MainWindowViewModel.cs

using System.Collections.ObjectModel;
using System.Linq;
using Avalonia.Controls;
using Avalonia.Controls.Models.TreeDataGrid;
using Avalonia.Controls.Selection;
using FlatTreeDataGridSample.Models;

namespace FlatTreeDataGridSample.ViewModels;

/// <summary>
/// Represents the view model for the main window, providing a data source for a flat TreeDataGrid
/// that displays a collection of <see cref="Country"/> objects.
/// </summary>
/// <remarks>
/// The data grid source includes predefined columns for displaying country information such as
/// name, region, population, area, and GDP. The columns support features like text search and
/// custom templates  for specific fields.
/// </remarks>
internal partial class MainWindowViewModel : ViewModelBase
{
    private readonly ObservableCollection<Country> _data = new(Countries.All);

    public MainWindowViewModel()
    {
        // Create a flat TreeDataGrid source and define the columns to show.
        Source = new FlatTreeDataGridSource<Country>(_data)
        {
            Columns =
            {
                // Define a row header column for row numbering. This can be made to be always
                // visible when scrolling horizontally by setting FrozenColumnCount = 1 on the
                // TreeDataGrid.
                new RowHeaderColumn<Country>(),

                // Define a read/write text column for the country name with text search enabled.
                new TextColumn<Country, string>(
                    "Country",
                    x => x.Name,
                    (o, v) => o.Name = v, 
                    new GridLength(6, GridUnitType.Star), new()
                {
                    IsTextSearchEnabled = true,
                }),

                // Define a template column for the region with custom cell and edit templates.
                new TemplateColumn<Country>("Region", "RegionCell", "RegionEditCell"),

                // Define read-only text columns for population and area.
                new TextColumn<Country, int>("Population", x => x.Population, new GridLength(3, GridUnitType.Star)),
                new TextColumn<Country, int>("Area", x => x.Area, new GridLength(3, GridUnitType.Star)),

                // Define a read-only text column for GDP with right-aligned text and a maximum width.
                new TextColumn<Country, int>("GDP", x => x.Gdp, new GridLength(3, GridUnitType.Star), new()
                {
                    TextAlignment = Avalonia.Media.TextAlignment.Right,
                    MaxWidth = new GridLength(150)
                }),
            }
        };

        Source.Selection = new TreeDataGridCellSelectionModel<Country>(Source);
        MaxPopulation = _data.Max(x => x.Population);
    }

    /// <summary>
    /// Gets the data source for the flat tree data grid, containing a collection of
    /// <see cref="Country"/> objects.
    /// </summary>
    public FlatTreeDataGridSource<Country> Source { get; }

    /// <summary>
    /// Gets the maximum population of all the countries.
    /// </summary>
    public int MaxPopulation { get; }
}