SimpleNote/MainWindow.axaml.cs

using System;
using System.ComponentModel;
using Avalonia;
using Avalonia.Controls;
using System.Reactive.Linq;
using Avalonia.Media;

namespace SimpleNote;

public partial class MainWindow : Window
{
    private MainViewModel ViewModel;
    public MainWindow(string[]? args)
    {
        if (args == null || args.Length < 1)
        {
            Console.WriteLine("Call with Note as Argument!");
            Environment.Exit(1);
        }
        InitializeComponent();
        ViewModel = new MainViewModel();
        DataContext = ViewModel;

        this.GetObservable(Window.WidthProperty)
            .Subscribe(width =>
            {
                if (DataContext is MainViewModel vm)
                {
                    vm.WindowWidth = (double)width;
                }
            });

        init_TextField(args[0]);
    }

    public void init_TextField(string text)
    {
        var textBlock = new TextBlock
        {
            Text = text,
            FontSize = ViewModel.FontSize,
            FontFamily = new FontFamily(FontFamily.DefaultFontFamilyName)
        };
        textBlock.Measure(Size.Infinity);
        ViewModel.WindowWidth = textBlock.DesiredSize.Width + 50;
        Note.Text = text;
    }
}

public class MainViewModel : INotifyPropertyChanged
{
    private double _windowWidth = 200;
    private double _fontSize = 24;

    public double WindowWidth
    {
        get => _windowWidth;
        set
        {
            _windowWidth = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(WindowWidth)));
        }
    }

    public double FontSize
    {
        get => _fontSize;
        set
        {
            _fontSize = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(FontSize)));
        }
    }

    public event PropertyChangedEventHandler? PropertyChanged;
}