QGraphicsLayoutItem Class

Detailed Description

QGraphicsLayoutItem is an abstract class that defines a set of virtual functions describing sizes, size policies, and size hints for any object arranged by QGraphicsLayout. The API contains functions relevant for both the item itself and for the user of the item as most of QGraphicsLayoutItem's functions are also part of the subclass' public API.

In most cases, existing layout-aware classes such as QGraphicsWidget and QGraphicsLayout already provide the functionality you require. However, subclassing these classes will enable you to create both graphical elements that work well with layouts (QGraphicsWidget) or custom layouts (QGraphicsLayout).

Subclassing QGraphicsLayoutItem

If you create a subclass of QGraphicsLayoutItem and reimplement its virtual functions, you will enable the layout to resize and position your item along with other QGraphicsLayoutItems including QGraphicsWidget and QGraphicsLayout.

You can start by reimplementing important functions: the protected sizeHint() function, as well as the public setGeometry() function. If you want your items to be aware of immediate geometry changes, you can also reimplement updateGeometry().

The geometry, size hint, and size policy affect the item's size and position. Calling setGeometry() will always resize and reposition the item immediately. Normally, this function is called by QGraphicsLayout after the layout has been activated, but it can also be called by the item's user at any time.

The sizeHint() function returns the item' minimum, preferred and maximum size hints. You can override these properties by calling setMinimumSize(), setPreferredSize() or setMaximumSize(). You can also use functions such as setMinimumWidth() or setMaximumHeight() to set only the width or height component if desired.

The effectiveSizeHint() function, on the other hand, returns a size hint for any given Qt::SizeHint, and guarantees that the returned size is bound to the minimum and maximum sizes and size hints. You can set the item's vertical and horizontal size policy by calling setSizePolicy(). The sizePolicy property is used by the layout system to describe how this item prefers to grow or shrink.

Nesting QGraphicsLayoutItems

QGraphicsLayoutItems can be nested within other QGraphicsLayoutItems, similar to layouts that can contain sublayouts. This is done either by passing a QGraphicsLayoutItem pointer to QGraphicsLayoutItem's protected constructor, or by calling setParentLayoutItem(). The parentLayoutItem() function returns a pointer to the item's layoutItem parent. If the item's parent is nullptr or if the parent does not inherit from QGraphicsItem, the parentLayoutItem() function then returns nullptr. isLayout() returns true if the QGraphicsLayoutItem subclass is itself a layout, or false otherwise.

Qt uses QGraphicsLayoutItem to provide layout functionality in the Graphics View Framework, but in the future its use may spread throughout Qt itself.