Qt图形视图框架：QGraphicsItem详解

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Chapter1 Qt图形视图框架：QGraphicsItem详解

原文链接：https://blog.csdn.net/kenfan1647/article/details/116991074

Chapter2 自定义QGraphicsItem实现平移、改变尺寸和旋转

原文链接：https://blog.csdn.net/douzhq/article/details/105017924

我们在使用QGraphicsView框架的时候，经常需要自定义QGraphicsItem，并且需要实现Item的平移、改变大小和旋转的效果。接下来介绍他们的一种实现方式

1. 平移

平移效果如下图所示：

实现方式有两种方法：

使用QGraphicsItem本身的移动标志实现。

this->setFlag(QGraphicsItem::ItemIsMovable, true);

通过重写鼠标的相关事件实现。
这里需要重写下面三个函数：

void mousePressEvent(QGraphicsSceneMouseEvent *event) override;
void mouseMoveEvent(QGraphicsSceneMouseEvent *event) override;
void mouseReleaseEvent(QGraphicsSceneMouseEvent *event) override;

这里只贴出关键部分实现代码：

void mousePressEvent(QGraphicsSceneMouseEvent *event)
{    // 获取场景坐标和本地坐标QPointF scenePos = event->scenePos();QPointF pos = event->pos();// 保存当前的一些信息m_pos = pos;m_pressedPos = scenePos;m_startPos = this->pos();return QGraphicsItem::mousePressEvent(event);
}void mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{// 获取场景坐标和本地坐标QPointF scenePos = event->scenePos();QPointF pos = event->pos();// 计算偏移qreal xInterval = scenePos.x() - m_pressedPos.x();qreal yInterval = scenePos.y() - m_pressedPos.y();// 设置在场景中位置this->setPos(m_startPos + QPointF(xInterval, yInterval));this->update();
}

这里 mousePressEvent 中保存了鼠标点击时的状态信息，包括鼠标点击时Item的本地坐标，场景坐标和该Item所在场景的坐标。 函数 mouseMoveEvent 中，获取鼠标移动的场景坐标位置计算偏移并设置新的Item的位置，从而实现平移效果。

2. 改变尺寸

改变尺寸效果如下图所示：

这里同样时通过重写 mousePressEvent 、 mouseMoveEvent 和 mouseReleaseEvent 实现。

关键部分代码如下：

void mousePressEvent(QGraphicsSceneMouseEvent *event)
{    // 获取场景坐标和本地坐标QPointF scenePos = event->scenePos();QPointF pos = event->pos();// 保存当前的一些信息m_pos = pos;m_pressedPos = scenePos;m_startPos = this->pos();return QGraphicsItem::mousePressEvent(event);
}void UICanvasItemBase::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{// 获取场景坐标和本地坐标QPointF scenePos = event->scenePos();QPointF loacalPos = event->pos();// 是否允许改变大小if (!m_isResizeable)return;// 是否为等比例修改大小qreal ratio = m_ratioValue;qreal itemWidth = abs(loacalPos.x()) * 2 - m_nInterval - m_nEllipseWidth;qreal itemHeight = abs(loacalPos.y()) * 2 - m_nInterval - m_nEllipseWidth;if (m_isRatioScale)itemHeight = itemWidth * 1.0 / ratio;// 设置图片的最小大小为10if (itemWidth < 10 || itemHeight < 10)return;m_size = QSize(itemWidth, itemHeight);this->update();
}

因为我这里的绘制的大小主要是通过 m_size ，改变 m_size 就是更改了 QGraphicsItem 的显示尺寸。本例子中的坐标系的中心点就是 m_size 的中心点。因此 itemWidth 的计算值为表示为：
qreal itemWidth = abs(loacalPos.x()) * 2 - m_nInterval - m_nEllipseWidth;
loacalPos.x() 为本地坐标系的 x 轴坐标， *2 正好为实际的宽度，这里的 m_nInterval 和 m_nEllipseWidth 表示图片和选择框之间的间距和拖拽手柄的半径。

3. 旋转

旋转效果如下图所示：

本篇文章讲述的旋转方法步骤如下：

1. 计算上一次鼠标移动和本次鼠标移动位置之间的角度。
2. 计算旋转的方向。
3. 根据计算的角度和方向，计算真正的选中角度（顺时针为正，逆时针为负），为 QGraphicsItem 本身设置变换矩阵。
   那么如何计算角度和方向呢？？

• 通过向量的 点乘 ，计算角度。单位向量点乘的值，正好为角度的余弦。
• 通过向量的 叉乘 ，计算旋转的方向。叉乘的结果为与这两个向量垂直的向量，可以通过Z轴结果判断，如果结果为正表示顺时针，结果为负表示逆时针。
  关键部分代码如下：

void mousePressEvent(QGraphicsSceneMouseEvent *event)
{    m_transform = this->transform();// 获取场景坐标和本地坐标QPointF scenePos = event->scenePos();QPointF pos = event->pos();// 保存当前的一些信息m_pos = pos;m_pressedPos = scenePos;m_startPos = this->pos();return QGraphicsItem::mousePressEvent(event);
}void UICanvasItemBase::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{// 获取场景坐标和本地坐标QPointF scenePos = event->scenePos();QPointF loacalPos = event->pos();// 获取并设置为单位向量QVector2D startVec(m_pos.x() - 0, m_pos.y() - 0);startVec.normalize();QVector2D endVec(loacalPos.x() - 0, loacalPos.y() - 0);endVec.normalize();// 单位向量点乘，计算角度qreal dotValue = QVector2D::dotProduct(startVec, endVec);if (dotValue > 1.0)dotValue = 1.0;else if (dotValue < -1.0)dotValue = -1.0;dotValue = qAcos(dotValue);if (isnan(dotValue))dotValue = 0.0;// 获取角度qreal angle = dotValue * 1.0 / (PI / 180);// 向量叉乘获取方向QVector3D crossValue = QVector3D::crossProduct( \QVector3D(startVec, 1.0), \QVector3D(endVec, 1.0));if (crossValue.z() < 0)angle = -angle;m_rotate += angle;// 设置变化矩阵m_transform.rotate(m_rotate);this->setTransform(m_transform);m_pos = loacalPos;this->update();
}

函数 normalize 表示转化为单位向量。
函数 QVector2D::dotProduct 计算两个向量的点乘结果。
函数 QVector3D::crossProduct 计算两个向量的叉乘，这里需要根据向量的Z值计算选装的方向，把2D的向量转化了3D向量作为函数的输入。

完整代码如下：

头文件

#ifndef UICANVASITEMBASE_H
#define UICANVASITEMBASE_H#include <QObject>
#include <QGraphicsItem>
#include <QPixmap>
#include <QGraphicsObject>class UICanvasItemBase : public QObject, public QGraphicsItem
{Q_OBJECTpublic:enum ItemOperator{t_none,t_move,t_resize,t_rotate};UICanvasItemBase(QGraphicsItem* parentItem = nullptr);~UICanvasItemBase() override;// 设置改变大小相关属性void setItemResizeable(bool resizeable);void setItemResizeRatio(bool resizeRation, qreal rationValue);private:// 初始化Iconvoid initIcon(void);static QImage m_closeIcon;static QImage m_resizeIcon;static QImage m_rotateIcon;QPixmap m_closePixmap;QPixmap m_resizePixmap;QPixmap m_rotatePixmap;// 设置是否能够更改尺寸bool m_isResizeable = true;bool m_isRatioScale = true;qreal m_ratioValue = 1.0;protected:QRectF boundingRect() const override;void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) final;QPainterPath shape() const override;void mousePressEvent(QGraphicsSceneMouseEvent *event) override;void mouseMoveEvent(QGraphicsSceneMouseEvent *event) override;void mouseReleaseEvent(QGraphicsSceneMouseEvent *event) override;QVariant itemChange(QGraphicsItem::GraphicsItemChange change, const QVariant &value) override;// 自定义元素绘制virtual void customPaint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);QSize m_size;ItemOperator m_itemOper = t_none;// 获取自定义绘制所需要的矩形QRectF getCustomRect(void) const;protected:// 处理Item上的类型virtual void mouseMoveMoveOperator(const QPointF& scenePos, const QPointF& loacalPos);virtual void mouseMoveResizeOperator(const QPointF& scenePos, const QPointF& loacalPos);virtual void mouseMoveRotateOperator(const QPointF& scenePos, const QPointF& loacalPos);QPointF m_pos;              // 本地所坐标点击的点QPointF m_pressedPos;       // 场景坐标点击的点QPointF m_startPos;         // Item再场景坐标的起始坐标QTransform m_transform;     // 变换矩阵qreal m_rotate = 0.0;       // 当前旋转角度signals:void onClickedCopyItem(void);private:int m_nInterval = 20;int m_nEllipseWidth = 12;    // 半径// 画笔设置QColor m_cPenColor;int m_nPenWidth = 1;// 画刷设置QColor m_cBrushColor;
};
#endif

源文件

#include "UICanvasItemBase.h"
#include "Utils.h"
#include <QPainter>
#include <QGraphicsSceneMouseEvent>
#include <QDebug>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QVector2D>
#include <QVector3D>#define PI 3.14159265358979QImage UICanvasItemBase::m_closeIcon;
QImage UICanvasItemBase::m_resizeIcon;
QImage UICanvasItemBase::m_rotateIcon;UICanvasItemBase::UICanvasItemBase(QGraphicsItem* parentItem):QGraphicsItem(parentItem),m_cPenColor(255, 0, 0),m_cBrushColor(200, 100, 100)
{this->setFlag(QGraphicsItem::ItemIsSelectable, true);initIcon();
}UICanvasItemBase::~UICanvasItemBase()
{}void UICanvasItemBase::setItemResizeable(bool resizeable)
{m_isResizeable = resizeable;
}void UICanvasItemBase::setItemResizeRatio(bool resizeRation, qreal rationValue)
{m_isRatioScale = resizeRation;m_ratioValue = rationValue;
}QRectF UICanvasItemBase::boundingRect() const
{QRectF rectF = getCustomRect();if (!this->isSelected())return rectF;rectF.adjust(-m_nInterval, -m_nInterval, m_nInterval, m_nInterval);rectF.adjust(-m_nEllipseWidth, -m_nEllipseWidth, m_nEllipseWidth, m_nEllipseWidth);return rectF;
}void UICanvasItemBase::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{painter->setRenderHint(QPainter::Antialiasing, true);painter->setRenderHint(QPainter::SmoothPixmapTransform, true);painter->setRenderHint(QPainter::TextAntialiasing, true);// 自定义绘制customPaint(painter, option, widget);if (!this->isSelected())return;// 设置画笔QPen pen;pen.setWidth(m_nPenWidth);pen.setColor(m_cPenColor);pen.setStyle(Qt::DashLine);painter->setPen(pen);QRectF itemRect = this->getCustomRect();// 绘制轮廓线QRectF outLintRect = itemRect.adjusted(-m_nInterval, -m_nInterval, m_nInterval, m_nInterval);painter->drawRect(outLintRect);painter->setPen(Qt::NoPen);painter->setBrush(m_cBrushColor);// 绘制控制点painter->drawEllipse(outLintRect.topRight(), m_nEllipseWidth, m_nEllipseWidth);if (!m_closePixmap.isNull())painter->drawPixmap(QRect(outLintRect.topRight().x() - m_nEllipseWidth / 2, \outLintRect.topRight().y() - m_nEllipseWidth / 2, \m_nEllipseWidth, m_nEllipseWidth), m_closePixmap);painter->drawEllipse(outLintRect.bottomLeft(), m_nEllipseWidth, m_nEllipseWidth);if (!m_rotatePixmap.isNull())painter->drawPixmap(QRect(outLintRect.bottomLeft().x() - m_nEllipseWidth / 2, \outLintRect.bottomLeft().y() - m_nEllipseWidth / 2, \m_nEllipseWidth, m_nEllipseWidth), m_rotatePixmap);painter->drawEllipse(outLintRect.bottomRight(), m_nEllipseWidth, m_nEllipseWidth);if (!m_resizePixmap.isNull())painter->drawPixmap(QRect(outLintRect.bottomRight().x() - m_nEllipseWidth / 2, \outLintRect.bottomRight().y() - m_nEllipseWidth / 2, \m_nEllipseWidth, m_nEllipseWidth), m_resizePixmap);
}QPainterPath UICanvasItemBase::shape() const
{QPainterPath path;path.addRect(boundingRect());return path;
}void UICanvasItemBase::mousePressEvent(QGraphicsSceneMouseEvent *event)
{m_transform = this->transform();QRectF itemRect = this->getCustomRect();QRectF outLintRect = itemRect.adjusted(-m_nInterval, -m_nInterval, m_nInterval, m_nInterval);// 获取当前模式QPointF pos = event->pos();QPointF scenePos = event->scenePos();if (itemRect.contains(pos))m_itemOper = t_move;else if (g_utilTool->getDistance(pos, outLintRect.topRight()) <= m_nEllipseWidth)emit onClickedCopyItem();else if (g_utilTool->getDistance(pos, outLintRect.bottomLeft()) <= m_nEllipseWidth)m_itemOper = t_rotate;else if (g_utilTool->getDistance(pos, outLintRect.bottomRight()) <= m_nEllipseWidth)m_itemOper = t_resize;// 保存当前的一些信息m_pos = pos;m_pressedPos = scenePos;m_startPos = this->pos();return QGraphicsItem::mousePressEvent(event);
}void UICanvasItemBase::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{// 获取场景坐标和本地坐标QPointF scenePos = event->scenePos();QPointF pos = event->pos();if (m_itemOper == t_move){// 处理移动mouseMoveMoveOperator(scenePos, pos);}else if (m_itemOper == t_resize){// 处理更改大小mouseMoveResizeOperator(scenePos, pos);}else if (m_itemOper == t_rotate){// 处理旋转mouseMoveRotateOperator(scenePos, pos);}return QGraphicsItem::mouseMoveEvent(event);
}void UICanvasItemBase::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
{m_itemOper = t_none;return QGraphicsItem::mouseReleaseEvent(event);
}QVariant UICanvasItemBase::itemChange(QGraphicsItem::GraphicsItemChange change, const QVariant &value)
{if (change == QGraphicsItem::ItemSelectedChange)prepareGeometryChange();return QGraphicsItem::itemChange(change, value);
}void UICanvasItemBase::mouseMoveMoveOperator(const QPointF& scenePos, const QPointF& loacalPos)
{qreal xInterval = scenePos.x() - m_pressedPos.x();qreal yInterval = scenePos.y() - m_pressedPos.y();this->setPos(m_startPos + QPointF(xInterval, yInterval));this->update();
}void UICanvasItemBase::mouseMoveResizeOperator(const QPointF& scenePos, const QPointF& loacalPos)
{if (!m_isResizeable)return;qreal ratio = m_ratioValue;qreal itemWidth = abs(loacalPos.x()) * 2 - m_nInterval - m_nEllipseWidth;qreal itemHeight = abs(loacalPos.y()) * 2 - m_nInterval - m_nEllipseWidth;if (m_isRatioScale)itemHeight = itemWidth * 1.0 / ratio;// 设置图片的最小大小为10if (itemWidth < 10 || itemHeight < 10)return;m_size = QSize(itemWidth, itemHeight);this->update();
}void UICanvasItemBase::mouseMoveRotateOperator(const QPointF& scenePos, const QPointF& loacalPos)
{// 获取并设置为单位向量QVector2D startVec(m_pos.x() - 0, m_pos.y() - 0);startVec.normalize();QVector2D endVec(loacalPos.x() - 0, loacalPos.y() - 0);endVec.normalize();// 单位向量点乘，计算角度qreal dotValue = QVector2D::dotProduct(startVec, endVec);if (dotValue > 1.0)dotValue = 1.0;else if (dotValue < -1.0)dotValue = -1.0;dotValue = qAcos(dotValue);if (isnan(dotValue))dotValue = 0.0;// 获取角度qreal angle = dotValue * 1.0 / (PI / 180);// 向量叉乘获取方向QVector3D crossValue = QVector3D::crossProduct(QVector3D(startVec, 1.0),QVector3D(endVec, 1.0));if (crossValue.z() < 0)angle = -angle;m_rotate += angle;// 设置变化矩阵m_transform.rotate(m_rotate);this->setTransform(m_transform);m_pos = loacalPos;this->update();
}void UICanvasItemBase::customPaint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{}QRectF UICanvasItemBase::getCustomRect(void) const
{QPointF centerPos(0, 0);return QRectF(centerPos.x() - m_size.width() / 2, centerPos.y() - m_size.height() / 2, \m_size.width(), m_size.height());
}void UICanvasItemBase::initIcon(void)
{if (m_closeIcon.isNull())m_closeIcon.load("./Images/close.png");if (m_resizeIcon.isNull())m_resizeIcon.load("./Images/resize.png");if (m_rotateIcon.isNull())m_rotateIcon.load("./Images/rotate.png");m_closePixmap = QPixmap::fromImage(m_closeIcon);m_resizePixmap = QPixmap::fromImage(m_resizeIcon);m_rotatePixmap = QPixmap::fromImage(m_rotateIcon);
}

函数 mouseMoveMoveOperator 、 mouseMoveResizeOperator 、 mouseMoveRotateOperator 就是平移、改变尺寸、旋转的处理函数。