How can I show a modeless dialog and display information in it immediately?

Question

I want to show a modeless dialog on the screen and display some information in it.

However if I use it the following way, it has some problems:

function()
{
showdialog(XXX).
//heavy work.
update the dialog..
//heavy work.
update the dialog...
}

It seems the dialog displayed, but it does not draw any information in it. It only draw all information when the function is over.

How can I modify the modeless dialog so it will display the information immediately?

Answer 1

There are a few things you can do.

(1) You could post the dialog a message from inside the CDialog::OnInitDialog method and then handle the long function in the message handler of that posted message. That way the dialog will first be displayed and then later the long function will get run.

(2) The second option is to make sure the message loop gets some processing time. So if your long function is some sort of loop just add the occasional call to the ProcessMessages to make sure the message queue is kept empty:

void ProcessMessages()
{
    MSG msg;
    CWinApp* pApp = AfxGetApp();
    while (PeekMessage(&msg, NULL, 0, 0, PM_NOREMOVE))
    {
        pApp->PumpMessage();
    }
}

Edit: It certainly is possible to use threads is such a situation, but doing so is not always without risk and complexity.

Using threads with a GUI means having to deal with multiple message queues which then means using API's like PostThreadMessage and that introduces a new set of issues to be wary of.

For an example of one such issue refer to this link:

http://msdn.microsoft.com/en-us/library/ms644946(VS.85).aspx

where is says:

Messages sent by PostThreadMessage are not associated with a window. As a general rule, messages that are not associated with a window cannot be dispatched by the DispatchMessage function. Therefore, if the recipient thread is in a modal loop (as used by MessageBox or DialogBox), the messages will be lost. To intercept thread messages while in a modal loop, use a thread-specific hook.

I use the process message approach in the Zeus IDE and it works very well at making sure the GUI remains responsive to the user. It is also has the advantage of being very easy to implement.