/// <summary>
/// Creates a PDF document with calculated interactive form field.
/// </summary>
/// <param name="filename">The filename.</param>
public static void CreateDocumentWithCalculatedInteractiveFormField(string filename)
{
    // create PDF document
    using (Vintasoft.Imaging.Pdf.PdfDocument document = new Vintasoft.Imaging.Pdf.PdfDocument())
    {
        // create interactive form in PDF document
        document.InteractiveForm = 
            new Vintasoft.Imaging.Pdf.Tree.InteractiveForms.PdfDocumentInteractiveForm(document);
            
        // specify that the viewer application must construct appearance streams and
        // appearance properties for all widget annotations
        document.InteractiveForm.NeedAppearances = true;
            
        // font and font size
        float fontSize = 12;
        Vintasoft.Imaging.Pdf.Tree.Fonts.PdfFont font = 
            document.FontManager.GetStandardFont(Vintasoft.Imaging.Pdf.Tree.Fonts.PdfStandardFontType.TimesRoman);
            
        // create an empty page
        Vintasoft.Imaging.Pdf.Tree.PdfPage page = new Vintasoft.Imaging.Pdf.Tree.PdfPage(
            document, Vintasoft.Imaging.PaperSizeKind.A4);
        // add page to the document
        document.Pages.Add(page);
            
        float width = 60;
        float height = 40;
        // create a rectangle that defines the object (first field) position on PDF page
        System.Drawing.RectangleF rect = new System.Drawing.RectangleF(
            (page.Size.Width - width * 5) / 2,
            ((page.Size.Height - height) / 3) * 2,
            width, height);
            
        // open PdfGraphics on page
        using (Vintasoft.Imaging.Pdf.Drawing.PdfGraphics graphics = page.GetGraphics())
        {
            // create calculator field: group of three fields
            Vintasoft.Imaging.Pdf.Tree.InteractiveForms.PdfInteractiveFormField calculator =
                new Vintasoft.Imaging.Pdf.Tree.InteractiveForms.PdfInteractiveFormField(document, "Calculator");
            calculator.SetTextDefaultAppearance(font, fontSize * 1.5f, System.Drawing.Color.Black);
            calculator.TextQuadding = Vintasoft.Imaging.Pdf.Tree.InteractiveForms.TextQuaddingType.Centered;
            
            // create the border style
            Vintasoft.Imaging.Pdf.Tree.Annotations.PdfAnnotationBorderStyle borderStyle = 
                new Vintasoft.Imaging.Pdf.Tree.Annotations.PdfAnnotationBorderStyle(document);
            borderStyle.Style = Vintasoft.Imaging.Pdf.Tree.Annotations.PdfAnnotationBorderStyleType.Inset;
            borderStyle.Width = 1;
            
            // create the appearance characteristics
            Vintasoft.Imaging.Pdf.Tree.Annotations.PdfAnnotationAppearanceCharacteristics appearanceCharacteristics =
                new Vintasoft.Imaging.Pdf.Tree.Annotations.PdfAnnotationAppearanceCharacteristics(document);
            appearanceCharacteristics.BorderColor = System.Drawing.Color.Gray;
            
            // create the left text box
            Vintasoft.Imaging.Pdf.Tree.InteractiveForms.PdfInteractiveFormTextField textField =
                new Vintasoft.Imaging.Pdf.Tree.InteractiveForms.PdfInteractiveFormTextField(document, "Left", rect, "2");
            textField.DefaultValue = textField.Value;
            textField.Annotation.BorderStyle = borderStyle;
            textField.Annotation.AppearanceCharacteristics = appearanceCharacteristics;
            // add the left text box to the calculator
            calculator.Kids.Add(textField);
            
            // update the rectangle that defines the object position on PDF page
            rect.X += rect.Width;
            
            // draw the symbol '+' on the page
            graphics.DrawString("+", font, fontSize * 1.5f,
                new Vintasoft.Imaging.Pdf.Drawing.PdfBrush(System.Drawing.Color.Black), rect, 
                Vintasoft.Imaging.Pdf.Drawing.PdfContentAlignment.Center, false);
            
            // update the rectangle that defines the object position on PDF page
            rect.X += rect.Width;
            
            // create the right text box
            textField = new Vintasoft.Imaging.Pdf.Tree.InteractiveForms.PdfInteractiveFormTextField(document, "Right", rect, "3");
            textField.DefaultValue = textField.Value;
            textField.Annotation.BorderStyle = borderStyle;
            textField.Annotation.AppearanceCharacteristics = appearanceCharacteristics;
            // add the right text box to the calculator
            calculator.Kids.Add(textField);
            
            // update the rectangle that defines the object position on PDF page
            rect.X += rect.Width;
            
            // draw the symbol '=' on the page
            graphics.DrawString("=", font, fontSize * 1.5f,
                new Vintasoft.Imaging.Pdf.Drawing.PdfBrush(System.Drawing.Color.Black), rect, 
                Vintasoft.Imaging.Pdf.Drawing.PdfContentAlignment.Center, false);
            
            // create the calculator program written on JavaScript
            System.Text.StringBuilder javaScriptCode = new System.Text.StringBuilder();
            javaScriptCode.Append("var left = this.getField('Calculator.Left');");
            javaScriptCode.Append("var right = this.getField('Calculator.Right');");
            javaScriptCode.Append("var result = this.getField('Calculator.Result');");
            javaScriptCode.Append("result.value = left.value + right.value;");
            Vintasoft.Imaging.Pdf.Tree.PdfJavaScriptAction calculatorProgram =
                new Vintasoft.Imaging.Pdf.Tree.PdfJavaScriptAction(document, javaScriptCode.ToString());
            
            // update the rectangle that defines the object position on PDF page
            rect.X += rect.Width;
            
            // create the result text box
            textField = new Vintasoft.Imaging.Pdf.Tree.InteractiveForms.PdfInteractiveFormTextField(document, "Result", rect);
            textField.IsReadOnly = true;
            textField.Annotation.BorderStyle = borderStyle;
            textField.Annotation.AppearanceCharacteristics = appearanceCharacteristics;
            // set a program that will calculate value of result field
            textField.AdditionalActions = 
                new Vintasoft.Imaging.Pdf.Tree.InteractiveForms.PdfInteractiveFormFieldAdditionalActions(document);
            textField.AdditionalActions.Calculate = calculatorProgram;
            // specify that calcualtor program must be executed when page is opened
            textField.Annotation.AdditionalActions.PageOpen = calculatorProgram;
            // add the result text box to the calculator
            calculator.Kids.Add(textField);
            
            // add result field to the calculated fields (calcualtion order) 
            // of the document interactive form fields
            document.InteractiveForm.CalculationOrder = 
                new Vintasoft.Imaging.Pdf.Tree.InteractiveForms.PdfInteractiveFormFieldList(document);
            document.InteractiveForm.CalculationOrder.Add(textField);
            
            // add field group to the document
            document.InteractiveForm.Fields.Add(calculator);
            
            // add annotations, associated with field group, to the page
            page.Annotations = new Vintasoft.Imaging.Pdf.Tree.Annotations.PdfAnnotationList(document);
            page.Annotations.AddRange(calculator.GetAnnotations());
            
            // update appearance of field group
            calculator.UpdateAppearance();
        }
            
        // save the document
        document.Save(filename);
    }
}