说实话,本来我是没有打算放一个很大的例子的,一则比较复杂,二来或许需要很多次才能说得完。不过,现在已经说完了绘图部分,所以计划还是上一个这样的例子。这里我会只做出一个简单的画板程序,大体上就是能够画直线和矩形吧。这样,我计划分成两种实现,一是使用普通的QWidget作为画板,第二则是使用Graphcis View Framework来实现。因为前面有朋友说不大明白Graphics View的相关内容,所以计划如此。
 
好了,现在先来看看我们的主体框架。我们的框架还是使用Qt Creator创建一个Gui Application工程。
 
简单的main()函数就不再赘述了,这里首先来看MainWindow。顺便说一下,我一般不会使用ui文件,所以这些内容都是手写的。首先先来看看最终的运行结果:
 
 
或许很简单,但是至少我们能够把前面所说的各种知识串连起来,这也就达到目的了。
 
现在先来看看MainWindow的代码:
 
mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QtGui>

#include "shape.h"
#include "paintwidget.h"

class MainWindow : public QMainWindow
{
        Q_OBJECT

public:
        MainWindow(QWidget *parent = 0);

signals:
        void changeCurrentShape(Shape::Code newShape);

private slots:
        void drawLineActionTriggered();
        void drawRectActionTriggered();

};

#endif // MAINWINDOW_H
 
mainwindow.cpp
#include "mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
        : QMainWindow(parent)
{
        QToolBar *bar = this->addToolBar("Tools");
        QActionGroup *group = new QActionGroup(bar);

        QAction *drawLineAction = new QAction("Line", bar);
        drawLineAction->setIcon(QIcon(":/line.png"));
        drawLineAction->setToolTip(tr("Draw a line."));
        drawLineAction->setStatusTip(tr("Draw a line."));
        drawLineAction->setCheckable(true);
        drawLineAction->setChecked(true);
        group->addAction(drawLineAction);

        bar->addAction(drawLineAction);
        QAction *drawRectAction = new QAction("Rectangle", bar);
        drawRectAction->setIcon(QIcon(":/rect.png"));
        drawRectAction->setToolTip(tr("Draw a rectangle."));
        drawRectAction->setStatusTip(tr("Draw a rectangle."));
        drawRectAction->setCheckable(true);
        group->addAction(drawRectAction);
        bar->addAction(drawRectAction);

        QLabel *statusMsg = new QLabel;
        statusBar()->addWidget(statusMsg);

        PaintWidget *paintWidget = new PaintWidget(this);
        setCentralWidget(paintWidget);

        connect(drawLineAction, SIGNAL(triggered()),
                        this, SLOT(drawLineActionTriggered()));
        connect(drawRectAction, SIGNAL(triggered()),
                        this, SLOT(drawRectActionTriggered()));
        connect(this, SIGNAL(changeCurrentShape(Shape::Code)),
                        paintWidget, SLOT(setCurrentShape(Shape::Code)));
}

void MainWindow::drawLineActionTriggered()
{
        emit changeCurrentShape(Shape::Line);
}

void MainWindow::drawRectActionTriggered()
{
        emit changeCurrentShape(Shape::Rect);
}
 
应该说,从以往的学习中可以看出,这里的代码没有什么奇怪的了。我们在MainWindow类里面声明了一个信号,changeCurrentShape(Shape::Code),用于按钮按下后通知画图板。注意,QActio的triggered()信号是没有参数的,因此,我们需要在QAction的槽函数中重新emit我们自己定义的信号。构造函数里面创建了两个QAction,一个是drawLineAction,一个是drawRectAction,分别用于绘制直线和矩形。MainWindow的中心组件是PainWidget,也就是我们的画图板。下面来看看PaintWidget类:
 
paintwidget.h
#ifndef PAINTWIDGET_H
#define PAINTWIDGET_H

#include <QtGui>
#include <QDebug>
#include "shape.h"
#include "line.h"
#include "rect.h"

class PaintWidget : public QWidget
{
        Q_OBJECT

public:
        PaintWidget(QWidget *parent = 0);

public slots:
        void setCurrentShape(Shape::Code s)
        {
                if(s != currShapeCode) {
                        currShapeCode = s;
                }
        }

protected:
        void paintEvent(QPaintEvent *event);
        void mousePressEvent(QMouseEvent *event);
        void mouseMoveEvent(QMouseEvent *event);
        void mouseReleaseEvent(QMouseEvent *event);

private:
        Shape::Code currShapeCode;
        Shape *shape;
        bool perm;
        QList<Shape*> shapeList;
};

#endif // PAINTWIDGET_H
 
paintwidget.cpp
#include "paintwidget.h"

PaintWidget::PaintWidget(QWidget *parent)
        : QWidget(parent), currShapeCode(Shape::Line), shape(NULL), perm(false)
{
        setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
}

void PaintWidget::paintEvent(QPaintEvent *event)
{
        QPainter painter(this);
        painter.setBrush(Qt::white);
        painter.drawRect(0, 0, size().width(), size().height());
        foreach(Shape * shape, shapeList) {
                shape->paint(painter);
        }
        if(shape) {
                shape->paint(painter);
        }
}

void PaintWidget::mousePressEvent(QMouseEvent *event)
{
        switch(currShapeCode)
        {
        case Shape::Line:
                {
                        shape = new Line;
                        break;
                }
        case Shape::Rect:
                {
                        shape = new Rect;
                        break;
                }
        }
        if(shape != NULL) {
                perm = false;
                shapeList<<shape;
                shape->setStart(event->pos());
                shape->setEnd(event->pos());
        }
}

void PaintWidget::mouseMoveEvent(QMouseEvent *event)
{
        if(shape && !perm) {
                shape->setEnd(event->pos());
                update();
        }
}

void PaintWidget::mouseReleaseEvent(QMouseEvent *event)
{
        perm = true;
}
 
PaintWidget类定义了一个slot,用于接收改变后的新的ShapeCode。最主要的是,PaintWidget重定义了三个关于鼠标的事件:mousePressEvent,mouseMoveEvent和mouseReleaseEvent。
 
我们来想象一下如何绘制一个图形:图形的绘制与鼠标操作息息相关。以画直线为例,首先我们需要按下鼠标,确定直线的第一个点,所以在mousePressEvent里面,我们让shape保存下start点。然后在鼠标按下的状态下移动鼠标,此时,直线就会发生变化,实际上是直线的终止点在随着鼠标移动,所以在mouseMoveEvent中我们让shape保存下end点,然后调用update()函数,这个函数会自动调用paintEvent()函数,显示出我们绘制的内容。最后,当鼠标松开时,图形绘制完毕,我们将一个标志位置为true,此时说明这个图形绘制完毕。
 
为了保存我们曾经画下的图形,我们使用了一个List。每次按下鼠标时,都会把图形存入这个List。可以看到,我们在paintEvent()函数中使用了foreach遍历了这个List,绘制出历史图形。foreach是Qt提供的一个宏,用于遍历集合中的元素。
 
最后我们来看看Shape类。
 
shape.h
#ifndef SHAPE_H
#define SHAPE_H

#include <QtGui>

class Shape
{
public:

        enum Code {
                Line,
                Rect
        };

        Shape();

        void setStart(QPoint s)
        {
                start = s;
        }

        void setEnd(QPoint e)
        {
                end = e;
        }

        QPoint startPoint()
        {
                return start;
        }

        QPoint endPoint()
        {
                return end;
        }

        void virtual paint(QPainter & painter) = 0;

protected:
        QPoint start;
        QPoint end;
};

#endif // SHAPE_H
 
shape.cpp
#include "shape.h"

Shape::Shape()
{
}
 
Shape类最重要的就是保存了start和end两个点。为什么只要这两个点呢?因为我们要绘制的是直线和矩形。对于直线来说,有了两个点就可以确定这条直线,对于矩形来说,有了两个点作为左上角的点和右下角的点也可以确定这个矩形,因此我们只要保存两个点,就足够保存这两种图形的位置和大小的信息。paint()函数是Shape类的一个纯虚函数,子类都必须实现这个函数。我们现在有两个子类:Line和Rect,分别定义如下:
 
line.h
#ifndef LINE_H
#define LINE_H

#include "shape.h"

class Line : public Shape
{
public:
        Line();

        void paint(QPainter &painter);
};

#endif // LINE_H
 
line.cpp
#include "line.h"

Line::Line()
{
}

void Line::paint(QPainter &painter)
{
        painter.drawLine(start, end);
}
 
rect.h
#ifndef RECT_H
#define RECT_H

#include "shape.h"

class Rect : public Shape
{
public:
        Rect();

        void paint(QPainter &painter);
};

#endif // RECT_H
 
rect.cpp
#include "rect.h"

Rect::Rect()
{
}

void Rect::paint(QPainter &painter)
{
        painter.drawRect(start.x(), start.y(),
                                         end.x() - start.x(), end.y() - start.y());
}
 
使用paint()函数,根据两个点的数据,Line和Rect都可以绘制出它们自身来。此时就可以看出,我们之所以要建立一个Shape作为父类,因为这两个类有几乎完全相似的数据对象,并且从语义上来说,Line、Rect与Shape也完全是一个is-a的关系。如果你想要添加颜色等的信息,完全可以在Shape类进行记录。这也就是类层次结构的好处。
 
代码很多也会比较乱,附件里面是整个工程的文件,有兴趣的朋友不妨看看哦!