Avalonia/samples/VirtualizationTest/ViewModels/MainWindowViewModel.cs

// Copyright (c) The Avalonia Project. All rights reserved.
// Licensed under the MIT license. See licence.md file in the project root for full license information.

using System;
using System.Linq;
using ReactiveUI;

namespace VirtualizationTest.ViewModels
{
    internal class MainWindowViewModel : ReactiveObject
    {
        private int _itemCount = 200;
        private IReactiveList<ItemViewModel> _items;
        private string _prefix = "Item";

        public MainWindowViewModel()
        {
            this.WhenAnyValue(x => x.ItemCount).Subscribe(ResizeItems);
            RecreateCommand = ReactiveCommand.Create();
            RecreateCommand.Subscribe(_ => Recreate());
        }

        public int ItemCount
        {
            get { return _itemCount; }
            set { this.RaiseAndSetIfChanged(ref _itemCount, value); }
        }

        public IReactiveList<ItemViewModel> Items
        {
            get { return _items; }
            private set { this.RaiseAndSetIfChanged(ref _items, value); }
        }

        public ReactiveCommand<object> RecreateCommand { get; private set; }

        private void ResizeItems(int count)
        {
            if (Items == null)
            {
                var items = Enumerable.Range(0, count)
                    .Select(x => new ItemViewModel(x));
                Items = new ReactiveList<ItemViewModel>(items);
            }
            else if (count > Items.Count)
            {
                var items = Enumerable.Range(Items.Count, count - Items.Count)
                    .Select(x => new ItemViewModel(x));
                Items.AddRange(items);
            }
            else if (count < Items.Count)
            {
                Items.RemoveRange(count, Items.Count - count - 1);
            }
        }

        private void Recreate()
        {
            _prefix = _prefix == "Item" ? "Recreated" : "Item";
            var items = Enumerable.Range(0, _itemCount)
                .Select(x => new ItemViewModel(x, _prefix));
            Items = new ReactiveList<ItemViewModel>(items);
        }
    }
}