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diff --git a/doc/src/examples/elasticnodes.qdoc b/doc/src/examples/elasticnodes.qdoc index f7b1c37..edc62d8 100644 --- a/doc/src/examples/elasticnodes.qdoc +++ b/doc/src/examples/elasticnodes.qdoc @@ -43,7 +43,396 @@ \example graphicsview/elasticnodes \title Elastic Nodes Example - This GraphicsView example shows how to implement edges between nodes in a graph. + This GraphicsView example shows how to implement edges between nodes in a + graph, with basic interaction. You can click to drag a node around, and + zoom in and out using the mouse wheel or the keyboard. Hitting the space + bar will randomize the nodes. The example is also resolution independent; + as you zoom in, the graphics remain crisp. \image elasticnodes-example.png + + Graphics View provides the QGraphicsScene class for managing and + interacting with a large number of custom-made 2D graphical items derived + from the QGraphicsItem class, and a QGraphicsView widget for visualizing + the items, with support for zooming and rotation. + + This example consists of a \c Node class, an \c Edge class, a \c + GraphWidget test, and a \c main function: the \c Node class represents + draggable yellow nodes in a grid, the \c Edge class represents the lines + between the nodes, the \c GraphWidget class represents the application + window, and the \c main() function creates and shows this window, and runs + the event loop. + + \section1 Node Class Definition + + The \c Node class serves three purposes: + + \list + \o Painting a yellow gradient "ball" in two states: sunken and raised. + \o Managing connections to other nodes. + \o Calculating forces pulling and pushing the nodes in the grid. + \endlist + + Let's start by looking at the \c Node class declaration. + + \snippet examples/graphicsview/elasticnodes/node.h 0 + + The \c Node class inherits QGraphicsItem, and reimplements the two + mandatory functions \l{QGraphicsItem::boundingRect()}{boundingRect()} and + \l{QGraphicsItem::paint()}{paint()} to provide its visual appearance. It + also reimplements \l{QGraphicsItem::shape()}{shape()} to ensure its hit + area has an elliptic shape (as opposed to the default bounding rectangle). + + For edge management purposes the node provides a simple API for adding + edges to a node, and for listing all connected edges. + + The \l{QGraphicsItem::advance()}{advance()} reimplementation is called + whenever the scene's state advances by one step. The calculateForces() + function is called to calculate the forces that push and pull on this node + and its neighbors. + + The \c Node class also reimplements + \l{QGraphicsItem::itemChange()}{itemChange()} to react to state changes (in + this case, position changes), and + \l{QGraphicsItem::mousePressEvent()}{mousePressEvent()} and + \l{QGraphicsItem::mouseReleaseEvent()}{mouseReleaseEvent()} to update the + item's visual appearance. + + We will start reviewing the \c Node implementation by looking at its + constructor: + + \snippet examples/graphicsview/elasticnodes/node.cpp 0 + + In the constructor, we set the + \l{QGraphicsItem::ItemIsMovable}{ItemIsMovable} flag to allow the item to + move in response to mouse dragging, and + \l{QGraphicsItem::ItemSendsGeometryChanges}{ItemSendsGeometryChanges} to + enable \l{QGraphicsItem::itemChange()}{itemChange()} notifications for + position and transformation changes. We also enable + \l{QGraphicsItem::DeviceCoordinateCache}{DeviceCoordinateCache} to speed up + rendering performance. To ensure that the nodes are always stacked on top + of edges, we finally set the item's Z value to -1. + + \c Node's constructor takes a \c GraphWidget pointer and stores this as a + member variable. We will revisit this pointer later on. + + \snippet examples/graphicsview/elasticnodes/node.cpp 1 + + The addEdge() function adds the input edge to a list of attached edges. The + edge is then adjusted so that the end points for the edge match the + positions of the source and destination nodes. + + The edges() function simply returns the list of attached edges. + + \snippet examples/graphicsview/elasticnodes/node.cpp 2 + + The \e calculateForces() function implements the elastic forces effect that + pulls and pushes on nodes in the grid. In addition to this algorithm, the + user can move one node around with the mouse. Because we do not want the + two to interfere, we start by checking if this \c Node is the current mouse + grabber item (i.e., QGraphicsScene::mouseGrabberItem()). Because we need to + find all neighboring (but not necessarily connected) nodes, we also make + sure the item is part of a scene in the first place. + + \snippet examples/graphicsview/elasticnodes/node.cpp 3 + + The algorithm has two steps: the first is to calculate the forces that push + the nodes apart, and the second is to subtract the forces that pull the + nodes together. First we need to find all the nodes in the graph. We call + QGraphicsScene::items() to find all items in the scene, and then use + qgraphicsitem_cast() to look for \c Node instances. + + We make use of \l{QGraphicsItem::mapFromItem()}{mapFromItem()} to create a + vector pointing from this node to each other node, in \l{The Graphics View + Coordinate System}{local coordinates}. We use the decomposed components of + this vector to determine the direction and strength of force that apply to + the node. The forces are added up for each node, and weighted so that the + closest nodes are given the strongest force. The sum of all forces are + stored in \e xvel (X-velocity) and \e yvel (Y-velocity). + + \snippet examples/graphicsview/elasticnodes/node.cpp 4 + + The edges between the nodes represent the forces that pull the nodes + together. By visiting each edge that is connected to this node, we can use + a similar approach as above to find the direction and strength of all + forces. These forces are subtracted from \e xvel and \e yvel. + + \snippet examples/graphicsview/elasticnodes/node.cpp 5 + + In theory, the sum of pushing and pulling forces should stabilize to + precisely 0. In practise, however, they never do. To circumvent errors in + numerical precision, we simply force the sum of forces to be 0 when they + are less than 0.1. + + \snippet examples/graphicsview/elasticnodes/node.cpp 6 + + The final step of \e calculateForces() determines the node's new position. + We add the force to the node's current position. We also make sure the new + position stays inside of our defined boundaries. We don't actually move the + item in this function; that's done in a separate step, from \e advance(). + + \snippet examples/graphicsview/elasticnodes/node.cpp 7 + + The \e advance() function updates the item's current position. It is called + from \e GraphWidget::timerEvent(). If the node's position changed, the + function returns true; otherwise false is returned. + + \snippet examples/graphicsview/elasticnodes/node.cpp 8 + + The \e Node's bounding rectangle is a 20x20 sized rectangle centered around + its origin (0, 0), adjusted by 2 units in all directions to compensate for + the node's outline stroke, and by 3 units down and to the right to make + room for a simple drop shadow. + + \snippet examples/graphicsview/elasticnodes/node.cpp 9 + + The shape is a simple ellipse. This ensures that you must click inside the + node's elliptic shape in order to drag it around. You can test this effect + by running the example, and zooming far enough in so that the nodes become + very large. Without reimplementing \l{QGraphicsItem::shape()}{shape()}, the + item's hit area would be identical to its bounding rectangle (i.e., + rectangular). + + \snippet examples/graphicsview/elasticnodes/node.cpp 10 + + This function implements the node's painting. We start by drawing a simple + dark gray elliptic drop shadow at (-7, -7), that is, (3, 3) units down and + to the right. + + We then draw an ellipse with a radial gradient fill. This fill is either + Qt::yellow to Qt::darkYellow when raised, or the opposite when sunken. In + sunken state we also shift the center and focal point by (3, 3) to + emphasize the impression that something has been pushed down. + + Drawing filled ellipses with gradients can be quite slow, especially when + using complex gradients such as QRadialGradient. This is why this example + uses \l{QGraphicsItem::DeviceCoordinateCache}{DeviceCoordinateCache}, a + simple yet effective measure that prevents unnecessary redrawing. + + \snippet examples/graphicsview/elasticnodes/node.cpp 11 + + We reimplement \l{QGraphicsItem::itemChange()}{itemChange()} to adjust the + position of all connected edges, and to notify the scene that an item has + moved (i.e., "something has happened"). This will trigger new force + calculations. + + This notification is the only reason why the nodes need to keep a pointer + back to the \e GraphWidget. Another approach could be to provide such + notification using a signal; in such case, \e Node would need to inherit + from QGraphicsObject. + + \snippet examples/graphicsview/elasticnodes/node.cpp 12 + + Because we have set the \l{QGraphicsItem::ItemIsMovable}{ItemIsMovable} + flag, we don't need to implement the logic that moves the node according to + mouse input; this is already provided for us. We still need to reimplement + the mouse press and release handlers though, to update the nodes' visual + appearance (i.e., sunken or raised). + + \section1 Edge Class Definition + + The \e Edge class represents the arrow-lines between the nodes in this + example. The class is very simple: it maintains a source- and destination + node pointer, and provides an \e adjust() function that makes sure the line + starts at the position of the source, and ends at the position of the + destination. The edges are the only items that change continuously as + forces pull and push on the nodes. + + Let's take a look at the class declaration: + + \snippet examples/graphicsview/elasticnodes/edge.h 0 + + \e Edge inherits from QGraphicsItem, as it's a simple class that has no use + for signals, slots, and properties (compare to QGraphicsObject). + + The constructor takes two node pointers as input. Both pointers are + mandatory in this example. We also provide get-functions for each node. + + The \e adjust() function repositions the edge, and the item also implements + \l{QGraphicsItem::boundingRect()}{boundingRect()} and + \{QGraphicsItem::paint()}{paint()}. + + We will now review its implementation. + + \snippet examples/graphicsview/elasticnodes/edge.cpp 0 + + The \e Edge constructor initializes its arrowSize data member to 10 units; + this determines the size of the arrow which is drawn in + \l{QGraphicsItem::paint()}{paint()}. + + In the constructor body, we call + \l{QGraphicsItem::setAcceptedMouseButtons()}{setAcceptedMouseButtons(0)}. + This ensures that the edge items are not considered for mouse input at all + (i.e., you cannot click the edges). Then, the source and destination + pointers are updated, this edge is registered with each node, and we call + \e adjust() to update this edge's start end end position. + + \snippet examples/graphicsview/elasticnodes/edge.cpp 1 + + The source and destination get-functions simply return the respective + pointers. + + \snippet examples/graphicsview/elasticnodes/edge.cpp 2 + + In \e adjust(), we define two points: \e sourcePoint, and \e destPoint, + pointing at the source and destination nodes' origins respectively. Each + point is calculated using \l{The Graphics View Coordinate System}{local + coordinates}. + + We want the tip of the edge's arrows to point to the exact outline of the + nodes, as opposed to the center of the nodes. To find this point, we first + decompose the vector pointing from the center of the source to the center + of the destination node into X and Y, and then normalize the components by + dividing by the length of the vector. This gives us an X and Y unit delta + that, when multiplied by the radius of the node (which is 10), gives us the + offset that must be added to one point of the edge, and subtracted from the + other. + + If the length of the vector is less than 20 (i.e., if two nodes overlap), + then we fix the source and destination pointer at the center of the source + node. In practise this case is very hard to reproduce manually, as the + forces between the two nodes is then at its maximum. + + It's important to notice that we call + \l{QGraphicsItem::prepareGeometryChange()}{prepareGeometryChange()} in this + function. The reason is that the variables \e sourcePoint and \e destPoint + are used directly when painting, and they are returned from the + \l{QGraphicsItem::boundingRect()}{boundingRect()} reimplementation. We must + always call + \l{QGraphicsItem::prepareGeometryChange()}{prepareGeometryChange()} before + changing what \l{QGraphicsItem::boundingRect()}{boundingRect()} returns, + and before these variables can be used by + \l{QGraphicsItem::paint()}{paint()}, to keep Graphics View's internal + bookkeeping clean. It's safest to call this function once, immediately + before any such variable is modified. + + \snippet examples/graphicsview/elasticnodes/edge.cpp 3 + + The edge's bounding rectangle is defined as the smallest rectangle that + includes both the start and the end point of the edge. Because we draw an + arrow on each edge, we also need to compensate by adjusting with half the + arrow size and half the pen width in all directions. The pen is used to + draw the outline of the arrow, and we can assume that half of the outline + can be drawn outside of the arrow's area, and half will be drawn inside. + + \snippet examples/graphicsview/elasticnodes/edge.cpp 4 + + We start the reimplementation of \l{QGraphicsItem::paint()}{paint()} by + checking a few preconditions. Firstly, if either the source or destination + node is not set, then we return immediately; there is nothing to draw. + + At the same time, we check if the length of the edge is approximately 0, + and if it is, then we also return. + + \snippet examples/graphicsview/elasticnodes/edge.cpp 5 + + We draw the line using a pen that has round joins and caps. If you run the + example, zoom in and study the edge in detail, you will see that there are + no sharp/square edges. + + \snippet examples/graphicsview/elasticnodes/edge.cpp 6 + + We proceed to drawing one arrow at each end of the edge. Each arrow is + drawn as a polygon with a black fill. The coordinates for the arrow are + determined using simple trigonometry. + + \section1 GraphWidget Class Definition + + \e GraphWidget is a subclass of QGraphicsView, which provides the main + window with scrollbars. + + \snippet examples/graphicsview/elasticnodes/graphwidget.h 0 + + It provides a basic constructor that initializes the scene, an \e + itemMoved() function to notify changes in the scene's node graph, a few + event handlers, a reimplementation of + \l{QGraphicsView::drawBackground()}{drawBackground()}, and a helper + function for scaling the view by mouse or keyboard. + + \snippet examples/graphicsview/elasticnodes/graphwidget.cpp 0 + + \e GraphicsWidget's constructor creates the scene, and because most items + move around most of the time, it sets QGraphicsScene::NoIndex. Then the + scene gets a fixed \l{QGraphicsScene::sceneRect}{scene rectangle}. + The scene is then assigned to the \e GraphWidget view. + + The view enables QGraphicsView::CacheBackground to cache rendering of its + static and somewhat complex background. Because the graph renders a close + collection of small items that all move around, it's unnecessary for + Graphics View to waste time finding accurate update regions, so we set the + QGraphicsView::BoundingRectViewportUpdate viewport update mode. The default + would work fine, but this mode is noticably faster for this example. + + To improve rendering quality, we set QPainter::Antialiasing. + + The transformation anchor decides how the view should scroll when you + transform the view, or in our case, when we zoom in or out. We have chosen + QGraphicsView::AnchorUnderMouse, which centers the view on the point under + the mouse cursor. This makes it easy to zoom towards a point in the scene + by moving the mouse over it, and then rolling the mouse wheel. + + Finally we give the window a minimum size that matches the scene's default + size, and set a suitable window title. + + \snippet examples/graphicsview/elasticnodes/graphwidget.cpp 1 + + The last part of the constructor creates the grid of nodes and edges, and + gives each node an initial position. + + \snippet examples/graphicsview/elasticnodes/graphwidget.cpp 2 + + \e GraphWidget is notified of node movement through this \e itemMoved() + function. Its job is simply to restart the main timer in case it's not + running already. The timer is designed to stop when the graph stabilizes, + and start once it's unstable again. + + \snippet examples/graphicsview/elasticnodes/graphwidget.cpp 3 + + This is \e GraphWidget's key event handler. The arrow keys move the center + node around, the '+' and '-' keys zoom in and out by calling \e + scaleView(), and the enter and space keys randomize the positions of the + nodes. All other key events (e.g., page up and page down) are handled by + QGraphicsView's default implementation. + + \snippet examples/graphicsview/elasticnodes/graphwidget.cpp 4 + + The timer event handler's job is to run the whole force calculation + machinery as a smooth animation. Each time the timer is triggered, the + handler will find all nodes in the scene, and call \e + Node::calculateForces() on each node, one at a time. Then, in a final step + it will call \e Node::advance() to move all nodes to their new positions. + By checking the return value of \e advance(), we can decide if the grid + stabilized (i.e., no nodes moved). If so, we can stop the timer. + + \snippet examples/graphicsview/elasticnodes/graphwidget.cpp 5 + + In the wheel event handler, we convert the mouse wheel delta to a scale + factor, and pass this factor to \e scaleView(). This approach takes into + account the speed that the wheel is rolled. The faster you roll the mouse + wheel, the faster the view will zoom. + + \snippet examples/graphicsview/elasticnodes/graphwidget.cpp 6 + + The view's background is rendered in a reimplementation of + QGraphicsView::drawBackground(). We draw a large rectangle filled with a + linear gradient, with a drop shadow, and then render text in top. The text + is rendered twice to give a similar simple drop-shadow effect. + + This background rendering is quite expensive; this is why the view enables + QGraphicsView::CacheBackground. + + \snippet examples/graphicsview/elasticnodes/graphwidget.cpp 7 + + The \e scaleView() helper function checks that the scale factor stays + within certain limits (i.e., you cannot zoom too far in nor too far out), + and then applies this scale. + + \section1 The main() Function + + In contrast to the complexity of the rest of this example, the \e main() + function is very simple: We create a QApplication instance, seed the + randomizer using qsrand(), and then create and show an instance of \e + GraphWidget. Because all nodes in the grid are moved initially, the \e + GraphWidget timer will start immediately after control has returned to the + event loop. */ |