在QT場景視圖中2D圖形項Item的基類為QGraphicsItem，如果我們需要自定義Item則可以從其派生，然后重寫boundingRect以及paint虛函數實現圖形項的外邊界定義以及內容繪制工作。如果需要將Qt基本的Widget組件加入到場景中，該框架為我們提供了QGraphicsWidget(QGraphicsProxyWidget)類，如果需要圖形項具有信號槽的功能，該框架又提供了QGraphicsObject類，方便我們根據需要選擇相關的類。
QgraphicsItem類提供了簡單方便的setRotation方法傳入旋轉角度（-360，360）來直接控制圖形項繞Z軸（垂直於屏幕的軸）的順時針以及逆時針旋轉，也可以通過更加綜合性的QTransform類來實現。旋轉的中心點則可以通過setTransformOriginPoint()方法來設定，默認是(0,0)坐標原點。
需要明白的是，對圖形項的旋轉實質上是對項坐標系的旋轉，圖形繪制的坐標相對於項坐標是不變的。
調用setRotation方法固然方便，但需要手動指定旋轉的角度值，而實際的情況中，我們常常需要通過鼠標選中並移動來實現旋轉，這就需要通過鼠標移動來不斷計算正確的旋轉角度值，這是重點，也是難點。
思路：
1) 確定旋轉中心坐標：centerPoint（O）
2) 在mousePressEvent事件響應中獲取按下時旋轉控制點坐標：pressPoint（A）
3) 在mouseMoveEvent事件響應中獲取當前移動點的坐標：movePoint（B）
4) centerPoint、pressPoint、movePoint三點確定的旋轉角度angle：角AOB
5) 在原始旋轉角度rotation()基礎上設置新的累積角度
setRotation(rotation() + angle);
難點：如何計算角AOB
inline qreal GetDegreeAngle(QVector2D vector2d) const
{
return fmod((atan2((qreal)vector2d.y(), (qreal)vector2d.x()) * AnglePerPI + 360.0), 360.0 );
}
C 語言里 double atan2(double y,double x) 返回的是原點至點(x,y)的方位角，即與 x 軸的夾角。返回值的單位為弧度，取值范圍為（-π, π]。結果為正表示從 X 軸逆時針旋轉的角度，結果為負表示從 X 軸順時針旋轉的角度。若要用度表示反正切值，請將結果再乘以 180/π。fmod是浮點數求余數運算，函數整體返回向量與+x軸的順時針角度值。
測試代碼：
僅供參考，如有錯誤，歡迎指正，歡迎改進幫助更加完善。
C++ Code
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#ifndef
ITEMBASE_H #define ITEMBASE_H #include <QGraphicsItem> #include <QTime> #include <QVector2D> #include <cmath> const qreal PI = 3 . 141592653 ; const qreal AnglePerPI = 180 . 0 / PI; class QGraphicsScene; class ItemBase : public QGraphicsItem { public : enum { Type = UserType + 1 }; ItemBase( int size, int x, int y); ~ItemBase() override; QRectF boundingRect() const override; void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override; inline qreal GetDegreeAngle(QVector2D vector2d) const { return fmod((atan2((qreal)vector2d.y(), (qreal)vector2d.x()) * AnglePerPI + 360 . 0 ), 360 . 0 ); } protected : virtual ItemBase *createNew( int size, int x, int y) = 0 ; void contextMenuEvent(QGraphicsSceneContextMenuEvent *event) override; void mouseMoveEvent(QGraphicsSceneMouseEvent *event) override; void hoverMoveEvent(QGraphicsSceneHoverEvent *event) override; void mousePressEvent(QGraphicsSceneMouseEvent *event) override; void mouseReleaseEvent(QGraphicsSceneMouseEvent *event) override; void keyPressEvent(QKeyEvent *event) override; void wheelEvent(QGraphicsSceneWheelEvent *event) override; int type() const override; bool isInResizeArea( const QPointF &pos); bool isInRotateArea( const QPointF &pos); static void duplicateSelectedItems(QGraphicsScene *scene); static void deleteSelectedItems(QGraphicsScene *scene); static void growSelectedItems(QGraphicsScene *scene); static void shrinkSelectedItems(QGraphicsScene *scene); int m_size; QTime m_startTime; bool m_isResizing; bool m_isRotating; }; #endif // ITEMBASE_H #include "ItemBase.h" #include <QStyleOptionGraphicsItem> #include <QPainter> #include <QGraphicsScene> #include <QGraphicsEllipseItem> #include <QMenu> #include <QGraphicsSceneMouseEvent> #include <QKeyEvent> #include <qmath.h> #include <GL/gl.h> #include <QDebug> #include <QRandomGenerator> const int MAX_ITEM_SIZE = 512 ; const int MIN_ITEM_SIZE = 16 ; //============================================================================// // ItemBase // //============================================================================// ItemBase::ItemBase( int size, int x, int y) : m_size(size) , m_isResizing( false ) , m_isRotating( false ) { setFlag(QGraphicsItem::ItemIsMovable, true ); setFlag(QGraphicsItem::ItemIsSelectable, true ); setFlag(QGraphicsItem::ItemIsFocusable, true ); setFlag(QGraphicsItem::ItemSendsGeometryChanges, true ); setAcceptHoverEvents( true ); setPos(x, y); m_startTime = QTime::currentTime(); } ItemBase::~ItemBase() { } QRectF ItemBase::boundingRect() const { return QRectF(-m_size / 2 , -m_size / 2 , m_size, m_size); } void ItemBase::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *) { if (option->state & QStyle::State_Selected) { painter->setRenderHint(QPainter::Antialiasing, true ); if (option->state & QStyle::State_HasFocus) { painter->setPen(Qt::yellow); } else { painter->setPen(Qt::white); } painter->drawRect(boundingRect()); // resize contrl area... painter->drawLine(m_size / 2 - 9 , m_size / 2 , m_size / 2 , m_size / 2 - 9 ); painter->drawLine(m_size / 2 - 6 , m_size / 2 , m_size / 2 , m_size / 2 - 6 ); painter->drawLine(m_size / 2 - 3 , m_size / 2 , m_size / 2 , m_size / 2 - 3 ); // rotate control point painter->setPen(Qt::blue); painter->drawLine( 0 , m_size / 2 , 0 , -m_size / 2 ); painter->setPen(QPen(Qt::blue, 5 )); painter->drawPoint( 0 , -m_size / 2 ); painter->setRenderHint(QPainter::Antialiasing, false ); } else { } } void ItemBase::contextMenuEvent(QGraphicsSceneContextMenuEvent *event) { if (!isSelected() && scene()) { scene()->clearSelection(); setSelected( true ); } QMenu menu; QAction *delAction = menu.addAction( "Delete" ); QAction *newAction = menu.addAction( "New" ); QAction *growAction = menu.addAction( "Grow" ); QAction *shrinkAction = menu.addAction( "Shrink" ); QAction *selectedAction = menu.exec(event->screenPos()); if (selectedAction == delAction) deleteSelectedItems(scene()); else if (selectedAction == newAction) duplicateSelectedItems(scene()); else if (selectedAction == growAction) growSelectedItems(scene()); else if (selectedAction == shrinkAction) shrinkSelectedItems(scene()); } void ItemBase::duplicateSelectedItems(QGraphicsScene *scene) { if (!scene) return ; QList<QGraphicsItem *> selected; selected = scene->selectedItems(); foreach (QGraphicsItem *item, selected) { ItemBase *itemBase = qgraphicsitem_cast<ItemBase *>(item); if (itemBase) scene->addItem(itemBase->createNew(itemBase->m_size, static_cast < int >(itemBase->pos().x()) + itemBase->m_size, static_cast < int >(itemBase->pos().y()))); } } void ItemBase::deleteSelectedItems(QGraphicsScene *scene) { if (!scene) return ; QList<QGraphicsItem *> selected; selected = scene->selectedItems(); foreach (QGraphicsItem *item, selected) { ItemBase *itemBase = qgraphicsitem_cast<ItemBase *>(item); if (itemBase) delete itemBase; } } void ItemBase::growSelectedItems(QGraphicsScene *scene) { if (!scene) return ; QList<QGraphicsItem *> selected; selected = scene->selectedItems(); foreach (QGraphicsItem *item, selected) { ItemBase *itemBase = qgraphicsitem_cast<ItemBase *>(item); if (itemBase) { itemBase->prepareGeometryChange(); itemBase->m_size *= 2 ; if (itemBase->m_size > MAX_ITEM_SIZE) itemBase->m_size = MAX_ITEM_SIZE; } } } void ItemBase::shrinkSelectedItems(QGraphicsScene *scene) { if (!scene) return ; QList<QGraphicsItem *> selected; selected = scene->selectedItems(); foreach (QGraphicsItem *item, selected) { ItemBase *itemBase = qgraphicsitem_cast<ItemBase *>(item); if (itemBase) { itemBase->prepareGeometryChange(); itemBase->m_size /= 2 ; if (itemBase->m_size < MIN_ITEM_SIZE) itemBase->m_size = MIN_ITEM_SIZE; } } } void ItemBase::mouseMoveEvent(QGraphicsSceneMouseEvent *event) { if (m_isResizing) { int dx = int ( 2 . 0 * event->pos().x()); int dy = int ( 2 . 0 * event->pos().y()); prepareGeometryChange(); m_size = (dx > dy ? dx : dy); if (m_size < MIN_ITEM_SIZE) m_size = MIN_ITEM_SIZE; else if (m_size > MAX_ITEM_SIZE) m_size = MAX_ITEM_SIZE; } else if (m_isRotating) { QPointF cursorPos = event->pos(); QVector2D vectorStart = QVector2D(QPointF( 0 . 0 , -m_size / 2 ) - QPointF( 0 . 0 , 0 . 0 )); // 起始向量 QVector2D vectorEnd = QVector2D(cursorPos - QPointF( 0 . 0 , 0 . 0 )); // 結束向量 // 計算起始向量和結束向量之間的角度 qreal angle = 0 . 0 ; qreal angleEnd = GetDegreeAngle(vectorEnd); qreal angleStart = GetDegreeAngle(vectorStart); angle = angleEnd - angleStart + rotation(); if (angle > 360 . 0 ) { while ( 1 ) { angle -= 360 . 0 ; if (angle < 360 . 0 ) break ; } } else if (angle < 0 . 0 ) { while ( 1 ) { angle += 360 . 0 ; if (angle > 0 . 0 ) break ; } } setRotation(angle); } else { QGraphicsItem::mouseMoveEvent(event); } } void ItemBase::hoverMoveEvent(QGraphicsSceneHoverEvent *event) { if (m_isResizing || (isInResizeArea(event->pos()) && isSelected())) setCursor(Qt::SizeFDiagCursor); else if ((isInRotateArea(event->pos()) && isSelected())) { QCursor *rotateCursor = new QCursor(QPixmap( ":/rotate_handler.png" )); setCursor(*rotateCursor); } else setCursor(Qt::ArrowCursor); QGraphicsItem::hoverMoveEvent(event); } void ItemBase::mousePressEvent(QGraphicsSceneMouseEvent *event) { static qreal z = 0 . 0 ; setZValue(z += 1 . 0 ); if (event->button() == Qt::LeftButton && isInResizeArea(event->pos())) { m_isResizing = true ; } else if (event->button() == Qt::LeftButton && isInRotateArea(event->pos())) { m_isRotating = true ; } else { QGraphicsItem::mousePressEvent(event); } } void ItemBase::mouseReleaseEvent(QGraphicsSceneMouseEvent *event) { if (event->button() == Qt::LeftButton && m_isResizing) { m_isResizing = false ; } else if (event->button() == Qt::LeftButton && m_isRotating) { m_isRotating = false ; } else { QGraphicsItem::mouseReleaseEvent(event); } } void ItemBase::keyPressEvent(QKeyEvent *event) { switch (event->key()) { case Qt::Key_Delete: deleteSelectedItems(scene()); break ; case Qt::Key_Insert: duplicateSelectedItems(scene()); break ; case Qt::Key_Plus: growSelectedItems(scene()); break ; case Qt::Key_Minus: shrinkSelectedItems(scene()); break ; default : QGraphicsItem::keyPressEvent(event); break ; } } void ItemBase::wheelEvent(QGraphicsSceneWheelEvent *event) { prepareGeometryChange(); m_size = int (m_size * qExp(-event->delta() / 600 . 0 )); if (m_size > MAX_ITEM_SIZE) m_size = MAX_ITEM_SIZE; else if (m_size < MIN_ITEM_SIZE) m_size = MIN_ITEM_SIZE; } int ItemBase::type() const { return Type; } bool ItemBase::isInResizeArea( const QPointF &pos) { return (-pos.y() < pos.x() - m_size + 9 ); } bool ItemBase::isInRotateArea( const QPointF &pos) { if ( - 5 <= pos.x() && pos.x() <= 5 && -m_size / 2 <= pos.y() && pos.y() <= -m_size / 2 + 5 ) { return true ; } else { return false ; } } #ifndef MYITEM_H #define MYITEM_H #include "ItemBase.h" class MyItem : public ItemBase { public : MyItem( int size, int x, int y); virtual ~MyItem() override; void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override; protected : ItemBase *createNew( int size, int x, int y) override; }; #endif // MYITEM_H #include "MyItem.h" #include <QPainter> MyItem::MyItem( int size, int x, int y) : ItemBase (size, x, y) { } MyItem::~MyItem() { } void MyItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) { painter->setRenderHint(QPainter::Antialiasing, true ); QVector<QLine> vec; QLine l1(-m_size * 0 . 4 , m_size * 0 . 4 , m_size * 0 . 4 , m_size * 0 . 4 ); QLine l2(-m_size * 0 . 4 , m_size * 0 . 2 , m_size * 0 . 4 , m_size * 0 . 2 ); QLine l3(-m_size * 0 . 4 , -m_size * 0 . 2 , m_size * 0 . 4 , -m_size * 0 . 2 ); QLine l4(-m_size * 0 . 4 , -m_size * 0 . 4 , m_size * 0 . 4 , -m_size * 0 . 4 ); vec.push_back(l1); vec.push_back(l2); vec.push_back(l3); vec.push_back(l4); painter->drawLines(vec); ItemBase::paint(painter, option, widget); } ItemBase *MyItem::createNew( int size, int x, int y) { return new MyItem(size, x, y); } #ifndef MAINWINDOW_H #define MAINWINDOW_H #include <QMainWindow> class QGraphicsView; class QGraphicsScene; class MainWindow : public QMainWindow { Q_OBJECT public : MainWindow(QWidget *parent = nullptr); ~MainWindow(); private : QGraphicsView *m_view; QGraphicsScene *m_scene; }; #endif // MAINWINDOW_H #include "MainWindow.h" #include <QGraphicsView> #include <QGraphicsScene> #include <QRandomGenerator> #include "MyItem.h" MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent) { m_scene = new QGraphicsScene( this ); m_scene->setSceneRect( 0 . 0 , 0 . 0 , 1024 . 0 , 768 . 0 ); m_view = new QGraphicsView( this ); m_view->setViewportUpdateMode(QGraphicsView::FullViewportUpdate); m_view->setScene(m_scene); setCentralWidget(m_view); QSize size = m_scene->sceneRect().size().toSize(); m_scene->addItem( new MyItem( 64 , QRandomGenerator::global()->bounded(size.width() - 64 ) + 32 , QRandomGenerator::global()->bounded(size.height() - 64 ) + 32 )); } MainWindow::~MainWindow() { } |
測試運行：
