Log out the undo and redo stack on an NSUndoManager using objc/runtime to access private ivars

logUndoStack.m

      @try
      {
         NSUndoManager* undoManager = [self undoManager];
        
         id undostack = object_getIvar(undoManager, class_getInstanceVariable([NSUndoManager class], "_undoStack"));
         id redostack = object_getIvar(undoManager, class_getInstanceVariable([NSUndoManager class], "_redoStack"));
         
         NSLog(@"%@",[NSString stringWithFormat:@"(%lu entries) %@", (unsigned long)[undostack count], [undostack description]]);
         NSLog(@"%@",[NSString stringWithFormat:@"(%lu entries) %@", (unsigned long)[redostack count], [redostack description]]);
      }
      @catch (NSException* e)
      {
      }

For Swift:

    let undoManager = NSDocumentController.shared().currentDocument?.undoManager
    let undostack = object_getIvar(undoManager, class_getInstanceVariable(UndoManager.self, "_undoStack"));
    print(undostack.debugDescription)