Описание тега qgraphicsitem

The QGraphicsItem class is the base class for all graphical items in a QGraphicsScene.

It provides a light-weight foundation for writing your own custom items. This includes defining the item's geometry, collision detection, its painting implementation and item interaction through its event handlers. QGraphicsItem is part of The Graphics View Framework

For convenience, Qt provides a set of standard graphics items for the most common shapes: ellipse, line, path, pixmap, polygon, rectangle, text.

All of an item's geometric information is based on its local coordinate system. The item's position, pos(), is the only function that does not operate in local coordinates, as it returns a position in parent coordinates. The Graphics View Coordinate System describes the coordinate system in detail.

You can set whether an item should be visible (i.e., drawn, and accepting events), by calling setVisible(). Hiding an item will also hide its children. Similarly, you can enable or disable an item by calling setEnabled(). If you disable an item, all its children will also be disabled. By default, items are both visible and enabled. To toggle whether an item is selected or not, first enable selection by setting the ItemIsSelectable flag, and then call setSelected(). Normally, selection is toggled by the scene, as a result of user interaction.

Features

Transformations

QGraphicsItem supports projective transformations in addition to its base position, pos(). There are several ways to change an item's transformation. For simple transformations, you can call either of the convenience functions setRotation() or setScale(), or you can pass any transformation matrix to setTransform(). For advanced transformation control you also have the option of setting several combined transformations by calling setTransformations().

Painting

The paint() function is called by QGraphicsView to paint the item's contents. The item has no background or default fill of its own; whatever is behind the item will shine through all areas that are not explicitly painted in this function. You can call update() to schedule a repaint, optionally passing the rectangle that needs a repaint. Depending on whether or not the item is visible in a view, the item may or may not be repainted; there is no equivalent to QWidget::repaint() in QGraphicsItem.

Sorting

All items are drawn in a defined, stable order, and this same order decides which items will receive mouse input first when you click on the scene. Normally you don't have to worry about sorting, as the items follow a "natural order", following the logical structure of the scene.

Events

QGraphicsItem receives events from QGraphicsScene through the virtual function sceneEvent(). This function distributes the most common events to a set of convenience event handlers.