在 std::thread 中打开 MFC 对话框

Question

I would like to show a dialog to inform the user that the application is busy. To avoid blocking of the main thread, I was thinking to use a std::thread to show the dialog. Consider the following code:

InProcDlg inProcess;
std::thread t([ &inProcess ] {      
    inProcess.DoModal();
    delete inProcess;
});
// wait till process has finished 
::PostMessage(inProcess.m_hWnd, WM_USER + 1, 0, 0);
if (t.joinable()){
    t.join();
}

InProcDlg.cpp

BEGIN_MESSAGE_MAP(InProcDlg, CDialogEx)
  ...
  ON_MESSAGE(WM_USER + 1, &InProcDlg::close)
END_MESSAGE_MAP()

LRESULT InProcDlg::close(WPARAM wParam, LPARAM lParam)
{
  UNREFERENCED_PARAMETER(wParam, lParam);
  EndDialog(1);
  return 0;
}

Running this code the dialog is shown properly. The dialog is also closed, but the main dialog is not shown, the application hangs in CreateRunDlgIndirect() . Trying to step in, while setting some breakpoints the main dialog is shown properly back again. Very strange. I would be very happy for any advices where I have to dive deeper in.

In the next step I would also like to show the process to the user, by sending an integer indicating the current state of process.

int *percent;
::PostMessage(inProcess.m_hWnd, WM_USER + 2, 0, reinterpret_cast<LPARAM>(percent));

How I can gain evidence that the dialog is already existing, before sending or posting a message? I'm using Visual Studio 2013.

Answer 1

I can think of two ways to do that:

Modeless dialog

https://www.codeproject.com/Articles/1651/Tutorial-Modeless-Dialogs-with-MFC

User thread (UI thread)

Creating a brother to the main UI thread (CWinApp) by using CWinThread. Most important is to assign CWinThread::m_pMainWnd member, with a pointer to a Dialog. If the dialog is Modal you return FALSE right after the call to DoModal, and return TRUE for Modeless.

class CMainFrame : public CFrameWnd {
    // pointer to thread
    CWinThread* m_pUserThread;
}

start thread

m_pUserThread = AfxBeginThread(RUNTIME_CLASS(CUserThread), THREAD_PRIORITY_NORMAL, 0, CREATE_SUSPENDED );
m_pUserThread->m_bAutoDelete = TRUE;
m_pUserThread->ResumeThread();

headr file**

class CUserThread : public CWinThread
{
    DECLARE_DYNCREATE(CUserThread)
public:
    CUserThread();

    // ClassWizard generated virtual function overrides
    //{{AFX_VIRTUAL(CUserThread)
    public:
    virtual BOOL InitInstance();
    virtual int ExitInstance();
    //}}AFX_VIRTUAL

protected:
    virtual ~CUserThread();
    // Generated message map functions
    //{{AFX_MSG(CUserThread)
        // NOTE - the ClassWizard will add and remove member functions here.
    //}}AFX_MSG
    DECLARE_MESSAGE_MAP()
private:
    CUserMsg* m_pDlg;
}

source file

#define DO_MODAL
BOOL CUserThread::InitInstance()
{

    #ifdef DO_MODAL
    // create and assign Modeless dialog
    CUserMsg dlg;
    m_pMainWnd = &m_pDlg;
    dlg.DoModal();
    return FALSE;
    #endif

    #ifdnef DO_MODAL
    // create and assign Modal dialog
    m_pDlg = new CUserMsg();
    m_pDlg->Create( IDD_USER_DLG, AfxGetMainWnd() );
    m_pMainWnd = m_pDlg;
    return TRUE;
    #endif
}

int CUserThread::ExitInstance()
{
    // check for null
    m_pDlg->SendMessage( WM_CLOSE ) ;
    delete m_pDlg;

    return CWinThread::ExitInstance();
}

to terminate the thread

// post quit message to thread
m_pUserThread->PostThreadMessage(WM_QUIT,0,0);

// wait until thread termineates
::WaitForSingleObject(m_pUserThread->m_hThread,INFINITE);

For both ways I would highly recommend making the dialog as a top most window:

BOOL CLicenseGenDlg::OnInitDialog() {
    CDialog::OnInitDialog();

    // TODO: Add extra initialization here
    SetWindowPos( &wndTopMost, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE | WS_EX_TOPMOST );
    return TRUE;
}