Page rating	93% for 3 votes	Useless Brilliant

Description.

The basic idea is to create an event that is attached to a registry key. When the key changes the event is triggered. It is then a simple task to poll the event to see if it has signalled and act upon this value.

How to do it.

    // Open a key.
    TCHAR szSubKey[] = _T("Software\\Microsoft\\Clock");
    HKEY   hKey;          // Key to the registery item to watch
    LONG   lErrorCode;    // Error code catcher
    lErrorCode = RegOpenKeyEx(HKEY_CURRENT_USER, 
                              szSubKey, 0, KEY_NOTIFY, &hKey);
    if( lErrorCode != ERROR_SUCCESS )
    {
        TRACE( _T("Error in RegOpenKeyEx.\n") );
        return false;
    }
    
    // Create an event.
    HANDLE hEvent;        // Event output for change
    hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
    if( hEvent == NULL )
    {
        TRACE( _T("Error in CreateEvent.\n") );
        return false;
    }
    

    // Watch the registry key for a change of value.
    DWORD  dwFilter = REG_NOTIFY_CHANGE_NAME |
                      REG_NOTIFY_CHANGE_ATTRIBUTES |
                      REG_NOTIFY_CHANGE_LAST_SET |
                      REG_NOTIFY_CHANGE_SECURITY; 

    lErrorCode = RegNotifyChangeKeyValue(
        hKey,            // handle to key to watch
        TRUE,            // subkey notification option
        dwFilter,        // changes to be reported
        hEvent,          // handle to event to be signaled
        TRUE);           // asynchronous reporting option

    if( lErrorCode != ERROR_SUCCESS )
    {
        TRACE( _T("Error in RegNotifyChangeKeyValue.\n") );
        return false;
    }

    // Check if the event has occured.
    switch( WaitForSingleObject( hEvent, 0 ) )
    {
    // Reg item has changed
    case WAIT_OBJECT_0:
        break;
    // Reg item has NOT changed
    case WAIT_TIMEOUT:
        break;
    // This an error condition that should not happen
    // if it does we best reopen the event and key.
    default:
        TRACE( _T("Error in WaitForSingleObject.\n") );
    }

    // Close the key.
    lErrorCode = RegCloseKey(hKey);
    if( lErrorCode != ERROR_SUCCESS )
        TRACE( _T("Error in RegCloseKey.\n") );
    
    // Close the handle.
    if( !CloseHandle( hEvent ) )
        TRACE( _T("Error in CloseHandle.\n") );