How does MFC's "Update Command UI" system work?

Question

I'd like to know more about how this system works, specifically when and how the framework actually decides to update a UI element.

My application has a 'tools' system where a single tool can be active at a time. I used the "ON_UPDATE_COMMAND_UI" message to 'check' the tool's icon/button in the UI, which affected both the application menu and the toolbars. Anyway, this was all working great until some point in the last couple of days, when the toolbar icons stopped getting highlighted properly.

I investigated a little and found that the update command was only being received when the icon was actually clicked. What's strange is this is only affecting the toolbars, not the menu, which is still working fine. Even when the buttons in the menu are updated the toolbar icon stays the same.

Obviously I've done something to break it - any ideas?

EDIT: Never mind. I'd overwritten the Application's OnIdle() method and hadn't called the original base class method - that is, CWinApp::OnIdle() - which I guess is where the update gets called most of the time. This code snippet from https://msdn.microsoft.com/en-us/library/3e077sxt.aspx illustrates:

BOOL CMyApp::OnIdle(LONG lCount)
{
   // CWinApp's original method is involved in the update message handling!
   // Removing this call will break things
   BOOL bMore = CWinApp::OnIdle(lCount);

   if (lCount == 0)
   {
      TRACE(_T("App idle for short period of time\n"));
      bMore = TRUE;
   }
   // ... do work

   return bMore;
   // return TRUE as long as there are any more idle tasks
}

Answer 1

Here's a good article that kinda explains how to do it. Don't use his code example with WM_KICKIDLE though, instead scroll down to the comments section. There are two code samples that explain how to do it better. I quote:

//Override WM_INITMENUPOPUP
void CDialog::OnInitMenuPopup(CMenu* pPopupMenu, UINT nIndex, BOOL bSysMenu)
{
    CDialog::OnInitMenuPopup(pPopupMenu, nIndex, bSysMenu);

    // TODO: Add your message handler code here
    if(pPopupMenu && 
        !bSysMenu)
    {
        CCmdUI CmdUI;
        CmdUI.m_nIndexMax = pPopupMenu->GetMenuItemCount();

        for(UINT i = 0; i < CmdUI.m_nIndexMax; i++)
        {
            CmdUI.m_nIndex = i;
            CmdUI.m_nID = pPopupMenu->GetMenuItemID(i);
            CmdUI.m_pMenu = pPopupMenu;

            // There are two options:

            // Option 1. All handlers are in dialog
            CmdUI.DoUpdate(this, FALSE);

            // Option 2. There are handlers in dialog and controls
/*
            CmdUI.DoUpdate( this, FALSE );

            // If dialog handler doesn't change state route update
            // request to child controls. The last DoUpdate will
            // disable menu item with no handler
            if( FALSE == CmdUI.m_bEnableChanged )
                CmdUI.DoUpdate( m_pControl_1, FALSE );

            ...

            if( FALSE == CmdUI.m_bEnableChanged )
                CmdUI.DoUpdate( m_pControl_Last, TRUE );
*/
        }       
    }
}