运行效果：

关键代码：

添加头文件：item_rectangle.hpp

//item_rectangle.hpp
#ifndef ITEM_RECTANGLE_HPP_INCLUDED
#define ITEM_RECTANGLE_HPP_INCLUDED
#include "item_i.hpp"

class RectangleItem : public IItem
{
public:
    RectangleItem()
        : _startPosition(0, 0), _endPosition(0, 0)
    {}

    virtual void Draw(wxDC& dc) const;

    void OnDrawStart(wxPoint const& point) override
    {
        _startPosition = point;
    }

    virtual void OnDrawEnd(wxPoint const& point)
    {
        _endPosition = point;
    }
private:
    wxPoint _startPosition, _endPosition;
};

#endif // ITEM_RECTANGLE_HPP_INCLUDED

添加源文件：item_rectangle.cpp

//item_rectangle.cpp
#include "item_rectangle.hpp"

#include <cmath> //需要一些数学函数abs（绝对值）,min（二者最小）

void RectangleItem::Draw(wxDC& dc) const
{
    //得到左上角的坐标
    int left_x = std::min(_startPosition.x, _endPosition.x);
    int top_y = std::min(_startPosition.y, _endPosition.y);

    //得到长和宽（绝对值）
    int width = std::abs(_endPosition.x - _startPosition.x);
    int height = std::abs(_endPosition.y - _startPosition.y);

    dc.DrawRectangle(left_x, top_y, width, height);
}

添加创建矩形的代码

CreateNewItem()函数中 155~158行