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\example graphicsview/elasticnodes
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\title Elastic Nodes Example
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\ingroup examples-graphicsview
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\brief Demonstrates how to interact with graphical items in a scene
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The Elastic Nodes example shows how to implement edges between nodes in a
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graph, with basic interaction. You can click to drag a node around, and
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zoom in and out using the mouse wheel or the keyboard. Hitting the space
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bar will randomize the nodes. The example is also resolution independent;
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as you zoom in, the graphics remain crisp.
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\image elasticnodes-example.png
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Graphics View provides the QGraphicsScene class for managing and
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interacting with a large number of custom-made 2D graphical items derived
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from the QGraphicsItem class, and a QGraphicsView widget for visualizing
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the items, with support for zooming and rotation.
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This example consists of a \c Node class, an \c Edge class, a \c
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GraphWidget test, and a \c main function: the \c Node class represents
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draggable yellow nodes in a grid, the \c Edge class represents the lines
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between the nodes, the \c GraphWidget class represents the application
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window, and the \c main() function creates and shows this window, and runs
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\section1 Node Class Definition
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The \c Node class serves three purposes:
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\li Painting a yellow gradient "ball" in two states: sunken and raised.
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\li Managing connections to other nodes.
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\li Calculating forces pulling and pushing the nodes in the grid.
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Let's start by looking at the \c Node class declaration.
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\snippet graphicsview/elasticnodes/node.h 0
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The \c Node class inherits QGraphicsItem, and reimplements the two
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mandatory functions \l{QGraphicsItem::boundingRect()}{boundingRect()} and
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\l{QGraphicsItem::paint()}{paint()} to provide its visual appearance. It
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also reimplements \l{QGraphicsItem::shape()}{shape()} to ensure its hit
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area has an elliptic shape (as opposed to the default bounding rectangle).
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For edge management purposes, the node provides a simple API for adding
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edges to a node, and for listing all connected edges.
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The \l{QGraphicsItem::advance()}{advance()} reimplementation is called
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whenever the scene's state advances by one step. The calculateForces()
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function is called to calculate the forces that push and pull on this node
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The \c Node class also reimplements
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\l{QGraphicsItem::itemChange()}{itemChange()} to react to state changes (in
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this case, position changes), and
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\l{QGraphicsItem::mousePressEvent()}{mousePressEvent()} and
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\l{QGraphicsItem::mouseReleaseEvent()}{mouseReleaseEvent()} to update the
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item's visual appearance.
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We will start reviewing the \c Node implementation by looking at its
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\snippet graphicsview/elasticnodes/node.cpp 0
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In the constructor, we set the
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\l{QGraphicsItem::ItemIsMovable}{ItemIsMovable} flag to allow the item to
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move in response to mouse dragging, and
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\l{QGraphicsItem::ItemSendsGeometryChanges}{ItemSendsGeometryChanges} to
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enable \l{QGraphicsItem::itemChange()}{itemChange()} notifications for
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position and transformation changes. We also enable
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\l{QGraphicsItem::DeviceCoordinateCache}{DeviceCoordinateCache} to speed up
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rendering performance. To ensure that the nodes are always stacked on top
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of edges, we finally set the item's Z value to -1.
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\c Node's constructor takes a \c GraphWidget pointer and stores this as a
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member variable. We will revisit this pointer later on.
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\snippet graphicsview/elasticnodes/node.cpp 1
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The addEdge() function adds the input edge to a list of attached edges. The
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edge is then adjusted so that the end points for the edge match the
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positions of the source and destination nodes.
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The edges() function simply returns the list of attached edges.
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\snippet graphicsview/elasticnodes/node.cpp 2
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There are two ways to move a node. The \c calculateForces() function
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implements the elastic effect that pulls and pushes on nodes in the grid.
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In addition, the user can directly move one node around with the mouse.
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Because we do not want the two approaches to operate at the same time on
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the same node, we start \c calculateForces() by checking if this \c Node is
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the current mouse grabber item (i.e., QGraphicsScene::mouseGrabberItem()).
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Because we need to find all neighboring (but not necessarily connected)
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nodes, we also make sure the item is part of a scene in the first place.
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\snippet graphicsview/elasticnodes/node.cpp 3
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The "elastic" effect comes from an algorithm that applies pushing and
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pulling forces. The effect is impressive, and surprisingly simple to
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The algorithm has two steps: the first is to calculate the forces that push
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the nodes apart, and the second is to subtract the forces that pull the
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nodes together. First we need to find all the nodes in the graph. We call
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QGraphicsScene::items() to find all items in the scene, and then use
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qgraphicsitem_cast() to look for \c Node instances.
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We make use of \l{QGraphicsItem::mapFromItem()}{mapFromItem()} to create a
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temporary vector pointing from this node to each other node, in \l{The
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Graphics View Coordinate System}{local coordinates}. We use the decomposed
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components of this vector to determine the direction and strength of force
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that should apply to the node. The forces accumulate for each node, and are
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then adjusted so that the closest nodes are given the strongest force, with
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rapid degradation when distance increases. The sum of all forces is stored
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in \c xvel (X-velocity) and \c yvel (Y-velocity).
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\snippet graphicsview/elasticnodes/node.cpp 4
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The edges between the nodes represent forces that pull the nodes together.
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By visiting each edge that is connected to this node, we can use a similar
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approach as above to find the direction and strength of all pulling forces.
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These forces are subtracted from \c xvel and \c yvel.
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\snippet graphicsview/elasticnodes/node.cpp 5
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In theory, the sum of pushing and pulling forces should stabilize to
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precisely 0. In practice, however, they never do. To circumvent errors in
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numerical precision, we simply force the sum of forces to be 0 when they
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\snippet graphicsview/elasticnodes/node.cpp 6
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The final step of \c calculateForces() determines the node's new position.
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We add the force to the node's current position. We also make sure the new
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position stays inside of our defined boundaries. We don't actually move the
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item in this function; that's done in a separate step, from \c advance().
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\snippet graphicsview/elasticnodes/node.cpp 7
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The \c advance() function updates the item's current position. It is called
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from \c GraphWidget::timerEvent(). If the node's position changed, the
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function returns true; otherwise false is returned.
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\snippet graphicsview/elasticnodes/node.cpp 8
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The \c Node's bounding rectangle is a 20x20 sized rectangle centered around
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its origin (0, 0), adjusted by 2 units in all directions to compensate for
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the node's outline stroke, and by 3 units down and to the right to make
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room for a simple drop shadow.
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\snippet graphicsview/elasticnodes/node.cpp 9
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The shape is a simple ellipse. This ensures that you must click inside the
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node's elliptic shape in order to drag it around. You can test this effect
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by running the example, and zooming far in so that the nodes are very
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large. Without reimplementing \l{QGraphicsItem::shape()}{shape()}, the
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item's hit area would be identical to its bounding rectangle (i.e.,
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\snippet graphicsview/elasticnodes/node.cpp 10
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This function implements the node's painting. We start by drawing a simple
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dark gray elliptic drop shadow at (-7, -7), that is, (3, 3) units down and
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to the right from the top-left corner (-10, -10) of the ellipse.
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We then draw an ellipse with a radial gradient fill. This fill is either
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Qt::yellow to Qt::darkYellow when raised, or the opposite when sunken. In
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sunken state we also shift the center and focal point by (3, 3) to
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emphasize the impression that something has been pushed down.
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Drawing filled ellipses with gradients can be quite slow, especially when
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using complex gradients such as QRadialGradient. This is why this example
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uses \l{QGraphicsItem::DeviceCoordinateCache}{DeviceCoordinateCache}, a
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simple yet effective measure that prevents unnecessary redrawing.
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\snippet graphicsview/elasticnodes/node.cpp 11
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We reimplement \l{QGraphicsItem::itemChange()}{itemChange()} to adjust the
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position of all connected edges, and to notify the scene that an item has
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moved (i.e., "something has happened"). This will trigger new force
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This notification is the only reason why the nodes need to keep a pointer
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back to the \c GraphWidget. Another approach could be to provide such
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notification using a signal; in such case, \c Node would need to inherit
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from QGraphicsObject.
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\snippet graphicsview/elasticnodes/node.cpp 12
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Because we have set the \l{QGraphicsItem::ItemIsMovable}{ItemIsMovable}
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flag, we don't need to implement the logic that moves the node according to
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mouse input; this is already provided for us. We still need to reimplement
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the mouse press and release handlers, though, to update the nodes' visual
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appearance (i.e., sunken or raised).
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\section1 Edge Class Definition
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The \c Edge class represents the arrow-lines between the nodes in this
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example. The class is very simple: it maintains a source- and destination
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node pointer, and provides an \c adjust() function that makes sure the line
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starts at the position of the source, and ends at the position of the
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destination. The edges are the only items that change continuously as
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forces pull and push on the nodes.
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Let's take a look at the class declaration:
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\snippet graphicsview/elasticnodes/edge.h 0
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\c Edge inherits from QGraphicsItem, as it's a simple class that has no use
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for signals, slots, and properties (compare to QGraphicsObject).
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The constructor takes two node pointers as input. Both pointers are
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mandatory in this example. We also provide get-functions for each node.
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The \c adjust() function repositions the edge, and the item also implements
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\l{QGraphicsItem::boundingRect()}{boundingRect()} and
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\l{QGraphicsItem::paint()}{paint()}.
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We will now review its implementation.
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\snippet graphicsview/elasticnodes/edge.cpp 0
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The \c Edge constructor initializes its \c arrowSize data member to 10 units;
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this determines the size of the arrow which is drawn in
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\l{QGraphicsItem::paint()}{paint()}.
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In the constructor body, we call
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\l{QGraphicsItem::setAcceptedMouseButtons()}{setAcceptedMouseButtons(0)}.
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This ensures that the edge items are not considered for mouse input at all
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(i.e., you cannot click the edges). Then, the source and destination
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pointers are updated, this edge is registered with each node, and we call
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\c adjust() to update this edge's start end end position.
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\snippet graphicsview/elasticnodes/edge.cpp 1
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The source and destination get-functions simply return the respective
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\snippet graphicsview/elasticnodes/edge.cpp 2
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In \c adjust(), we define two points: \c sourcePoint, and \c destPoint,
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pointing at the source and destination nodes' origins respectively. Each
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point is calculated using \l{The Graphics View Coordinate System}{local
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We want the tip of the edge's arrows to point to the exact outline of the
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nodes, as opposed to the center of the nodes. To find this point, we first
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decompose the vector pointing from the center of the source to the center
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of the destination node into X and Y, and then normalize the components by
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dividing by the length of the vector. This gives us an X and Y unit delta
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that, when multiplied by the radius of the node (which is 10), gives us the
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offset that must be added to one point of the edge, and subtracted from the
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If the length of the vector is less than 20 (i.e., if two nodes overlap),
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then we fix the source and destination pointer at the center of the source
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node. In practice this case is very hard to reproduce manually, as the
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forces between the two nodes is then at its maximum.
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It's important to notice that we call
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\l{QGraphicsItem::prepareGeometryChange()}{prepareGeometryChange()} in this
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function. The reason is that the variables \c sourcePoint and \c destPoint
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are used directly when painting, and they are returned from the
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\l{QGraphicsItem::boundingRect()}{boundingRect()} reimplementation. We must
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\l{QGraphicsItem::prepareGeometryChange()}{prepareGeometryChange()} before
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changing what \l{QGraphicsItem::boundingRect()}{boundingRect()} returns,
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and before these variables can be used by
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\l{QGraphicsItem::paint()}{paint()}, to keep Graphics View's internal
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bookkeeping clean. It's safest to call this function once, immediately
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before any such variable is modified.
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\snippet graphicsview/elasticnodes/edge.cpp 3
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The edge's bounding rectangle is defined as the smallest rectangle that
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includes both the start and the end point of the edge. Because we draw an
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arrow on each edge, we also need to compensate by adjusting with half the
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arrow size and half the pen width in all directions. The pen is used to
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draw the outline of the arrow, and we can assume that half of the outline
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can be drawn outside of the arrow's area, and half will be drawn inside.
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\snippet graphicsview/elasticnodes/edge.cpp 4
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We start the reimplementation of \l{QGraphicsItem::paint()}{paint()} by
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checking a few preconditions. Firstly, if either the source or destination
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node is not set, then we return immediately; there is nothing to draw.
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At the same time, we check if the length of the edge is approximately 0,
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and if it is, then we also return.
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\snippet graphicsview/elasticnodes/edge.cpp 5
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We draw the line using a pen that has round joins and caps. If you run the
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example, zoom in and study the edge in detail, you will see that there are
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no sharp/square edges.
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\snippet graphicsview/elasticnodes/edge.cpp 6
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We proceed to drawing one arrow at each end of the edge. Each arrow is
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drawn as a polygon with a black fill. The coordinates for the arrow are
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determined using simple trigonometry.
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\section1 GraphWidget Class Definition
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\c GraphWidget is a subclass of QGraphicsView, which provides the main
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window with scrollbars.
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\snippet graphicsview/elasticnodes/graphwidget.h 0
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The class provides a basic constructor that initializes the scene, an \c
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itemMoved() function to notify changes in the scene's node graph, a few
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event handlers, a reimplementation of
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\l{QGraphicsView::drawBackground()}{drawBackground()}, and a helper
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function for scaling the view by using the mouse wheel or keyboard.
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\snippet graphicsview/elasticnodes/graphwidget.cpp 0
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\c GraphicsWidget's constructor creates the scene, and because most items
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move around most of the time, it sets QGraphicsScene::NoIndex. The scene
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then gets a fixed \l{QGraphicsScene::sceneRect}{scene rectangle}, and is
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assigned to the \c GraphWidget view.
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The view enables QGraphicsView::CacheBackground to cache rendering of its
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static, and somewhat complex, background. Because the graph renders a close
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collection of small items that all move around, it's unnecessary for
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Graphics View to waste time finding accurate update regions, so we set the
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QGraphicsView::BoundingRectViewportUpdate viewport update mode. The default
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would work fine, but this mode is noticably faster for this example.
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To improve rendering quality, we set QPainter::Antialiasing.
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The transformation anchor decides how the view should scroll when you
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transform the view, or in our case, when we zoom in or out. We have chosen
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QGraphicsView::AnchorUnderMouse, which centers the view on the point under
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the mouse cursor. This makes it easy to zoom towards a point in the scene
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by moving the mouse over it, and then rolling the mouse wheel.
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Finally we give the window a minimum size that matches the scene's default
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size, and set a suitable window title.
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\snippet graphicsview/elasticnodes/graphwidget.cpp 1
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The last part of the constructor creates the grid of nodes and edges, and
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gives each node an initial position.
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\snippet graphicsview/elasticnodes/graphwidget.cpp 2
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\c GraphWidget is notified of node movement through this \c itemMoved()
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function. Its job is simply to restart the main timer in case it's not
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running already. The timer is designed to stop when the graph stabilizes,
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and start once it's unstable again.
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\snippet graphicsview/elasticnodes/graphwidget.cpp 3
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This is \c GraphWidget's key event handler. The arrow keys move the center
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node around, the '+' and '-' keys zoom in and out by calling \c
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scaleView(), and the enter and space keys randomize the positions of the
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nodes. All other key events (e.g., page up and page down) are handled by
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QGraphicsView's default implementation.
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\snippet graphicsview/elasticnodes/graphwidget.cpp 4
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The timer event handler's job is to run the whole force calculation
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machinery as a smooth animation. Each time the timer is triggered, the
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handler will find all nodes in the scene, and call \c
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Node::calculateForces() on each node, one at a time. Then, in a final step
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it will call \c Node::advance() to move all nodes to their new positions.
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By checking the return value of \c advance(), we can decide if the grid
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stabilized (i.e., no nodes moved). If so, we can stop the timer.
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\snippet graphicsview/elasticnodes/graphwidget.cpp 5
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In the wheel event handler, we convert the mouse wheel delta to a scale
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factor, and pass this factor to \c scaleView(). This approach takes into
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account the speed that the wheel is rolled. The faster you roll the mouse
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wheel, the faster the view will zoom.
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\snippet graphicsview/elasticnodes/graphwidget.cpp 6
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The view's background is rendered in a reimplementation of
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QGraphicsView::drawBackground(). We draw a large rectangle filled with a
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linear gradient, add a drop shadow, and then render text on top. The text
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is rendered twice for a simple drop-shadow effect.
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This background rendering is quite expensive; this is why the view enables
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QGraphicsView::CacheBackground.
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\snippet graphicsview/elasticnodes/graphwidget.cpp 7
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The \c scaleView() helper function checks that the scale factor stays
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within certain limits (i.e., you cannot zoom too far in nor too far out),
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and then applies this scale to the view.
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\section1 The main() Function
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In contrast to the complexity of the rest of this example, the \c main()
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function is very simple: We create a QApplication instance, seed the
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randomizer using qsrand(), and then create and show an instance of \c
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GraphWidget. Because all nodes in the grid are moved initially, the \c
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GraphWidget timer will start immediately after control has returned to the