MFC 无边框窗体实现用鼠标拖动窗体边缘实现窗体大小变化

实现 MFC 无边框窗体可以通过以下步骤：

1. 创建一个派生自 CWnd 的类，作为无边框窗口的基类。
2. 重载 OnNcCalcSize 函数，计算非客户区的大小。
3. 重载 OnNcPaint 函数，绘制非客户区的背景和边框。
4. 重载 OnNcLButtonDown 函数，处理鼠标左键按下事件，如果鼠标位于窗口边缘，则将鼠标捕获，并设置窗口大小变化标志。
5. 重载 OnNcLButtonUp 函数，处理鼠标左键释放事件，释放鼠标捕获，并清除窗口大小变化标志。
6. 重载 OnMouseMove 函数，处理鼠标移动事件，如果窗口大小变化标志被设置，则根据鼠标位置改变窗口大小。

下面是一个简单的示例代码：

class CMyWnd : public CWnd
{
public:
    CMyWnd()
    {
        m_bSizing = FALSE;
    }

protected:
    BOOL m_bSizing;

    afx_msg void OnNcCalcSize(BOOL bCalcValidRects, NCCALCSIZE_PARAMS* lpncsp)
    {
        lpncsp->rgrc[0].left += 2;
        lpncsp->rgrc[0].top += 2;
        lpncsp->rgrc[0].right -= 2;
        lpncsp->rgrc[0].bottom -= 2;
    }

    afx_msg void OnNcPaint()
    {
        CWindowDC dc(this);
        CRect rect;
        GetWindowRect(rect);
        rect.OffsetRect(-rect.left, -rect.top);
        rect.DeflateRect(2, 2);
        dc.FillSolidRect(rect, RGB(192, 192, 192));
        dc.Draw3dRect(rect, RGB(0, 0, 0), RGB(0, 0, 0));
    }

    afx_msg void OnNcLButtonDown(UINT nHitTest, CPoint point)
    {
        if (nHitTest == HTLEFT || nHitTest == HTRIGHT ||
            nHitTest == HTTOP || nHitTest == HTBOTTOM ||
            nHitTest == HTTOPLEFT || nHitTest == HTTOPRIGHT ||
            nHitTest == HTBOTTOMLEFT || nHitTest == HTBOTTOMRIGHT)
        {
            SetCapture();
            m_bSizing = TRUE;
        }
        else
        {
            CWnd::OnNcLButtonDown(nHitTest, point);
        }
    }

    afx_msg void OnNcLButtonUp(UINT nHitTest, CPoint point)
    {
        if (m_bSizing)
        {
            ReleaseCapture();
            m_bSizing = FALSE;
        }
        else
        {
            CWnd::OnNcLButtonUp(nHitTest, point);
        }
    }

    afx_msg void OnMouseMove(UINT nFlags, CPoint point)
    {
        if (m_bSizing)
        {
            CRect rect;
            GetWindowRect(rect);
            switch (GetHitTest(point))
            {
            case HTLEFT:
                rect.left = point.x;
                break;
            case HTRIGHT:
                rect.right = point.x;
                break;
            case HTTOP:
                rect.top = point.y;
                break;
            case HTBOTTOM:
                rect.bottom = point.y;
                break;
            case HTTOPLEFT:
                rect.left = point.x;
                rect.top = point.y;
                break;
            case HTTOPRIGHT:
                rect.right = point.x;
                rect.top = point.y;
                break;
            case HTBOTTOMLEFT:
                rect.left = point.x;
                rect.bottom = point.y;
                break;
            case HTBOTTOMRIGHT:
                rect.right = point.x;
                rect.bottom = point.y;
                break;
            }
            MoveWindow(rect);
        }
        else
        {
            CWnd::OnMouseMove(nFlags, point);
        }
    }

    int GetHitTest(CPoint point)
    {
        CRect rect;
        GetWindowRect(rect);
        int cx = rect.Width() / 3;
        int cy = rect.Height() / 3;
        if (point.x < rect.left + cx)
        {
            if (point.y < rect.top + cy)
            {
                return HTTOPLEFT;
            }
            else if (point.y > rect.bottom - cy)
            {
                return HTBOTTOMLEFT;
            }
            else
            {
                return HTLEFT;
            }
        }
        else if (point.x > rect.right - cx)
        {
            if (point.y < rect.top + cy)
            {
                return HTTOPRIGHT;
            }
            else if (point.y > rect.bottom - cy)
            {
                return HTBOTTOMRIGHT;
            }
            else
            {
                return HTRIGHT;
            }
        }
        else
        {
            if (point.y < rect.top + cy)
            {
                return HTTOP;
            }
            else if (point.y > rect.bottom - cy)
            {
                return HTBOTTOM;
            }
            else
            {
                return HTCLIENT;
            }
        }
    }

    DECLARE_MESSAGE_MAP()
};

BEGIN_MESSAGE_MAP(CMyWnd, CWnd)
    ON_WM_NCCALCSIZE()
    ON_WM_NCPAINT()
    ON_WM_NCLBUTTONDOWN()
    ON_WM_NCLBUTTONUP()
    ON_WM_MOUSEMOVE()
END_MESSAGE_MAP()

在这个示例代码中，GetHitTest 函数用于确定鼠标位置相对于窗口的位置，以便按照相应的方法改变窗口大小。这个函数是基于窗口分成 9 个区域的思想，每个区域都对应一个鼠标消息。当鼠标位于边缘时，根据鼠标位置判断应该执行哪个消息。