QT：使用“状态模式”绘制界面

QT与很多GUI库不同（如MFC），它不能随时随地地在界面上画图，只能在界面类的painterEvent中画图，如此一来，想在绘制QT界面时使用状态模式（GOF的23种设计模式之一）就有点困难了，作为解决方案，我先把要界面上的图片绘制在一张图片上（QPixmap），然后再在painterEvent中将Pixmap“画”到界面上。以下是这种方法的一个小例子。

截图：

源代码：

#include

//状态类的基类，定义了各个公用接口，
//其中，SetPixmap是一个纯虚接口
class BasePen
{
protected:
    //这三个类成员理应是BasePen的私有成员，然后通过接口访问
    //我这里为了方便，直接把它们设为保护成员了
    QPixmap m_Pixmap;
    QPoint m_StartPoint;
    QPoint m_EndPoint;
    virtual void SetPixmap() = 0;
public:
    BasePen()
    {
        m_StartPoint = m_EndPoint = QPoint(0, 0);
        m_Pixmap = QPixmap(500, 500);
    }
    void SetStartPoint(QPoint point) { m_StartPoint = point; }
    void SetEndPoint(QPoint point)
    {
        m_EndPoint = point;
        SetPixmap();
    }
    QPixmap GetPixmap() { return m_Pixmap; }
};

//矩形类，在界面上画一个红色的矩形
class RectPen : public BasePen
{
protected:
    void SetPixmap()
    {
        m_Pixmap.fill(Qt::white);
        QPainter painter(&m_Pixmap);
        QRect rect(m_StartPoint, m_EndPoint);
        painter.setPen(Qt::red);
        painter.drawRect(rect);
    }
};

//直线类，在界面上画一条蓝色的直线
class LinePen : public BasePen
{
protected:
    void SetPixmap()
    {
        m_Pixmap.fill(Qt::white);
        QPainter painter(&m_Pixmap);
        painter.setPen(Qt::blue);
        painter.drawLine(m_StartPoint, m_EndPoint);
    }
};

//圆形类，在界面上画一个绿色的椭圆
class CirclePen : public BasePen
{
protected:
    void SetPixmap()
    {
        m_Pixmap.fill(Qt::white);
        QPainter painter(&m_Pixmap);
        QRect rect(m_StartPoint, m_EndPoint);
        painter.setPen(Qt::green);
        painter.drawEllipse(rect);
    }
};

class Widget : public QWidget
{
    Q_OBJECT
private:
    bool m_MouseDown;
    BasePen *m_BasePen;
    RectPen *m_RectPen;
    LinePen *m_LinePen;
    CirclePen *m_CirclePen;
    //在界面上放三个按钮，用来控制画图状态
    QRadioButton *m_LineButton;
    QRadioButton *m_RectButton;
    QRadioButton *m_CircleButton;
protected:
    void mousePressEvent(QMouseEvent *event);
    void mouseMoveEvent(QMouseEvent *event);
    void mouseReleaseEvent(QMouseEvent *event);
    void paintEvent(QPaintEvent *event);
public:
    Widget(QWidget *parent = 0);
    ~Widget();
    private slots:
        void ClickedLineButton() { m_BasePen = m_LinePen; }
        void ClickedRectButton() { m_BasePen = m_RectPen; }
        void ClickedCircleButton() { m_BasePen = m_CirclePen; }
};

Widget::Widget(QWidget *parent /* = 0 */)
: QWidget(parent)
{
    m_MouseDown = false;
    m_RectPen = new RectPen;
    m_LinePen = new LinePen;
    m_CirclePen = new CirclePen;
    m_LineButton = new QRadioButton("Line", this);
    m_RectButton = new QRadioButton("Rect", this);
    m_CircleButton = new QRadioButton("Circle", this);
    m_LineButton->move(10, 10);
    m_RectButton->move(100, 10);
    m_CircleButton->move(200, 10);
    connect(m_LineButton, SIGNAL(clicked()), this, SLOT(ClickedLineButton()));
    connect(m_RectButton, SIGNAL(clicked()), this, SLOT(ClickedRectButton()));
    connect(m_CircleButton, SIGNAL(clicked()), this, SLOT(ClickedCircleButton()));
    m_BasePen = m_LinePen;
    m_LineButton->setChecked(true);
    setFixedSize(500, 500);
}

Widget::~Widget()
{
    delete m_LinePen;
    delete m_RectPen;
    delete m_CirclePen;
}

void Widget::mousePressEvent(QMouseEvent *event)
{
    if( event->button() == Qt::LeftButton )
    {
        m_MouseDown = true;
        m_BasePen->SetStartPoint(event->pos());
    }
}

void Widget::mouseMoveEvent(QMouseEvent *event)
{
    if( m_MouseDown )
    {
        m_BasePen->SetEndPoint(event->pos());
        update();
    }
}

void Widget::mouseReleaseEvent(QMouseEvent *event)
{
    if( event->button() == Qt::LeftButton )
    {
        m_MouseDown = false;
    }
}

void Widget::paintEvent(QPaintEvent *event)
{
    QPixmap temp = m_BasePen->GetPixmap();
    QPainter painter(this);
    painter.drawPixmap(0, 0, temp);
}

#include "main.moc"

int main(int argc, char **argv)
{
    QApplication app(argc, argv);
    Widget *ww = new Widget;
    ww->show();
    return app.exec();
}