win32 DialogBox app: how to show text from callback on the DialogBox?

Question

I'm working on a win32 DialogBox based app. This uses DialogBox() to create the dialog box, and has a dialog box procedure which does all the usual things.

The dialog box has some static text controls on it, and generally I'm showing text in these via SendDlgItemMessage() from within the dialog box procedure.

However at one point the DialogBox initiates a lengthy operation. This operation has a callback with a series of status messages. I'm having some trouble showing these messages on the dialog box, for two reasons:

1. The callback function doesn't know what the dialog box HWND is, because it gets called from the code which carries out the lengthy operation. I suppose I can define a file scope HWND variable and copy the dialog box HWND into it from the dialog box procedure just before the lengthy operation is started. That way, the callback function could have access to the dialog box HWND. But that seems awfully kludgy: is there a more elegant way?

2. The dialog box procedure is blocked while the lengthy operation happens. This doesn't matter because it's an embedded system. But will Windows even show the text in the dialog box if I issue a SendDlgItemMessage() while the dialog box procedure is blocked?

edit I've done some investigations using SendDlgItemMessage() to send a WM_SETTEXT to a static text control on a dialog. The text is displayed immediately even if the dialog box procedure is blocked.

Answer 1

Well, your dialog HWND is a singleton so it isn't the end of the world. But yes, the standard way this is done is by passing an opaque pointer to the code that gets the job done. Compare with the lParam argument of EnumWindows() for example, the callback gets that pointer back.

Whether a control repaints itself immediately is an implementation detail. I only know of progress bar doing this. You could call UpdateWindow on the dialog window handle to get any pending paint updates flushed to the screen.

The all-around better mouse trap is to perform long running tasks on a worker thread. Avoids Windows displaying the "Not Responding" ghost window, avoids timeouts on broadcast messages and numerous potential deadlock problems. But tends to be tricky to get right, you cannot update the window directly from the worker thread.