VC中为工具栏创建下拉式按扭

实现步骤:
1、toolbar在cmainframe::oncreate()中被创建之后，扩展toolbar的样式使其能够映射箭头按钮的消息。
dword dwexstyle = tbstyle_ex_drawddarrows;
m_wndtoolbar.gettoolbarctrl().sendmessage(tb_setextendedstyle, 0, (lparam)dwexstyle);     

2、将箭头加到指定的按钮中，通过setbuttonstyle()方法实现：
dword dwstyle = m_wndtoolbar.getbuttonstyle(m_wndtoolbar.commandtoindex(id_sel_mode));
dwstyle |= tbstyle_dropdown;
m_wndtoolbar.setbuttonstyle(m_wndtoolbar.commandtoindex(id_sel_mode), dwstyle);
m_wndtoolbar.setbuttonstyle(m_wndtoolbar.commandtoindex(id_view_mode), dwstyle);

3、加入箭头按钮的消息映射：
begin_message_map(cmainframe, cmdiframewnd)
         on_notify(tbn_dropdown, afx_idw_toolbar, ontoolbardropdown)
end_message_map()

声明消息响应函数
protected:
afx_msg void ontoolbardropdown(nmtoolbar* pnmh, lresult* plres);

消息响应函数：
void cmainframe::ontoolbardropdown(nmtoolbar* pnmtb, lresult *plr)
{
    cwnd *pwnd;
    uint nid;

    // switch on button command id's.
    switch (pnmtb->iitem)
    {
    case id_sel_mode:
        pwnd = &m_wndtoolbar;
        nid  = idr_sel_mode;
        break;

 case id_view_mode:
  pwnd = &m_wndtoolbar;
  nid = idr_view_mode;
  break;

    default:
        return;
    }
   
    // load and display popup menu
    cmenu menu;
    menu.loadmenu(nid);
    cmenu* ppopup = menu.getsubmenu(0);
    assert(ppopup);
   
    crect rc;
    pwnd->sendmessage(tb_getrect, pnmtb->iitem, (lparam)&rc);
    pwnd->clienttoscreen(&rc);
   
    ppopup->trackpopupmenu( tpm_leftalign | tpm_leftbutton | tpm_vertical,
        rc.left, rc.bottom, this, &rc);
}