ViewModel => Viewのイベント通知

MainWindow.xaml

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApplication1"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Button Content="Hello!" Command="{Binding DoSomething}"/>
    </Grid>
</Window>

MainWindow.xaml.cs

using System;
using System.Windows;
using System.Windows.Data;
using System.Windows.Input;
using Prism.Commands;

namespace WpfApplication1
{
    /// <summary>
    /// MainWindow.xaml の相互作用ロジック
    /// </summary>
    public partial class MainWindow : Window
    {
        public ICommand CloseCommand
        {
            get { return (ICommand)this.GetValue(MainWindow.CloseCommandProperty); }
            set { this.SetValue(MainWindow.CloseCommandProperty, value); }
        }
        public static readonly DependencyProperty CloseCommandProperty =
            DependencyProperty.Register(nameof(CloseCommand), typeof(ICommand), typeof(MainWindow), new PropertyMetadata(default(ICommand)));

        public MainWindow()
        {

            this.DataContext = new ViewModel();
            this.InitializeComponent();

            this.SetBinding(CloseCommandProperty, new Binding()
            {
                Path = new PropertyPath("Close"),
                Mode = BindingMode.OneWayToSource, 
            });
            this.CloseCommand = new DelegateCommand(() =>
            {
                this.Close();
            });
        }
    }
}

VewModel.cs

using System;
using System.Windows;
using System.Windows.Input;
using Prism.Commands;

namespace WpfApplication1
{
    public class ViewModel
    {
        public ICommand DoSomething { get; }

        public ICommand Close { get; set; }

        public ViewModel()
        {
            this.DoSomething = new DelegateCommand(() =>
            {
                MessageBox.Show("Hello, world!");
                this.Close?.Execute(null);
            });
        }
    }
}