[Feature] TokenizingTextBox - Make TokenItemAdding event to be invoked even if the adding object is not a string to allow pre-check and cancellation

[kaismic]

Describe the problem

Currently, TokenizingTextBox does not fire TokenItemAdding event when the data parameter in AddTokenAsync(object data) is not a string.

So when the user selects the already added item from SuggestedItemsSource, only QuerySubmitted event is invoked and I cannot prevent the duplicate item adding.

Thus, allowing TokenItemAdding to be invoked even if the data parameter is not a string would allow me to prevent duplicate items adding by checking the adding object and cancelling the event.

Describe the solution

Modify AddTokenAsync(object data)

        if (TokenItemAdding != null)
        {
            TokenItemAddingEventArgs tiaea;
            if (data is string str)
            {
                tiaea = new TokenItemAddingEventArgs(str);
            }
            else
            {
                tiaea = new TokenItemAddingEventArgs(null) { Item = data };
            }
            await TokenItemAdding.InvokeAsync(this, tiaea);
            ...
        }

Alternatives

1. Create new TokenizingTextBoxQuerySubmittedEventArgs class and either remove sealed from AutoSuggestBoxQuerySubmittedEventArgs and derive from it like this:

   public class TokenizingTextBoxQuerySubmittedEventArgs : AutoSuggestBoxQuerySubmittedEventArgs
   {
       public bool Cancel { get; set; }
   }

   Or just create a similar class with additional Cancel property:

   public class TokenizingTextBoxQuerySubmittedEventArgs
       {
       public object ChosenSuggestion { get; }
       public string QueryText { get; }
       public bool Cancel { get; set; } = false;
   }

2. Create a new TokenizingTextBoxQuerySubmitted event in TokenizingTextBox.Events.cs

public event TypedEventHandler<AutoSuggestBox, TokenizingTextBoxQuerySubmittedEventArgs> TokenizingTextBoxQuerySubmitted;

3. Modify RaiseQuerySubmitted

    internal void RaiseQuerySubmitted(AutoSuggestBox sender, AutoSuggestBoxQuerySubmittedEventArgs 
 args, TokenizingTextBoxQuerySubmittedEventArgs args)
    {
        TokenizingTextBoxQuerySubmitted?.Invoke(sender, ttbArgs);
        QuerySubmitted?.Invoke(sender, args);
    }

4. Modify AutoSuggestBox_QuerySubmitted

   private async void AutoSuggestBox_QuerySubmitted(AutoSuggestBox sender, AutoSuggestBoxQuerySubmittedEventArgs args)
   {
       var ttbArgs = new TokenizingTextBoxQuerySubmittedEventArgs()
       {
           ChosenSuggestion = args.ChosenSuggestion,
           QueryText = args.QueryText
       };
       Owner.RaiseQuerySubmitted(sender, args, ttbArgs);
       if (ttbArgs.Cancel)
       {
           return;
       }
       object? chosenItem = null;
       ....
   }

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