Qt图形视图框架：QGraphicsItem详解

Chapter1 Qt图形视图框架：QGraphicsItem详解
Chapter2 自定义QGraphicsItem实现平移、改变尺寸和旋转
- 1. 平移
- 2. 改变尺寸
- 3. 旋转
- 完整代码如下：
- - 头文件
  - 源文件

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：
在这里插入图片描述
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：
在这里插入图片描述
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10
    if (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：
在这里插入图片描述
本篇文章讲述的旋转方法步骤如下：

计算上一次鼠标移动和本次鼠标移动位置之间的角度。
计算旋转的方向。
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_OBJECT

public:
    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Icon
    void 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 3.14159265358979

QImage 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10
    if (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);
}