/// <summary>
/// Checks printer(s) for notifications.
/// </summary>
private void ListenForChanges()
{
// Check for cancellation.
_cancelSource.Token.ThrowIfCancellationRequested();
SafePrinterChangeNotificationHandle hChange = null;
try
{
hChange = WaitForMultipleObjects(_hChanges);
}
catch (TimeoutException)
{
// No object changed within the timeout period.
// This could mean that there was really no change,
// or because the spooler is no longer sending updates to us.
// Since we can't tell the difference, clean up existing structures and sign up for events again.
// If the cancel token has been set, it might be a bit before we get back to checking it again,
// so check it here before resetting. If it has been set, bail out so the monitor can stop.
_cancelSource.Token.ThrowIfCancellationRequested();
LogDebug("No job changes detected. Resetting listener.");
Reset();
Initialize();
return;
}
catch (ArgumentOutOfRangeException ex)
{
// Unsuccessful in getting the change notification handle. Bail out.
LogDebug(ex);
return;
}
// WaitForMultipleObjects could have taken a while, so check for cancellation again.
_cancelSource.Token.ThrowIfCancellationRequested();
// Get the notify information that changed.
PrinterNotifyInfoReader infoReader = FindNextPrinterChangeNotification(hChange);
if (infoReader == null)
{
LogDebug("Lost job data. Trying a refresh.");
infoReader = FindNextPrinterChangeNotification(hChange, _options);
}
using (infoReader)
{
foreach (PrinterNotifyInfoData infoData in infoReader.ReadInfoData())
{
if (infoData.NotifyType == NotifyType.Job)
{
JobNotificationReceived?.Invoke(this, new JobNotificationEventArgs(infoData));
}
}
}
}
/// <summary>
/// Retrieves information about the most recent notification for a change notification object associated with a printer or print server.
/// Call this function when a wait operation on the change notification object is satisfed.
/// </summary>
/// <param name="hChange">A handle to a change notification object associated with a printer or print server.</param>
/// <param name="options">The <see cref="PrinterNotifyOptions" />.</param>
/// <returns>A <see cref="PrinterNotifyInfoReader" /> containing the notification details.</returns>
/// <exception cref="Win32Exception">The underlying operation failed.</exception>
/// <exception cref="System.Exception">
/// No notification available at this time.
/// or
/// Info has been discarded.
/// </exception>
private static PrinterNotifyInfoReader FindNextPrinterChangeNotification(SafePrinterChangeNotificationHandle hChange, PrinterNotifyOptions options = new PrinterNotifyOptions())
{
if (!NativeMethods.FindNextPrinterChangeNotification(hChange, out _, ref options, out SafePrinterNotifyInfoHandle infoHandle))
{
throw new Win32Exception();
}
if (infoHandle == null || infoHandle.IsInvalid)
{
// Apparently there was no notification after all. Just return an empty reader.
return(new PrinterNotifyInfoReader());
}
PrinterNotifyInfoReader result = new PrinterNotifyInfoReader(infoHandle);
// Check to see if data has been discarded.
if (result.InfoDiscarded)
{
result.Dispose();
return(null);
}
return(result);
}