如何在 Win32 应用程序中拖动纯色矩形而不会出现白色闪烁且不干扰屏幕上的其他对象？

Question

I've been messing around with a little toy Win32 app to try and get the hang of basic drawing and processing mouse and keyboard messages. Basically, the goal is to click around the client area to place rectangles, alt click them to cycle through 4 colors, right click them to reassign a color directly with one of the f keys, or right click and hold to reposition with drag and drop, with the currently selected rectangle having a dotted outline instead of solid.

Dragging correctly is proving to be difficult. While dragging, the selected rectangle being dragged flashes white / flickers, and also temporarily erases the pen drawn borders of the other rectangles. Overcoming these two issues is what I need help with.

A c++ vector is being used to collect and process dynamically allocated CRect objects, in case it matters.

    //relevant code from WndProc

        case WM_RBUTTONDOWN:
        {
            ClipCursor(&rcClip);
            if (prcSelected)
            {
                prcSelected->deselect();
                prcSelected = nullptr;
                InvalidateRect(hWnd, NULL, TRUE);
            }
            int x{ LOWORD(lParam) }, y{ HIWORD(lParam) };
            for (auto rc : vRect)
            {
                if (rc->IsClicked(x, y))
                {
                    rc->select();
                    prcSelected = rc;
                    InvalidateRect(hWnd, NULL, TRUE);
                    break;
                }
            }
        }
            break;

        case WM_RBUTTONUP:
            ClipCursor(&rcOldClip);
            InvalidateRect(hWnd, NULL, TRUE);
            break;

        case WM_MOUSEMOVE:
        {
            if (wParam & MK_RBUTTON && prcSelected)
            {
                CRect rcPrev{ *prcSelected };
                CRect crIsect{};
                int x{ LOWORD(lParam) }, y{ HIWORD(lParam) };
                int xShift{ x - prcSelected->r.left }, yShift{ y - prcSelected->r.top };
                prcSelected->shift(xShift, yShift);     
                rcPrev.SetFill(CR_WHITE);
                rcPrev.SetOutline(CR_WHITE);
                rcPrev.draw();

                for (auto rc : vRect)
                    if (IntersectRect(&crIsect.r, &(rc->r), &(rcPrev.r)))
                    {
                        crIsect.SetFill(rc->GetFill());
                        crIsect.SetOutline(rc->GetFill());
                        crIsect.draw();
                    }

                prcSelected->draw();
            }
        }
            break;

        case WM_PAINT:
        {
            PAINTSTRUCT ps;
            HDC hdc = BeginPaint(hWnd, &ps);

            for (auto rc : vRect)
                rc->draw(hdc);

            EndPaint(hWnd, &ps);
        }
            break;


    //Rectangle drawing code

    void CRect::draw()
    {
        HDC hdc{ GetDC(hWnd) };
        HPEN hpenDot;
        SelectObject(hdc, GetStockObject(DC_BRUSH));
        SetDCBrushColor(hdc, crBrush);
        if (!fSelected)
        {
            SelectObject(hdc, GetStockObject(DC_PEN)); // Can be set to any color. No need to release.
            SetDCPenColor(hdc, crPen);
            Rectangle(hdc, r.left, r.top, r.right, r.bottom);
        }
        else
        {
            hpenDot = CreatePen(PS_DOT, 1, crPen);
            SelectObject(hdc, hpenDot);
            Rectangle(hdc, r.left, r.top, r.right, r.bottom);
            DeleteObject(hpenDot);
        }
        ReleaseDC(hWnd, hdc);
    }

    void CRect::draw(HDC& hdc)
    {
        HPEN hpenDot;
        SelectObject(hdc, GetStockObject(DC_BRUSH));
        SetDCBrushColor(hdc, crBrush);
        if (!fSelected)
        {
            SelectObject(hdc, GetStockObject(DC_PEN)); 
            SetDCPenColor(hdc, crPen);
            Rectangle(hdc, r.left, r.top, r.right, r.bottom);
        }
        else
        {
            hpenDot = CreatePen(PS_DOT, 1, crPen);
            SelectObject(hdc, hpenDot);
            Rectangle(hdc, r.left, r.top, r.right, r.bottom);
            DeleteObject(hpenDot);
        }
    }


    void CRect::shift(int x, int y)
    {
        r.left   += x;
        r.top    += y;
        r.right  += x;
        r.bottom += y;
    }

    bool CRect::IsClicked(int x, int y)
    {
        POINT pt{ x, y };
        return (bool)PtInRect(&r, pt);

    }

Answer 1

TL;DR: you should do painting only in WM_PAINT handler.

Instead of painting directly on screen surface in WM_MOUSEMOVE handler you should do the following:

1. In WM_MOUSEMOVE just call InvalidateRect(,prcSelected) to invalidate area occupied by your drag rectangle.

2. In your WM_PAINT handler

   case WM_PAINT: { PAINTSTRUCT ps; HDC hdc = BeginPaint(hWnd, &ps); // clear surface FillRect(hdc, ps.rcPaint, COLORREF(0xFFFFFF)); for (auto rc : vRect) rc->draw(hdc); if(prcSelected) DrawDragRectangle(...); // drawing code from your WM_MOUSEMOVE EndPaint(hWnd, &ps); }

This way you will do drawing in WM_PAINT only and in correct order.

You may still have flickering even in this case. If so enable double buffering on your window by adding WS_EX_COMPOSITED flag to it.