elasticnodes

(You can also check this code in the repository)

import sys
import weakref
import math

from PySide6.QtCore import (QLineF, QPointF, QRandomGenerator, QRectF, QSizeF,
                            Qt, qAbs)
from PySide6.QtGui import (QColor, QBrush, QPainter, QPainterPath, QPen,
                           QPolygonF, QRadialGradient)
from PySide6.QtWidgets import (QApplication, QGraphicsItem, QGraphicsScene,
                               QGraphicsView, QStyle, QWidget)


def random(boundary):
    return QRandomGenerator.global_().bounded(boundary)


class Edge(QGraphicsItem):

    item_type = QGraphicsItem.UserType + 2

    def __init__(self, sourceNode, destNode):
        super().__init__()

        self._arrow_size = 10.0
        self._source_point = QPointF()
        self._dest_point = QPointF()
        self.setAcceptedMouseButtons(Qt.NoButton)
        self.source = weakref.ref(sourceNode)
        self.dest = weakref.ref(destNode)
        self.source().add_edge(self)
        self.dest().add_edge(self)
        self.adjust()

    def item_type(self):
        return Edge.item_type

    def source_node(self):
        return self.source()

    def set_source_node(self, node):
        self.source = weakref.ref(node)
        self.adjust()

    def dest_node(self):
        return self.dest()

    def set_dest_node(self, node):
        self.dest = weakref.ref(node)
        self.adjust()

    def adjust(self):
        if not self.source() or not self.dest():
            return

        line = QLineF(self.mapFromItem(self.source(), 0, 0),
                      self.mapFromItem(self.dest(), 0, 0))
        length = line.length()

        if length == 0.0:
            return

        edge_offset = QPointF((line.dx() * 10) / length, (line.dy() * 10) / length)

        self.prepareGeometryChange()
        self._source_point = line.p1() + edge_offset
        self._dest_point = line.p2() - edge_offset

    def boundingRect(self):
        if not self.source() or not self.dest():
            return QRectF()

        pen_width = 1
        extra = (pen_width + self._arrow_size) / 2.0

        width = self._dest_point.x() - self._source_point.x()
        height = self._dest_point.y() - self._source_point.y()
        rect = QRectF(self._source_point, QSizeF(width, height))
        return rect.normalized().adjusted(-extra, -extra, extra, extra)

    def paint(self, painter, option, widget):
        if not self.source() or not self.dest():
            return

        # Draw the line itself.
        line = QLineF(self._source_point, self._dest_point)

        if line.length() == 0.0:
            return

        painter.setPen(QPen(Qt.black, 1, Qt.SolidLine, Qt.RoundCap, Qt.RoundJoin))
        painter.drawLine(line)

        # Draw the arrows if there's enough room.
        angle = math.acos(line.dx() / line.length())
        if line.dy() >= 0:
            angle = 2 * math.pi - angle

        arrow_head1 = QPointF(math.sin(angle + math.pi / 3) * self._arrow_size,
                              math.cos(angle + math.pi / 3) * self._arrow_size)
        source_arrow_p1 = self._source_point + arrow_head1
        arrow_head2 = QPointF(math.sin(angle + math.pi - math.pi / 3) * self._arrow_size,
                              math.cos(angle + math.pi - math.pi / 3) * self._arrow_size)
        source_arrow_p2 = self._source_point + arrow_head2

        arrow_head1 = QPointF(math.sin(angle - math.pi / 3) * self._arrow_size,
                              math.cos(angle - math.pi / 3) * self._arrow_size)
        dest_arrow_p1 = self._dest_point + arrow_head1
        arrow_head2 = QPointF(math.sin(angle - math.pi + math.pi / 3) * self._arrow_size,
                              math.cos(angle - math.pi + math.pi / 3) * self._arrow_size)
        dest_arrow_p2 = self._dest_point + arrow_head2

        painter.setBrush(Qt.black)
        painter.drawPolygon(QPolygonF([line.p1(), source_arrow_p1, source_arrow_p2]))
        painter.drawPolygon(QPolygonF([line.p2(), dest_arrow_p1, dest_arrow_p2]))


class Node(QGraphicsItem):
    item_type = QGraphicsItem.UserType + 1

    def __init__(self, graphWidget):
        super().__init__()

        self.graph = weakref.ref(graphWidget)
        self._edge_list = []
        self._new_pos = QPointF()
        self.setFlag(QGraphicsItem.ItemIsMovable)
        self.setFlag(QGraphicsItem.ItemSendsGeometryChanges)
        self.setCacheMode(self.DeviceCoordinateCache)
        self.setZValue(-1)

    def item_type(self):
        return Node.item_type

    def add_edge(self, edge):
        self._edge_list.append(weakref.ref(edge))
        edge.adjust()

    def edges(self):
        return self._edge_list

    def calculate_forces(self):
        if not self.scene() or self.scene().mouseGrabberItem() is self:
            self._new_pos = self.pos()
            return

        # Sum up all forces pushing this item away.
        xvel = 0.0
        yvel = 0.0
        for item in self.scene().items():
            if not isinstance(item, Node):
                continue

            line = QLineF(self.mapFromItem(item, 0, 0), QPointF(0, 0))
            dx = line.dx()
            dy = line.dy()
            l = 2.0 * (dx * dx + dy * dy)
            if l > 0:
                xvel += (dx * 150.0) / l
                yvel += (dy * 150.0) / l

        # Now subtract all forces pulling items together.
        weight = (len(self._edge_list) + 1) * 10.0
        for edge in self._edge_list:
            if edge().source_node() is self:
                pos = self.mapFromItem(edge().dest_node(), 0, 0)
            else:
                pos = self.mapFromItem(edge().source_node(), 0, 0)
            xvel += pos.x() / weight
            yvel += pos.y() / weight

        if qAbs(xvel) < 0.1 and qAbs(yvel) < 0.1:
            xvel = yvel = 0.0

        scene_rect = self.scene().sceneRect()
        self._new_pos = self.pos() + QPointF(xvel, yvel)
        self._new_pos.setX(min(max(self._new_pos.x(), scene_rect.left() + 10),
                               scene_rect.right() - 10))
        self._new_pos.setY(min(max(self._new_pos.y(), scene_rect.top() + 10),
                               scene_rect.bottom() - 10))

    def advance(self):
        if self._new_pos == self.pos():
            return False

        self.setPos(self._new_pos)
        return True

    def boundingRect(self):
        adjust = 2.0
        return QRectF(-10 - adjust, -10 - adjust,
                             23 + adjust, 23 + adjust)

    def shape(self):
        path = QPainterPath()
        path.addEllipse(-10, -10, 20, 20)
        return path

    def paint(self, painter, option, widget):
        painter.setPen(Qt.NoPen)
        painter.setBrush(Qt.darkGray)
        painter.drawEllipse(-7, -7, 20, 20)

        gradient = QRadialGradient(-3, -3, 10)
        if option.state & QStyle.State_Sunken:
            gradient.setCenter(3, 3)
            gradient.setFocalPoint(3, 3)
            gradient.setColorAt(1, QColor(Qt.yellow).lighter(120))
            gradient.setColorAt(0, QColor(Qt.darkYellow).lighter(120))
        else:
            gradient.setColorAt(0, Qt.yellow)
            gradient.setColorAt(1, Qt.darkYellow)

        painter.setBrush(QBrush(gradient))
        painter.setPen(QPen(Qt.black, 0))
        painter.drawEllipse(-10, -10, 20, 20)

    def itemChange(self, change, value):
        if change == QGraphicsItem.ItemPositionChange:
            for edge in self._edge_list:
                edge().adjust()
            self.graph().item_moved()

        return QGraphicsItem.itemChange(self, change, value)

    def mousePressEvent(self, event):
        self.update()
        QGraphicsItem.mousePressEvent(self, event)

    def mouseReleaseEvent(self, event):
        self.update()
        QGraphicsItem.mouseReleaseEvent(self, event)


class GraphWidget(QGraphicsView):
    def __init__(self):
        super().__init__()

        self._timer_id = 0

        scene = QGraphicsScene(self)
        scene.setItemIndexMethod(QGraphicsScene.NoIndex)
        scene.setSceneRect(-200, -200, 400, 400)
        self.setScene(scene)
        self.setCacheMode(QGraphicsView.CacheBackground)
        self.setRenderHint(QPainter.Antialiasing)
        self.setTransformationAnchor(QGraphicsView.AnchorUnderMouse)
        self.setResizeAnchor(QGraphicsView.AnchorViewCenter)

        node1 = Node(self)
        node2 = Node(self)
        node3 = Node(self)
        node4 = Node(self)
        self._center_node = Node(self)
        node6 = Node(self)
        node7 = Node(self)
        node8 = Node(self)
        node9 = Node(self)
        scene.addItem(node1)
        scene.addItem(node2)
        scene.addItem(node3)
        scene.addItem(node4)
        scene.addItem(self._center_node)
        scene.addItem(node6)
        scene.addItem(node7)
        scene.addItem(node8)
        scene.addItem(node9)
        scene.addItem(Edge(node1, node2))
        scene.addItem(Edge(node2, node3))
        scene.addItem(Edge(node2, self._center_node))
        scene.addItem(Edge(node3, node6))
        scene.addItem(Edge(node4, node1))
        scene.addItem(Edge(node4, self._center_node))
        scene.addItem(Edge(self._center_node, node6))
        scene.addItem(Edge(self._center_node, node8))
        scene.addItem(Edge(node6, node9))
        scene.addItem(Edge(node7, node4))
        scene.addItem(Edge(node8, node7))
        scene.addItem(Edge(node9, node8))

        node1.setPos(-50, -50)
        node2.setPos(0, -50)
        node3.setPos(50, -50)
        node4.setPos(-50, 0)
        self._center_node.setPos(0, 0)
        node6.setPos(50, 0)
        node7.setPos(-50, 50)
        node8.setPos(0, 50)
        node9.setPos(50, 50)

        self.scale(0.8, 0.8)
        self.setMinimumSize(400, 400)
        self.setWindowTitle(self.tr("Elastic Nodes"))

    def item_moved(self):
        if not self._timer_id:
            self._timer_id = self.startTimer(1000 / 25)

    def keyPressEvent(self, event):
        key = event.key()

        if key == Qt.Key_Up:
            self._center_node.moveBy(0, -20)
        elif key == Qt.Key_Down:
            self._center_node.moveBy(0, 20)
        elif key == Qt.Key_Left:
            self._center_node.moveBy(-20, 0)
        elif key == Qt.Key_Right:
            self._center_node.moveBy(20, 0)
        elif key == Qt.Key_Plus:
            self.scale_view(1.2)
        elif key == Qt.Key_Minus:
            self.scale_view(1 / 1.2)
        elif key == Qt.Key_Space or key == Qt.Key_Enter:
            for item in self.scene().items():
                if isinstance(item, Node):
                    item.setPos(-150 + random(300), -150 + random(300))
        else:
            QGraphicsView.keyPressEvent(self, event)

    def timerEvent(self, event):
        nodes = [item for item in self.scene().items() if isinstance(item, Node)]

        for node in nodes:
            node.calculate_forces()

        items_moved = False
        for node in nodes:
            if node.advance():
                items_moved = True

        if not items_moved:
            self.killTimer(self._timer_id)
            self._timer_id = 0

    def wheelEvent(self, event):
        delta = event.angleDelta().y()
        self.scale_view(math.pow(2.0, -delta / 240.0))

    def draw_background(self, painter, rect):
        # Shadow.
        scene_rect = self.sceneRect()
        right_shadow = QRectF(scene_rect.right(), scene_rect.top() + 5,
                              5, scene_rect.height())
        bottom_shadow = QRectF(scene_rect.left() + 5, scene_rect.bottom(),
                               scene_rect.width(), 5)
        if right_shadow.intersects(rect) or right_shadow.contains(rect):
                painter.fillRect(right_shadow, Qt.darkGray)
        if bottom_shadow.intersects(rect) or bottom_shadow.contains(rect):
                painter.fillRect(bottom_shadow, Qt.darkGray)

        # Fill.
        gradient = QLinearGradient(scene_rect.topLeft(), scene_rect.bottomRight())
        gradient.setColorAt(0, Qt.white)
        gradient.setColorAt(1, Qt.lightGray)
        painter.fillRect(rect.intersected(scene_rect), QBrush(gradient))
        painter.setBrush(Qt.NoBrush)
        painter.drawRect(scene_rect)

        # Text.
        text_rect = QRectF(scene_rect.left() + 4, scene_rect.top() + 4,
                                 scene_rect.width() - 4, scene_rect.height() - 4)
        message = self.tr("Click and drag the nodes around, and zoom with the "
                          "mouse wheel or the '+' and '-' keys")

        font = painter.font()
        font.setBold(True)
        font.setPointSize(14)
        painter.setFont(font)
        painter.setPen(Qt.lightGray)
        painter.drawText(text_rect.translated(2, 2), message)
        painter.setPen(Qt.black)
        painter.drawText(text_rect, message)

    def scale_view(self, scaleFactor):
        factor = self.transform().scale(scaleFactor, scaleFactor).mapRect(QRectF(0, 0, 1, 1)).width()

        if factor < 0.07 or factor > 100:
            return

        self.scale(scaleFactor, scaleFactor)


if __name__ == "__main__":
    app = QApplication(sys.argv)

    widget = GraphWidget()
    widget.show()

    sys.exit(app.exec())