MainViewModel.cs

using ImageFilePickerSample.Models;
using System;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Runtime.CompilerServices;

namespace ImageFilePickerSample.ViewModels
{
    public class MainViewModel : INotifyPropertyChanged
    {
        private ImageModel Model = new ImageModel();

        public ObservableCollection<TItem> Items
        {
            get { return this.Model.Items; }
            set { this.Model.Items = value; }
        }

        public event PropertyChangedEventHandler PropertyChanged;
        protected virtual void OnPropertyChanged([CallerMemberName] String propertyName = "")
        {
            this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}