide how to store the items, and the visual arrangement does not have to be reflected through this function.

See also count() and removeAt().

[pure virtual] void QGraphicsLayout::removeAt(int index)

This pure virtual function must be reimplemented in a subclass of QGraphicsLayout to remove the item at index. The reimplementation can assume that index is valid (i.e., it respects the value of count()).

The implementation must ensure that the parentLayoutItem() of the removed item does not point to this layout, since the item is considered to be removed from the layout hierarchy.

If the layout is to be reused between applications, we recommend that the layout deletes the item, but the graphics view framework does not depend on this.

The subclass is free to decide how to store the items.

See also itemAt() and count().

void QGraphicsLayout::setContentsMargins(qreal left, qreal top, qreal right, qreal bottom)

Sets the contents margins to left, top, right and bottom. The default contents margins for toplevel layouts are style dependent (by querying the pixelMetric for QStyle::PM_LayoutLeftMargin, QStyle::PM_LayoutTopMargin, QStyle::PM_LayoutRightMargin and QStyle::PM_LayoutBottomMargin).

For sublayouts the default margins are 0.

Changing the contents margins automatically invalidates the layout.

See also invalidate().

[override virtual] void QGraphicsLayout::updateGeometry()

Reimplements: QGraphicsLayoutItem::updateGeometry().

[virtual] void QGraphicsLayout::widgetEvent(QEvent *e)

This virtual event handler receives all events for the managed widget. QGraphicsLayout uses this event handler to listen for layout related events such as geometry changes, layout changes or layout direction changes.

e is a pointer to the event.

You can reimplement this event handler to track similar events for your own custom layout.

See also QGraphicsWidget::event() and QGraphicsItem::sceneEvent().

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