コレクションの要素が1つでも true だったら Execute 可能な ReactiveCommand の作り方

MainPage.xaml

<Page x:Class="App1.MainPage"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:local="using:App1"
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
      mc:Ignorable="d">
    <Page.DataContext>
        <local:MainPageViewModel />
    </Page.DataContext>
    <Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition />
        </Grid.RowDefinitions>
        <Button Content="Add"
                Command="{x:Bind ViewModel.AddCommand}" />
        <Button Content="{x:Bind ViewModel.Message.Value, Mode=OneWay}"
                Command="{x:Bind ViewModel.InvokedCommand}" 
                Grid.Row="1" />
        <ListView ItemsSource="{x:Bind ViewModel.Items}"
                  Grid.Row="2">
            <ListView.ItemTemplate>
                <DataTemplate x:DataType="local:Item">
                    <Grid>
                        <CheckBox Content="{x:Bind Text}"
                                  IsChecked="{x:Bind local:MainPageViewModel.Convert(IsChecked), BindBack=SetChecked, Mode=TwoWay}" />
                    </Grid>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
    </Grid>
</Page>

MainPage.xaml.cs

using Reactive.Bindings;
using Reactive.Bindings.Extensions;
using System;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.Reactive.Linq;
using Windows.UI.Xaml.Controls;

// 空白ページの項目テンプレートについては、https://go.microsoft.com/fwlink/?LinkId=402352&clcid=0x411 を参照してください

namespace App1
{
    /// <summary>
    /// それ自体で使用できる空白ページまたはフレーム内に移動できる空白ページ。
    /// </summary>
    public sealed partial class MainPage : Page
    {
        private MainPageViewModel ViewModel => this.DataContext as MainPageViewModel;

        public MainPage()
        {
            this.InitializeComponent();
        }
    }

    public class MainPageViewModel
    {
        private ObservableCollection<Item> ItemsSource { get; } = new ObservableCollection<Item>(Enumerable
            .Range(1, 10)
            .Select(x => new Item()));

        public ReadOnlyReactiveCollection<Item> Items { get; }

        public ReactiveCommand InvokedCommand { get; }
        public ReactiveCommand AddCommand { get; }

        public ReadOnlyReactiveProperty<string> Message { get; }

        public MainPageViewModel()
        {
            this.Items = this.ItemsSource.ToReadOnlyReactiveCollection();
            this.AddCommand = new ReactiveCommand();
            this.AddCommand.Subscribe(_ =>
            {
                this.ItemsSource.Add(new Item());
            });
            this.InvokedCommand = this.Items.ObserveElementProperty(x => x.IsChecked)
                    .Select(_ => this.Items.Any(x => x.IsChecked))
                    .ToReactiveCommand();
            this.Message = this.InvokedCommand
                .Select(_ => DateTime.Now.ToString())
                .ToReadOnlyReactiveProperty("None");
        }

        public static bool? Convert(bool x) => x;
    }

    public class Item : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        private bool isChecked;

        public bool IsChecked
        {
            get => this.isChecked;
            set
            {
                this.isChecked = value;
                this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Item.IsChecked)));
            }
        }

        public void SetChecked(bool? x) => this.IsChecked = x ?? false;

        public string Text { get; } = Guid.NewGuid().ToString();
    }
}