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QGraphicsView view; view.setScene( &scene ); view.show(); return app.exec(); } When working with graphics items, you can use the Z value to control the order in which the items are drawn. You can set each item using the setZValue(qreal) method. The default Z value for any item is 0. When drawing the scene, items with a high Z value appear in front of items with lower Z values. For items with the same Z value, the order is undefined.
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With custom items you can create the kind of behavior you want by using graphics view. This flexibility and ease of implementing custom shapes are what make graphics view such a nice tool to use. The aim of this section is to create a set of handles: one central handle for moving shapes and two edge handles for resizing them. Figure 7-30 shows the handles in action. Notice that you can apply handles to several shapes at once and that the shapes used are standard shapes: QGraphicsRectItem and QGraphicsEllipseItem.
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Figure 7-30. The handles in action
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Let s start looking at the code, beginning from the main function of the application. This shows how the handles are created, configured, and used. The main function is shown in Listing 7-22. The function starts by creating the Qt classes that you need: a QApplication, a QGraphicsScene, and the two shapes represented through a QGraphicsRectItem and a QGraphicsEllipseItem. When these shapes have been added to the scene, it s time to create six HandleItem objects three for each of the shapes. Each handle s constructor takes the following arguments: an item to act upon, a scene, a color, and a role. The available roles are TopHandle, RightHandle, and CenterHandle. When you create a CenterHandle you have to pass a QList with pointers to the two other handles. That is, if you choose to have other handles, the CenterHandle works perfectly on its own, as do the other two variants. The main function then continues by creating a QGraphicsView and sets it up to show the scene. The main loop is then started by calling the exec method on the QApplication object. However, you do not return the result from this directly. Because the handles refer to the other shapes without being child nodes, it is important that you delete the handles first. The remaining shapes are then deleted when the QGraphicsScene is destroyed. Listing 7-22. Using the HandleItem class in a scene int main( int argc, char **argv ) { QApplication app( argc, argv ); QGraphicsScene scene( 0, 0, 200, 200 ); QGraphicsRectItem *rectItem = new QGraphicsRectItem( QRect( 10, 10, 50, 100 ), 0, &scene ); QGraphicsEllipseItem *elItem = new QGraphicsEllipseItem( QRect( 80, 40, 100, 80 ), 0, &scene ); HandleItem *trh = new HandleItem( rectItem, &scene, Qt::red, HandleItem::TopHandle ); HandleItem *rrh = new HandleItem( rectItem, &scene, Qt::red, HandleItem::RightHandle ); HandleItem *crh = new HandleItem( rectItem, &scene, Qt::red, HandleItem::CenterHandle, QList<HandleItem*>() << trh << rrh ); HandleItem *teh = new HandleItem( elItem, &scene, Qt::green, HandleItem::TopHandle ); HandleItem *reh = new HandleItem( elItem, &scene, Qt::green, HandleItem::RightHandle ); HandleItem *ceh = new HandleItem( elItem, &scene, Qt::green, HandleItem::CenterHandle, QList<HandleItem*>() << teh << reh );
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QGraphicsView view; view.setScene( &scene ); view.show(); return app.exec();
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} Now that you know how the handles look and how the class is used in a scene, it s time to have a look at the actual class. Listing 7-23 shows the class declaration. The listing starts with a forward declaration of the class because the class will contain pointers to instances of itself. Then it defines an enumeration of the different available roles: CenterHandle, RightHandle, and TopHandle. The constructor that follows the enum contains all the expected arguments, as discussed earlier. However, the role and list of handles have default values. The default role is a center handle, and the list is empty by default. The next two methods are required when inheriting from QGraphicsItem. The paint method is responsible for painting the shape upon request, while boundingRect tells the scene how large the shape is. The class declaration then continues with a set of protected methods. You can override these methods to interact with the user through the shape. The mousePressEvent and mouseReleaseEvent methods react to the mouse buttons, while the itemChange method can be used to filter and react to all changes to the item. You use it to react to and limit the moving of the widget. The private section ends the class declaration. It contains all the local states and variables that are needed. The following list summarizes their roles and uses (you will look more closely at how they are used in the rest of this section): m_item: The QGraphicsItem that the handles acts on. m_role: The role of the handle. m_color: The color of the handle. m_handles: A list of other handles acting on the same m_item required by center handles. m_pressed: A Boolean that indicates whether the mouse button is pressed. This is important because you need to be able to tell whether the handle is moving because of user interaction or programmatic changes.
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Listing 7-23. The handle class class HandleItem; class HandleItem : public QGraphicsItem { public:
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