Added transfercanceled event like PR #35.

src/NTwain/TransferCanceledEventArgs.cs

@@ -0,0 +1,10 @@
+using NTwain.Data;
+using System;
+
+namespace NTwain
+{
+  public class TransferCanceledEventArgs : EventArgs
+  {
+
+  }
+}

src/NTwain/TwainAppSession.PropEvents.cs

@@ -156,6 +156,11 @@ namespace NTwain
     /// </summary>
     public event TwainEventDelegate<TransferReadyEventArgs>? TransferReady;
 
+    /// <summary>
+    /// Fires when there's a transfer cancellation, e.g. if the user pressed the "Cancel" button.
+    /// </summary>
+    public event TwainEventDelegate<TransferCanceledEventArgs>? TransferCanceled;
+
     /// <summary>
     /// Fires when transferred data is available for app to use.
     /// </summary>

src/NTwain/TwainAppSession.Xfers.cs

@@ -176,6 +176,14 @@ namespace NTwain
           // ok to keep going
           break;
         case TWRC.CANCEL:
+          // might eventually have option to cancel this or all like transfer ready
+          if (TransferCanceled != null)
+          {
+            _uiThreadMarshaller.Invoke(() =>
+            {
+              TransferCanceled.Invoke(this, new TransferCanceledEventArgs());
+            });
+          };
           TW_PENDINGXFERS pending = default;
           DGControl.PendingXfers.EndXfer(ref _appIdentity, ref _currentDS, ref pending);
           // todo: also reset?