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.
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:
| Function | Description |
|---|---|
| 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().