SimpleDraw/Views/MainWindow.axaml.cs

using System;
using System.IO;
using System.Threading.Tasks;
using Avalonia;
using Avalonia.Controls;
using Avalonia.Input;
using Avalonia.Interactivity;
using Avalonia.Media.Imaging;
using Avalonia.Platform.Storage;
using SimpleDraw.ViewModels;

namespace SimpleDraw.Views;

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        DataContext = new MainWindowViewModel();
        Topmost = true;
        Topmost = false;
    }

    // MVVM like implementations burst the bounds -> implemented the old school way
    private async void SaveFileButton_Clicked(object sender, RoutedEventArgs args)
    {
        TopLevel? topLevel = TopLevel.GetTopLevel(this);
        if(topLevel == null) return;

        var file = await topLevel.StorageProvider.SaveFilePickerAsync(new FilePickerSaveOptions
        {
            Title = "Save Drawing",
            DefaultExtension = ".png",
            SuggestedFileName = "unnamed",
            SuggestedStartLocation = await topLevel.StorageProvider.TryGetFolderFromPathAsync(Environment.GetFolderPath(Environment.SpecialFolder.MyPictures)),
        });

        if (file is not null)
        {
            SaveCanvasAsPngAsync(Canvas, file.Path.AbsolutePath);
        }
    }

    public async Task SaveCanvasAsPngAsync(Canvas canvas, string filePath)
    {
        Rect bounds = canvas.Bounds;
        PixelSize pixelSize = new PixelSize((int)bounds.Width, (int)bounds.Height);
        Vector dpi = new Vector(96, 96);

        RenderTargetBitmap renderBitmap = new RenderTargetBitmap(pixelSize, dpi);

        renderBitmap.Render(canvas);

        await using (FileStream stream = File.OpenWrite(filePath))
        {
            renderBitmap.Save(stream);
        }
    }
}