Invalidates the entire client area.
這句話說的很有意思，就是使無效就是重繪。
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;
}