MFC 滑鼠拖動畫圓

先計算出圓心, 半徑, 然後再根據圓心半徑計算出矩形(正方形)的左上方跟右下角的頂點, 然後用Ellipse函數畫圓。

原始碼如下:
在**view類標頭檔裡添加如下變數private:
    bool m_bLButtonDown;
       bool m_bErase;
    CPen* pGrayPen;
    CPen* pLinePen;
    CPoint old_center;
    CPoint m_pStart;
    int nRadius;

**View類建構函式添加如下初始化代碼：    pGrayPen = new CPen(0, 1, RGB(100, 100, 100));
   
    pLinePen = new CPen(0, 1, RGB(250, 0, 0));

**View類為滑鼠添加如下訊息響應函數； 按下左鍵,  按下左鍵滑鼠移動, 釋放左鍵

// CmfcTestView 訊息處理常式

void CmfcTestView::OnLButtonDown(UINT nFlags, CPoint point)
...{
    // TODO: 在此添加訊息處理常式代碼和/或調用預設值

    m_bLButtonDown = TRUE; // 設左滑鼠鍵按下為真
    m_pStart  = point;
    SetCapture(); // 設定滑鼠捕獲
    CView::OnLButtonDown(nFlags, point);
}

void CmfcTestView::OnMouseMove(UINT nFlags, CPoint point)
...{
    // TODO: 在此添加訊息處理常式代碼和/或調用預設值
    if(m_bLButtonDown==TRUE) ...{

        CDC* pDC = GetDC(); // 擷取裝置上下文        
        pDC->SelectObject(pGrayPen); // 選取灰色筆
        pDC->SelectStockObject(NULL_BRUSH);
        pDC->SetROP2(R2_XORPEN); // 設定為異或繪圖方式
        if (m_bErase) ...{ // need to erase
            pDC->Ellipse(old_center.x-nRadius, old_center.y-nRadius,
            old_center.x+nRadius, old_center.y+nRadius);
        }

        else // 需要擦除為假
            m_bErase = TRUE; // 設需要擦除為真
        CPoint center;
        center.x=(float(m_pStart.x+point.x))/2;
        center.y=(float(m_pStart.y+point.y))/2;
        nRadius=sqrt((double)(point.y-m_pStart.y)*(point.y-m_pStart.y)+
        (point.x-m_pStart.x)*(point.x-m_pStart.x))/2;
        pDC->Ellipse(center.x-nRadius, center.y-nRadius, 
            center.x+nRadius, center.y+nRadius);
        old_center=center;
        ReleaseDC(pDC); // 釋放裝置上下文
    }
    CView::OnMouseMove(nFlags, point);

}

void CmfcTestView::OnLButtonUp(UINT nFlags, CPoint point)
...{
    // TODO: 在此添加訊息處理常式代碼和/或調用預設值
    ReleaseCapture();
    if (m_bLButtonDown) ...{
        CDC* pDC = GetDC(); // 擷取裝置上下文
        pDC->SelectStockObject(NULL_BRUSH);
        pDC->SelectObject(pLinePen); // 選擇筆
        pDC->SetROP2(R2_COPYPEN); // 設定為覆蓋繪圖方式
        CPoint center; // 圓心
        //calculate the center
        center.x=(float(m_pStart.x+point.x))/2;
        center.y=(float(m_pStart.y+point.y))/2;
        // nRadius is a class member, double
        nRadius=sqrt(double(point.y-m_pStart.y)*(point.y-m_pStart.y)+
        (point.x-m_pStart.x)*(point.x-m_pStart.x))/2;
        pDC->Ellipse(center.x-nRadius, center.y-nRadius, center.x+nRadius, center.y+nRadius);
        ReleaseDC(pDC); // 釋放裝置上下文
        m_bLButtonDown = FALSE; // 重設左滑鼠鍵按下為假
        m_bErase = FALSE; // 重需要擦除為假
    }
    CView::OnLButtonUp(nFlags, point);
}