Question

When using ATL, what is the best way to display the progress of a background thread (e.g. when it’s searching for a file) without blocking the UI?

I still want to be able to process messages, to allow for a Cancel button and to possibly allow the user to continue working with the program while the search happens.

Answer 1

There is no ATL specific here. One of the ways to do is to update progress details into member variable and post a message to GUI window, then handle the message by pulling the data from member variable and updating GUI, such as updating static and/or progress bar.

Worker thread pseudo-code:

m_DataCriticalSection.Lock();
m_nProgress = (INT) (nCurrent * 100 / nTotal);
m_DataCriticalSection.Unlock();
PostMessage(WM_MYUPDATEPROGRESS);

Window:

OnMyUpdateProgress()
{
  m_DataCriticalSection.Lock();
  INT nProgress = m_nProgress;
  m_DataCriticalSection.Unlock();
  m_ProgressBar.SetPos(nProgress);
}

UPD. A real code snippet, AddText is called on background thread, :

    VOID AddText(const CString& sText)
    {
            _A(sText.Find(_T('\n')) < 0);
            BOOL bIsTextEmpty;
            {
                    CRoCriticalSectionLock TextLock(m_TextCriticalSection);
                    bIsTextEmpty = m_sText.IsEmpty();
                    m_sText.Append(sText);
                    m_sText.Append(_T("\r\n"));
            }
            if(bIsTextEmpty)
                    PostPrivateMessage(WM_UPDATETEXT);
    }

And the code handler:

BEGIN_MSG_MAP_EX(CMainDialog)
        // ...
        MESSAGE_HANDLER_EX(WM_UPDATETEXT, OnUpdateText)

    LRESULT OnUpdateText(UINT, WPARAM, LPARAM)
    {
            CString sText;
            {
                    CRoCriticalSectionLock TextLock(m_TextCriticalSection);
                    sText = m_sText;
                    m_sText.Empty();
            }
            if(!sText.IsEmpty())
            {
                    m_TextEdit.SetValue(m_TextEdit.GetValue() + sText);
                    const INT nTextLength = m_TextEdit.GetWindowTextLength();
                    m_TextEdit.SetSel(nTextLength, nTextLength);
            }
            return 0;
    }

This uses custom classes (not ‘pure’ ATL), but I hope you get the idea.