Add some demo examples.

.gitignore

@@ -1 +1,2 @@
 .DS_Store
+.vscode

src/app/CMakeLists.txt

@@ -3,6 +3,10 @@ qt_add_executable(scribbles
     main.cpp
     MainWindow.cpp 
     MainWindow.ui
+    ScribbleArea.cpp
+    Edge.cpp 
+    Node.cpp 
+    GraphWidget.cpp 
 )
 
 target_link_libraries(scribbles PRIVATE Qt6::Widgets)

src/app/Edge.cpp

@@ -0,0 +1,101 @@
+#include "Edge.h"
+#include "Node.h"
+
+#include <QPainter>
+#include <QtMath>
+
+//! [0]
+Edge::Edge(Node *sourceNode, Node *destNode)
+    : source(sourceNode), dest(destNode)
+{
+    setAcceptedMouseButtons(Qt::NoButton);
+    source->addEdge(this);
+    dest->addEdge(this);
+    adjust();
+}
+//! [0]
+
+//! [1]
+Node *Edge::sourceNode() const
+{
+    return source;
+}
+
+Node *Edge::destNode() const
+{
+    return dest;
+}
+//! [1]
+
+//! [2]
+void Edge::adjust()
+{
+    if (!source || !dest)
+        return;
+
+    QLineF line(mapFromItem(source, 0, 0), mapFromItem(dest, 0, 0));
+    qreal length = line.length();
+
+    prepareGeometryChange();
+
+    if (length > qreal(20.)) {
+        QPointF edgeOffset((line.dx() * 10) / length, (line.dy() * 10) / length);
+        sourcePoint = line.p1() + edgeOffset;
+        destPoint = line.p2() - edgeOffset;
+    } else {
+        sourcePoint = destPoint = line.p1();
+    }
+}
+//! [2]
+
+//! [3]
+QRectF Edge::boundingRect() const
+{
+    if (!source || !dest)
+        return QRectF();
+
+    qreal penWidth = 1;
+    qreal extra = (penWidth + arrowSize) / 2.0;
+
+    return QRectF(sourcePoint, QSizeF(destPoint.x() - sourcePoint.x(),
+                                      destPoint.y() - sourcePoint.y()))
+        .normalized()
+        .adjusted(-extra, -extra, extra, extra);
+}
+//! [3]
+
+//! [4]
+void Edge::paint(QPainter *painter, const QStyleOptionGraphicsItem *, QWidget *)
+{
+    if (!source || !dest)
+        return;
+
+    QLineF line(sourcePoint, destPoint);
+    if (qFuzzyCompare(line.length(), qreal(0.)))
+        return;
+//! [4]
+
+//! [5]
+    // Draw the line itself
+    painter->setPen(QPen(Qt::black, 1, Qt::SolidLine, Qt::RoundCap, Qt::RoundJoin));
+    painter->drawLine(line);
+//! [5]
+
+//! [6]
+    // Draw the arrows
+    double angle = std::atan2(-line.dy(), line.dx());
+
+    QPointF sourceArrowP1 = sourcePoint + QPointF(sin(angle + M_PI / 3) * arrowSize,
+                                                  cos(angle + M_PI / 3) * arrowSize);
+    QPointF sourceArrowP2 = sourcePoint + QPointF(sin(angle + M_PI - M_PI / 3) * arrowSize,
+                                                  cos(angle + M_PI - M_PI / 3) * arrowSize);
+    QPointF destArrowP1 = destPoint + QPointF(sin(angle - M_PI / 3) * arrowSize,
+                                              cos(angle - M_PI / 3) * arrowSize);
+    QPointF destArrowP2 = destPoint + QPointF(sin(angle - M_PI + M_PI / 3) * arrowSize,
+                                              cos(angle - M_PI + M_PI / 3) * arrowSize);
+
+    painter->setBrush(Qt::black);
+    painter->drawPolygon(QPolygonF() << line.p1() << sourceArrowP1 << sourceArrowP2);
+    painter->drawPolygon(QPolygonF() << line.p2() << destArrowP1 << destArrowP2);
+}
+//! [6]

src/app/Edge.h

@@ -0,0 +1,35 @@
+#ifndef EDGE_H
+#define EDGE_H
+
+#include <QGraphicsItem>
+
+class Node;
+
+//! [0]
+class Edge : public QGraphicsItem
+{
+public:
+    Edge(Node *sourceNode, Node *destNode);
+
+    Node *sourceNode() const;
+    Node *destNode() const;
+
+    void adjust();
+
+    enum { Type = UserType + 2 };
+    int type() const override { return Type; }
+
+protected:
+    QRectF boundingRect() const override;
+    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override;
+
+private:
+    Node *source, *dest;
+
+    QPointF sourcePoint;
+    QPointF destPoint;
+    qreal arrowSize = 10;
+};
+//! [0]
+
+#endif // EDGE_H

src/app/GraphWidget.cpp

@@ -0,0 +1,215 @@
+#include "GraphWidget.h"
+#include "Edge.h"
+#include "Node.h"
+
+#include <math.h>
+
+#include <QKeyEvent>
+#include <QRandomGenerator>
+
+//! [0]
+GraphWidget::GraphWidget(QWidget *parent)
+    : QGraphicsView(parent) 
+{
+    QGraphicsScene *scene = new QGraphicsScene(this);
+    scene->setItemIndexMethod(QGraphicsScene::NoIndex);
+    scene->setSceneRect(-200, -200, 400, 400);
+    setScene(scene);
+    setCacheMode(CacheBackground);
+    setViewportUpdateMode(BoundingRectViewportUpdate);
+    setRenderHint(QPainter::Antialiasing);
+    setTransformationAnchor(AnchorUnderMouse);
+    scale(qreal(0.8), qreal(0.8));
+    setMinimumSize(400, 400);
+    setWindowTitle(tr("Elastic Nodes"));
+//! [0]
+
+//! [1]
+    Node *node1 = new Node(this);
+    Node *node2 = new Node(this);
+    Node *node3 = new Node(this);
+    Node *node4 = new Node(this);
+    centerNode = new Node(this);
+    Node *node6 = new Node(this);
+    Node *node7 = new Node(this);
+    Node *node8 = new Node(this);
+    Node *node9 = new Node(this);
+    scene->addItem(node1);
+    scene->addItem(node2);
+    scene->addItem(node3);
+    scene->addItem(node4);
+    scene->addItem(centerNode);
+    scene->addItem(node6);
+    scene->addItem(node7);
+    scene->addItem(node8);
+    scene->addItem(node9);
+    scene->addItem(new Edge(node1, node2));
+    scene->addItem(new Edge(node2, node3));
+    scene->addItem(new Edge(node2, centerNode));
+    scene->addItem(new Edge(node3, node6));
+    scene->addItem(new Edge(node4, node1));
+    scene->addItem(new Edge(node4, centerNode));
+    scene->addItem(new Edge(centerNode, node6));
+    scene->addItem(new Edge(centerNode, node8));
+    scene->addItem(new Edge(node6, node9));
+    scene->addItem(new Edge(node7, node4));
+    scene->addItem(new Edge(node8, node7));
+    scene->addItem(new Edge(node9, node8));
+
+    node1->setPos(-50, -50);
+    node2->setPos(0, -50);
+    node3->setPos(50, -50);
+    node4->setPos(-50, 0);
+    centerNode->setPos(0, 0);
+    node6->setPos(50, 0);
+    node7->setPos(-50, 50);
+    node8->setPos(0, 50);
+    node9->setPos(50, 50);
+}
+//! [1]
+
+//! [2]
+void GraphWidget::itemMoved()
+{
+    if (!timerId)
+        timerId = startTimer(1000 / 25);
+}
+//! [2]
+
+//! [3]
+void GraphWidget::keyPressEvent(QKeyEvent *event)
+{
+    switch (event->key()) {
+    case Qt::Key_Up:
+        centerNode->moveBy(0, -20);
+        break;
+    case Qt::Key_Down:
+        centerNode->moveBy(0, 20);
+        break;
+    case Qt::Key_Left:
+        centerNode->moveBy(-20, 0);
+        break;
+    case Qt::Key_Right:
+        centerNode->moveBy(20, 0);
+        break;
+    case Qt::Key_Plus:
+        zoomIn();
+        break;
+    case Qt::Key_Minus:
+        zoomOut();
+        break;
+    case Qt::Key_Space:
+    case Qt::Key_Enter:
+        shuffle();
+        break;
+    default:
+        QGraphicsView::keyPressEvent(event);
+    }
+}
+//! [3]
+
+//! [4]
+void GraphWidget::timerEvent(QTimerEvent *event)
+{
+    Q_UNUSED(event);
+
+    QList<Node *> nodes;
+    const QList<QGraphicsItem *> items = scene()->items();
+    for (QGraphicsItem *item : items) {
+        if (Node *node = qgraphicsitem_cast<Node *>(item))
+            nodes << node;
+    }
+
+    for (Node *node : std::as_const(nodes))
+        node->calculateForces();
+
+    bool itemsMoved = false;
+    for (Node *node : std::as_const(nodes)) {
+        if (node->advancePosition())
+            itemsMoved = true;
+    }
+
+    if (!itemsMoved) {
+        killTimer(timerId);
+        timerId = 0;
+    }
+}
+//! [4]
+
+#if QT_CONFIG(wheelevent)
+//! [5]
+void GraphWidget::wheelEvent(QWheelEvent *event)
+{
+    scaleView(pow(2., -event->angleDelta().y() / 240.0));
+}
+//! [5]
+#endif
+
+//! [6]
+void GraphWidget::drawBackground(QPainter *painter, const QRectF &rect)
+{
+    Q_UNUSED(rect);
+
+    // Shadow
+    QRectF sceneRect = this->sceneRect();
+    QRectF rightShadow(sceneRect.right(), sceneRect.top() + 5, 5, sceneRect.height());
+    QRectF bottomShadow(sceneRect.left() + 5, sceneRect.bottom(), sceneRect.width(), 5);
+    if (rightShadow.intersects(rect) || rightShadow.contains(rect))
+        painter->fillRect(rightShadow, Qt::darkGray);
+    if (bottomShadow.intersects(rect) || bottomShadow.contains(rect))
+        painter->fillRect(bottomShadow, Qt::darkGray);
+
+    // Fill
+    QLinearGradient gradient(sceneRect.topLeft(), sceneRect.bottomRight());
+    gradient.setColorAt(0, Qt::white);
+    gradient.setColorAt(1, Qt::lightGray);
+    painter->fillRect(rect.intersected(sceneRect), gradient);
+    painter->setBrush(Qt::NoBrush);
+    painter->drawRect(sceneRect);
+
+    // Text
+    QRectF textRect(sceneRect.left() + 4, sceneRect.top() + 4,
+                    sceneRect.width() - 4, sceneRect.height() - 4);
+    QString message(tr("Click and drag the nodes around, and zoom with the mouse "
+                       "wheel or the '+' and '-' keys"));
+
+    QFont font = painter->font();
+    font.setBold(true);
+    font.setPointSize(14);
+    painter->setFont(font);
+    painter->setPen(Qt::lightGray);
+    painter->drawText(textRect.translated(2, 2), message);
+    painter->setPen(Qt::black);
+    painter->drawText(textRect, message);
+}
+//! [6]
+
+//! [7]
+void GraphWidget::scaleView(qreal scaleFactor)
+{
+    qreal factor = transform().scale(scaleFactor, scaleFactor).mapRect(QRectF(0, 0, 1, 1)).width();
+    if (factor < 0.07 || factor > 100)
+        return;
+
+    scale(scaleFactor, scaleFactor);
+}
+//! [7]
+
+void GraphWidget::shuffle()
+{
+    const QList<QGraphicsItem *> items = scene()->items();
+    for (QGraphicsItem *item : items) {
+        if (qgraphicsitem_cast<Node *>(item))
+            item->setPos(-150 + QRandomGenerator::global()->bounded(300), -150 + QRandomGenerator::global()->bounded(300));
+    }
+}
+
+void GraphWidget::zoomIn()
+{
+    scaleView(qreal(1.2));
+}
+
+void GraphWidget::zoomOut()
+{
+    scaleView(1 / qreal(1.2));
+}

src/app/GraphWidget.h

@@ -0,0 +1,39 @@
+#ifndef GRAPHWIDGET_H
+#define GRAPHWIDGET_H
+
+#include <QGraphicsView>
+
+class Node;
+
+//! [0]
+class GraphWidget : public QGraphicsView
+{
+    Q_OBJECT
+
+public:
+    GraphWidget(QWidget *parent = nullptr);
+
+    void itemMoved();
+
+public slots:
+    void shuffle();
+    void zoomIn();
+    void zoomOut();
+
+protected:
+    void keyPressEvent(QKeyEvent *event) override;
+    void timerEvent(QTimerEvent *event) override;
+#if QT_CONFIG(wheelevent)
+    void wheelEvent(QWheelEvent *event) override;
+#endif
+    void drawBackground(QPainter *painter, const QRectF &rect) override;
+
+    void scaleView(qreal scaleFactor);
+
+private:
+    int timerId = 0;
+    Node *centerNode;
+};
+//! [0]
+
+#endif // GRAPHWIDGET_H

src/app/MainWindow.cpp

@@ -1,11 +1,6 @@
 #include "MainWindow.h"
 #include "ui_MainWindow.h"
 
-//#include "viewerfactory.h"
-//#include "abstractviewer.h"
-//#include "recentfiles.h"
-//#include "recentfilemenu.h"
-
 #include <QFileDialog>
 #include <QToolButton>
 #include <QMessageBox>
@@ -13,16 +8,27 @@
 #include <QDir>
 #include <QSettings>
 
+//#include "viewerfactory.h"
+//#include "abstractviewer.h"
+//#include "recentfiles.h"
+//#include "recentfilemenu.h"
+
+#include "ScribbleArea.h"
+#include "GraphWidget.h"
+
+
 using namespace Qt::StringLiterals;
 
 MainWindow::MainWindow(QWidget *parent) :
     QMainWindow(parent),
-    ui(new Ui::MainWindow)
+    //ui(new Ui::MainWindow),
+    m_scribble_area(new ScribbleArea),
+    m_graph_widget(new GraphWidget)
 {
-    ui->setupUi(this);
-    connect(ui->actionOpen, &QAction::triggered, this, &MainWindow::onActionOpenTriggered);
-    connect(ui->actionAbout, &QAction::triggered, this, &MainWindow::onActionAboutTriggered);
-    connect(ui->actionAboutQt, &QAction::triggered, this, &MainWindow::onActionAboutQtTriggered);
+    setCentralWidget(m_graph_widget.get());
+
+    setWindowTitle(tr("Scribbles"));
+    //resize(500, 500);
 
     /*
     m_recentFiles.reset(new RecentFiles(ui->actionRecent));

src/app/MainWindow.h

@@ -1,5 +1,4 @@
-#ifndef MAINWINDOW_H
-#define MAINWINDOW_H
+#pragma once
 
 #include <QMainWindow>
 #include <QDir>
@@ -9,11 +8,12 @@ class AbstractViewer;
 class RecentFiles;
 class ViewerFactory;
 
-QT_BEGIN_NAMESPACE
 namespace Ui {
 class MainWindow;
 }
-QT_END_NAMESPACE
+
+class ScribbleArea;
+class GraphWidget;
 
 class MainWindow : public QMainWindow
 {
@@ -38,9 +38,12 @@ private:
     void resetViewer() const;
     void saveViewerSettings() const;
 
+    std::unique_ptr<ScribbleArea> m_scribble_area;
+    std::unique_ptr<GraphWidget> m_graph_widget;
+
     QDir m_currentDir;
     AbstractViewer *m_viewer = nullptr;
-    std::unique_ptr<Ui::MainWindow> ui;
+    //std::unique_ptr<Ui::MainWindow> ui;
     //std::unique_ptr<RecentFiles> m_recentFiles;
     //std::unique_ptr<ViewerFactory> m_factory;
 
@@ -49,5 +52,3 @@ private:
     static constexpr QLatin1StringView settingsViewers = QLatin1StringView("Viewers");
     static constexpr QLatin1StringView settingsFiles = QLatin1StringView("RecentFiles");
 };
-
-#endif // MAINWINDOW_H

src/app/MainWindow.ui

@@ -11,7 +11,7 @@
    </rect>
   </property>
   <property name="windowTitle">
-   <string>Document Viewer Demo</string>
+   <string>Scribbles</string>
   </property>
   <property name="windowIcon">
    <iconset resource="documentviewer.qrc">

src/app/Node.cpp

@@ -0,0 +1,172 @@
+#include "Edge.h"
+#include "Node.h"
+#include "GraphWidget.h"
+
+#include <QGraphicsScene>
+#include <QGraphicsSceneMouseEvent>
+#include <QPainter>
+#include <QStyleOption>
+
+//! [0]
+Node::Node(GraphWidget *graphWidget)
+    : graph(graphWidget)
+{
+    setFlag(ItemIsMovable);
+    setFlag(ItemSendsGeometryChanges);
+    setCacheMode(DeviceCoordinateCache);
+    setZValue(-1);
+}
+//! [0]
+
+//! [1]
+void Node::addEdge(Edge *edge)
+{
+    edgeList << edge;
+    edge->adjust();
+}
+
+QList<Edge *> Node::edges() const
+{
+    return edgeList;
+}
+//! [1]
+
+//! [2]
+void Node::calculateForces()
+{
+    if (!scene() || scene()->mouseGrabberItem() == this) {
+        newPos = pos();
+        return;
+    }
+//! [2]
+
+//! [3]
+    // Sum up all forces pushing this item away
+    qreal xvel = 0;
+    qreal yvel = 0;
+    const QList<QGraphicsItem *> items = scene()->items();
+    for (QGraphicsItem *item : items) {
+        Node *node = qgraphicsitem_cast<Node *>(item);
+        if (!node)
+            continue;
+
+        QPointF vec = mapToItem(node, 0, 0);
+        qreal dx = vec.x();
+        qreal dy = vec.y();
+        double l = 2.0 * (dx * dx + dy * dy);
+        if (l > 0) {
+            xvel += (dx * 150.0) / l;
+            yvel += (dy * 150.0) / l;
+        }
+    }
+//! [3]
+
+//! [4]
+    // Now subtract all forces pulling items together
+    double weight = (edgeList.size() + 1) * 10;
+    for (const Edge *edge : std::as_const(edgeList)) {
+        QPointF vec;
+        if (edge->sourceNode() == this)
+            vec = mapToItem(edge->destNode(), 0, 0);
+        else
+            vec = mapToItem(edge->sourceNode(), 0, 0);
+        xvel -= vec.x() / weight;
+        yvel -= vec.y() / weight;
+    }
+//! [4]
+
+//! [5]
+    if (qAbs(xvel) < 0.1 && qAbs(yvel) < 0.1)
+        xvel = yvel = 0;
+//! [5]
+
+//! [6]
+    QRectF sceneRect = scene()->sceneRect();
+    newPos = pos() + QPointF(xvel, yvel);
+    newPos.setX(qMin(qMax(newPos.x(), sceneRect.left() + 10), sceneRect.right() - 10));
+    newPos.setY(qMin(qMax(newPos.y(), sceneRect.top() + 10), sceneRect.bottom() - 10));
+}
+//! [6]
+
+//! [7]
+bool Node::advancePosition()
+{
+    if (newPos == pos())
+        return false;
+
+    setPos(newPos);
+    return true;
+}
+//! [7]
+
+//! [8]
+QRectF Node::boundingRect() const
+{
+    qreal adjust = 2;
+    return QRectF( -10 - adjust, -10 - adjust, 23 + adjust, 23 + adjust);
+}
+//! [8]
+
+//! [9]
+QPainterPath Node::shape() const
+{
+    QPainterPath path;
+    path.addEllipse(-10, -10, 20, 20);
+    return path;
+}
+//! [9]
+
+//! [10]
+void Node::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *)
+{
+    painter->setPen(Qt::NoPen);
+    painter->setBrush(Qt::darkGray);
+    painter->drawEllipse(-7, -7, 20, 20);
+
+    QRadialGradient gradient(-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(gradient);
+
+    painter->setPen(QPen(Qt::black, 0));
+    painter->drawEllipse(-10, -10, 20, 20);
+}
+//! [10]
+
+//! [11]
+QVariant Node::itemChange(GraphicsItemChange change, const QVariant &value)
+{
+    switch (change) {
+    case ItemPositionHasChanged:
+        for (Edge *edge : std::as_const(edgeList))
+            edge->adjust();
+        graph->itemMoved();
+        break;
+    default:
+        break;
+    };
+
+    return QGraphicsItem::itemChange(change, value);
+}
+//! [11]
+
+//! [12]
+void Node::mousePressEvent(QGraphicsSceneMouseEvent *event)
+{
+    update();
+    QGraphicsItem::mousePressEvent(event);
+}
+
+void Node::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
+{
+    update();
+    QGraphicsItem::mouseReleaseEvent(event);
+}
+//! [12]

src/app/Node.h

@@ -0,0 +1,42 @@
+#ifndef NODE_H
+#define NODE_H
+
+#include <QGraphicsItem>
+#include <QList>
+
+class Edge;
+class GraphWidget;
+
+//! [0]
+class Node : public QGraphicsItem
+{
+public:
+    Node(GraphWidget *graphWidget);
+
+    void addEdge(Edge *edge);
+    QList<Edge *> edges() const;
+
+    enum { Type = UserType + 1 };
+    int type() const override { return Type; }
+
+    void calculateForces();
+    bool advancePosition();
+
+    QRectF boundingRect() const override;
+    QPainterPath shape() const override;
+    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override;
+
+protected:
+    QVariant itemChange(GraphicsItemChange change, const QVariant &value) override;
+
+    void mousePressEvent(QGraphicsSceneMouseEvent *event) override;
+    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event) override;
+
+private:
+    QList<Edge *> edgeList;
+    QPointF newPos;
+    GraphWidget *graph;
+};
+//! [0]
+
+#endif // NODE_H

src/app/ScribbleArea.cpp

@@ -0,0 +1,178 @@
+#include "ScribbleArea.h"
+
+#include <QMouseEvent>
+#include <QPainter>
+
+#if defined(QT_PRINTSUPPORT_LIB)
+#include <QtPrintSupport/qtprintsupportglobal.h>
+#if QT_CONFIG(printdialog)
+#include <QPrinter>
+#include <QPrintDialog>
+#endif
+#endif
+
+//! [0]
+ScribbleArea::ScribbleArea(QWidget *parent)
+    : QWidget(parent)
+{
+    setAttribute(Qt::WA_StaticContents);
+}
+//! [0]
+
+//! [1]
+bool ScribbleArea::openImage(const QString &fileName)
+//! [1] //! [2]
+{
+    QImage loadedImage;
+    if (!loadedImage.load(fileName))
+        return false;
+
+    QSize newSize = loadedImage.size().expandedTo(size());
+    resizeImage(&loadedImage, newSize);
+    image = loadedImage;
+    modified = false;
+    update();
+    return true;
+}
+//! [2]
+
+//! [3]
+bool ScribbleArea::saveImage(const QString &fileName, const char *fileFormat)
+//! [3] //! [4]
+{
+    QImage visibleImage = image;
+    resizeImage(&visibleImage, size());
+
+    if (visibleImage.save(fileName, fileFormat)) {
+        modified = false;
+        return true;
+    }
+    return false;
+}
+//! [4]
+
+//! [5]
+void ScribbleArea::setPenColor(const QColor &newColor)
+//! [5] //! [6]
+{
+    myPenColor = newColor;
+}
+//! [6]
+
+//! [7]
+void ScribbleArea::setPenWidth(int newWidth)
+//! [7] //! [8]
+{
+    myPenWidth = newWidth;
+}
+//! [8]
+
+//! [9]
+void ScribbleArea::clearImage()
+//! [9] //! [10]
+{
+    image.fill(qRgb(255, 255, 255));
+    modified = true;
+    update();
+}
+//! [10]
+
+//! [11]
+void ScribbleArea::mousePressEvent(QMouseEvent *event)
+//! [11] //! [12]
+{
+    if (event->button() == Qt::LeftButton) {
+        lastPoint = event->position().toPoint();
+        scribbling = true;
+    }
+}
+
+void ScribbleArea::mouseMoveEvent(QMouseEvent *event)
+{
+    if ((event->buttons() & Qt::LeftButton) && scribbling)
+        drawLineTo(event->position().toPoint());
+}
+
+void ScribbleArea::mouseReleaseEvent(QMouseEvent *event)
+{
+    if (event->button() == Qt::LeftButton && scribbling) {
+        drawLineTo(event->position().toPoint());
+        scribbling = false;
+    }
+}
+
+//! [12] //! [13]
+void ScribbleArea::paintEvent(QPaintEvent *event)
+//! [13] //! [14]
+{
+    QPainter painter(this);
+    QRect dirtyRect = event->rect();
+    painter.drawImage(dirtyRect, image, dirtyRect);
+}
+//! [14]
+
+//! [15]
+void ScribbleArea::resizeEvent(QResizeEvent *event)
+//! [15] //! [16]
+{
+    if (width() > image.width() || height() > image.height()) {
+        int newWidth = qMax(width() + 128, image.width());
+        int newHeight = qMax(height() + 128, image.height());
+        resizeImage(&image, QSize(newWidth, newHeight));
+        update();
+    }
+    QWidget::resizeEvent(event);
+}
+//! [16]
+
+//! [17]
+void ScribbleArea::drawLineTo(const QPoint &endPoint)
+//! [17] //! [18]
+{
+    QPainter painter(&image);
+    painter.setPen(QPen(myPenColor, myPenWidth, Qt::SolidLine, Qt::RoundCap,
+                        Qt::RoundJoin));
+    painter.drawLine(lastPoint, endPoint);
+    modified = true;
+
+    int rad = (myPenWidth / 2) + 2;
+    update(QRect(lastPoint, endPoint).normalized()
+                                     .adjusted(-rad, -rad, +rad, +rad));
+    lastPoint = endPoint;
+}
+//! [18]
+
+//! [19]
+void ScribbleArea::resizeImage(QImage *image, const QSize &newSize)
+//! [19] //! [20]
+{
+    if (image->size() == newSize)
+        return;
+
+    QImage newImage(newSize, QImage::Format_RGB32);
+    newImage.fill(qRgb(255, 255, 255));
+    QPainter painter(&newImage);
+    painter.drawImage(QPoint(0, 0), *image);
+    *image = newImage;
+}
+//! [20]
+
+//! [21]
+void ScribbleArea::print()
+{
+#if defined(QT_PRINTSUPPORT_LIB) && QT_CONFIG(printdialog)
+    QPrinter printer(QPrinter::HighResolution);
+
+    QPrintDialog printDialog(&printer, this);
+//! [21] //! [22]
+    if (printDialog.exec() == QDialog::Accepted) {
+        QPainter painter(&printer);
+        QRect rect = painter.viewport();
+        QSize size = image.size();
+        size.scale(rect.size(), Qt::KeepAspectRatio);
+        painter.setViewport(rect.x(), rect.y(), size.width(), size.height());
+        painter.setWindow(image.rect());
+        painter.drawImage(0, 0, image);
+    }
+#endif // QT_CONFIG(printdialog)
+}

src/app/ScribbleArea.h

@@ -0,0 +1,50 @@
+#ifndef SCRIBBLEAREA_H
+#define SCRIBBLEAREA_H
+
+#include <QColor>
+#include <QImage>
+#include <QPoint>
+#include <QWidget>
+
+//! [0]
+class ScribbleArea : public QWidget
+{
+    Q_OBJECT
+
+public:
+    ScribbleArea(QWidget *parent = nullptr);
+
+    bool openImage(const QString &fileName);
+    bool saveImage(const QString &fileName, const char *fileFormat);
+    void setPenColor(const QColor &newColor);
+    void setPenWidth(int newWidth);
+
+    bool isModified() const { return modified; }
+    QColor penColor() const { return myPenColor; }
+    int penWidth() const { return myPenWidth; }
+
+public slots:
+    void clearImage();
+    void print();
+
+protected:
+    void mousePressEvent(QMouseEvent *event) override;
+    void mouseMoveEvent(QMouseEvent *event) override;
+    void mouseReleaseEvent(QMouseEvent *event) override;
+    void paintEvent(QPaintEvent *event) override;
+    void resizeEvent(QResizeEvent *event) override;
+
+private:
+    void drawLineTo(const QPoint &endPoint);
+    void resizeImage(QImage *image, const QSize &newSize);
+
+    bool modified = false;
+    bool scribbling = false;
+    int myPenWidth = 1;
+    QColor myPenColor = Qt::blue;
+    QImage image;
+    QPoint lastPoint;
+};
+//! [0]
+
+#endif