Invalidates the entire client area.
这句话说的很有意思，就是使无效就是重绘。
void CBounceWnd::OnLButtonDown(UINT, CPoint point)
{
Invalidate();
CRect rcClient;
GetClientRect(rcClient);
point.x = min(point.x, rcClient.right - 2*m_sizeRadius.cx);
point.x = max(point.x, 2*m_sizeRadius.cx);
point.y = min(point.y, rcClient.bottom - 2*m_sizeRadius.cx);
point.y = max(point.y, 2*m_sizeRadius.cy);
m_ptCenter = point;
}