01. CAD双缓冲绘图

# CAD

1. 什么时候绘制

    1. WM_PAINT时绘制，MFC中是CMFCApplication1View::OnDraw时绘制

 

2. 在哪里绘制

   1. DC

SDK	MFC	绘制区域
BeginPaint	CPaintDC	无效区
GetDC	CClientDC	客户区
GetWindowDC	CWindowDC	整个窗口
CreateCompatibleDC	CDC	内存

 

3. 如何绘制

   1. InvalidateRect(hWnd, nullptr, TRUE);如果调用时传TRUE清除背景，就会导致闪烁。

   2. 捕获鼠标SetCapture, ReleaseCapture

   3. SDK用MoveToEx, MoveToEx绘制直线

   4. 双缓冲步骤

 

SDK例子：

case WM_PAINT: {
        PAINTSTRUCT ps;
        HDC hdc = BeginPaint(hWnd, &ps);
        
        RECT rect = {};
        GetClientRect(hWnd, &rect);
        auto nWidth = rect.right - rect.left;
        auto nHeight = rect.bottom - rect.top;
        HDC hdcMem = CreateCompatibleDC(hdc);
        HBITMAP hBitMap = CreateCompatibleBitmap(hdc, nWidth, nHeight);
        SelectObject(hdcMem, hBitMap);
        FillRect(hdcMem, &rect, (HBRUSH)(RGB(255, 255, 255) + 1));

        for (auto &pts : vec) {
            MoveToEx(hdcMem, pts.first.x, pts.first.y, nullptr);
            LineTo(hdcMem, pts.second.x, pts.second.y);
        }
        MoveToEx(hdcMem, ptBeg.x, ptBeg.y, nullptr);
        LineTo(hdcMem, ptEnd.x, ptEnd.y);

        BitBlt(hdc, 0, 0, nWidth, nHeight, hdcMem, 0, 0, SRCCOPY);
        DeleteObject(hBitMap);
        DeleteDC(hdcMem);

        EndPaint(hWnd, &ps);
        break;
    }

　　

MFC例子：

void CMFCApplication1View::OnDraw(CDC *pDC) {
	CMFCApplication1Doc *pDoc = GetDocument();
	ASSERT_VALID(pDoc);
	if (!pDoc)
		return;

	CRect rcClient;
	GetClientRect(&rcClient);

	CDC dcMem;
	dcMem.CreateCompatibleDC(pDC);

	CBitmap bmpMem;
	bmpMem.CreateCompatibleBitmap(pDC, rcClient.Width(), rcClient.Height());

	dcMem.SelectObject(&bmpMem);
	dcMem.FillSolidRect(rcClient, RGB(255, 255, 255));

	auto pos = m_lstLines.GetHeadPosition();
	while (pos) {
		auto line = m_lstLines.GetNext(pos);
		dcMem.MoveTo(line.first);
		dcMem.LineTo(line.second);
	}

	dcMem.MoveTo(m_ptBegin);
	dcMem.LineTo(m_ptEnd);

	pDC->BitBlt(0, 0, rcClient.Width(), rcClient.Height(), &dcMem, 0, 0, SRCCOPY);
}