C ++事件信号和后台线程

Question

Here is the issue that I have in c++ code implementation.

In the main thread , created a dialogbox showing static text with cancel button which also spins a child thread.

In the back ground thread ( or the child thread) check the database to see if the certain status field is updated. If updated, then return true, otherwise continue to poll the database at regular intervals.

Expected behavior - Show dialog box with cancel button , continue showing it until the event is signaled by child thread or the user hits cancel button.

I created an event in the main thread with unsignalled state , this event state will be modified by the child tread ( when the status field is updated ).

The problem is if i block the main thread until it gets response from the background thread, the user cannot hit the cancel button in the dialog , it is always accompanied with hour glass symbol.

Not sure what is wrong with the below code.

HANDLE mainThread = NULL;
HANDLE ghWriteEvent; 

MainMethod()
{
mainThread = GetCurrentThread() ;
dlgCancelDialog dog // dialog with cancel button .

    ghWriteEvent = CreateEvent( NULL, TRUE,  FALSE,  TEXT("WriteEvent") ); 
HANDLE hThread = CreateThread(0,0, childThread, &threadData,0,NULL);
dlg.showDialog(); // Show dialog with cancel button .

    DWORD  dwWaitResult = WaitForSingleObject( ghWriteEvent, INFINITE); 
 //AT THIS POINT i WANT TO SAY WAIT FOR SIGNAL EVENT OR WAIT UNTIL CANCEL OPERATION IS HIT , BUT THE DIALOG IS SHOWN WITH HOURGLASS SYMBOL.
  switch (dwWaitResult) 
   {
    // Event object was signaled
    case WAIT_OBJECT_0: 
   break;
  }
dlg.hideDialog();

} //Child thread code.

childThread(LPVOID lpParam)
{
while(!databaseIsUpdated) // Check database.
{   
    Sleep(1000);

}
if (! SetEvent(ghWriteEvent) ) 
    {

        return 0;
    }
return 0;
}

Answer 1

You should not wait with INFINITE because you block your dialog thread.

So my proposal is, create timer using SetTimer(hDlg, 500, 1, NULL) and handle WM_TIMER inside DlgProc like this:

case WM_TIMER:
{
 DWORD  dwWaitResult = WaitForSingleObject( ghWriteEvent, 0);  //We will not wait

 if(dwWaitResult  == WAIT_OBJECT_0)  //We are signaled, exit now
    dlg.hideDialog();
}
break;

This will check every 500ms state of event.If signaled, dialog ends, if not, continue.

Answer 2

你也可以使用MsgWaitForMultipleObjectsEx（）： http://msdn.microsoft.com/en-us/library/windows/desktop/ms684245%28v=vs.85%29.aspx