[Programming Visual C++]Ex05c-CScrollView Revisited

1. Mouse Move
void CEx05cView::OnMouseMove(UINT nFlags, CPoint point)
{
    if (m_bCaptured) {
        CClientDC dc(this);
        OnPrepareDC(&dc);
        CRect rectOld(m_pointTopLeft, m_sizeEllipse);
        dc.LPtoDP(rectOld);
        InvalidateRect(rectOld, TRUE);
        m_pointTopLeft = point - m_sizeOffset;
        dc.DPtoLP(&m_pointTopLeft);
        CRect rectNew(m_pointTopLeft, m_sizeEllipse);
        dc.LPtoDP(rectNew);
        InvalidateRect(rectNew, TRUE);
    }
}

The OnMouseMove Coordinate Transformation Code

As you can see, this function contains several translation statements. The logic can be summarized by the following steps:

1. Construct the previous ellipse rectangle and convert it from logical to device coordinates.
2. Invalidate the previous rectangle.
3. Update the top left coordinate of the ellipse rectangle.
4. Construct the new rectangle and convert it to device coordinates.
5. Invalidate the new rectangle.

The function calls InvalidateRect twice. Windows "saves up" the two invalid rectangles and computes a new invalid rectangle that is the union of the two, intersected with the client rectangle.

2. Set Cursor Type
void CEx05cView::OnLButtonDown(UINT nFlags, CPoint point) {
   ...
    if (circle.PtInRegion(point)) {
        // Capturing the mouse ensures subsequent LButtonUp message
        SetCapture();
        ...
        // New mouse cursor is active while mouse is captured
        ::SetCursor(::LoadCursor(NULL, IDC_CROSS));
    }
}

3.The SetCapture and ReleaseCapture Functions

SetCapture is the CWnd member function that "captures" the mouse, such that mouse movement messages are sent to this window even if the mouse cursor is outside the window. An unfortunate side effect of this function is that the ellipse can be moved outside the window and "lost." A desirable and necessary effect is that all subsequent mouse messages are sent to the window, including the WM_LBUTTONUP message, which would otherwise be lost. The Win32 ReleaseCapture function turns off mouse capture.

void CEx05cView::OnLButtonUp(UINT nFlags, CPoint point)
{
    if (m_bCaptured) {
        ::ReleaseCapture();
        m_bCaptured = FALSE;
    }
}

4.The m_sizeOffset Data Member

When OnMouseMove moves the ellipse, the relative position of the mouse within the ellipse must be the same as it was when the user first pressed the left mouse button. The m_sizeOffset object stores this original offset of the mouse from the top left corner of the ellipse rectangle.