c++ - 为什么 CDC::LineTo() 不在 Visual C++ 2015 MFC 对话框中绘制？

Question

我正在学习 MFC，我正在尝试在基于 MFC 对话框的应用程序主窗口上绘制一些线条，这应该是一项相当简单的任务，但在运行时我看不到对话框上绘制的线条。

下面是我写的方法:

// draw corner of a rectangle on specified device context
void CTestDrawCornerDlg::DrawCorner(
    CDC* pDC,
    const CornerType& type,
    const CPoint& position,
    const unsigned int& size
    )
{
    CPen pen(PS_SOLID, 5, RGB(0, 0, 0));
    CPen* pOldPen = pDC->SelectObject(&pen);

    CPoint pH, pV;
    // I could make following lines simply with a 2-lines block,
    //  but I'd leave it as it was to make it easier to understand.
    switch (type)
    {
    case LEFT_TOP:
        pH.x = position.x + size;
        pH.y = position.y;
        pV.x = position.x;
        pV.y = position.y + size;
        break;
    case LEFT_BOTTOM:
        pH.x = position.x - size;
        pH.y = position.y;
        pV.x = position.x;
        pV.y = position.y + size;
        break;
    case RIGHT_TOP:
        pH.x = position.x + size;
        pH.y = position.y;
        pV.x = position.x;
        pV.y = position.y - size;
        break;
    case RIGHT_BOTTOM:
        pH.x = position.x - size;
        pH.y = position.y;
        pV.x = position.x;
        pV.y = position.y - size;
        break;
    default: break;
    }
    pDC->MoveTo(position);
    pDC->LineTo(pH);
    pDC->MoveTo(position);
    pDC->LineTo(pV);

    pDC->SelectObject(pOldPen);
}

我在 Dialog 类的 OnPaint 方法中调用了这个方法:

void CTestDrawCornerDlg::OnPaint()
{
    if (IsIconic())
    {
        CPaintDC dc(this); // device context for painting
        // lines generated automatically when creating
        // MFC project are truncated for brevity
    }
    else
    {
        CDialogEx::OnPaint();
    }

    CPaintDC pDC(this);
    DrawCorner(&pDC, LEFT_TOP, CPoint(50, 50), 50);
}

我想这是一个新手错误，但我只是不知道错误是什么。感谢您的帮助!

附言请从以下链接下载 MFC 项目以重现此问题: https://www.dropbox.com/s/exeehci9kopvgsn/TestDrawCorner.zip?dl=0

Answer 1

您可以更改代码以使用 CDialogEx::OnPaint() + CClientDC，如下所示:

void CTestDrawCornerDlg::OnPaint()
{
    CDialogEx::OnPaint();
    CClientDC pDC(this);
    DrawCorner(&pDC, LEFT_TOP, CPoint(50, 50), 50);
}

或者只使用CPaintDC:

void CTestDrawCornerDlg::OnPaint()
{
    CPaintDC pDC(this);
    DrawCorner(&pDC, LEFT_TOP, CPoint(50, 50), 50);
}

但不要使用OnPaint + CPaintDC

要查看问题，请注意 OnPaint 和 CPaintDC 是如何在 MFC 中定义的:

void CDialog::OnPaint()
{
    CPaintDC dc(this);
    if (PaintWindowlessControls(&dc))
       return;
    Default();
}

CPaintDC::CPaintDC(CWnd* pWnd)
{
    if (!Attach(::BeginPaint(m_hWnd = pWnd->m_hWnd, &m_ps)))
        AfxThrowResourceException();
}

::BeginPaint 是核心 WinAPI 函数。它只应调用一次以响应 WM_PAINT，并且不能在其他任何地方使用。

另一方面，

CClientDC 使用 ::GetDC，只要窗口句柄可用，它几乎可以在任何地方使用。