public DocumentEditorViewModel(IDocumentEditor documentEditor, IExporter exporter)
        {
            Ensure.ArgumentNotNull(documentEditor, "documentEditor");
            Ensure.ArgumentNotNull(exporter, "exporter");

            _documentEditor = documentEditor;
            _fontSize = DefaultFontSize;
            _documentEditor.New();
            _document = new TextDocument();
            _exporter = exporter;
        }