Avalonia/src/Avalonia.Controls/Repeater/ItemsSourceView.cs

// This source file is adapted from the WinUI project.
// (https://github.com/microsoft/microsoft-ui-xaml)
//
// Licensed to The Avalonia Project under MIT License, courtesy of The .NET Foundation.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Linq;

namespace Avalonia.Controls
{
    /// <summary>
    /// Represents a standardized view of the supported interactions between a given ItemsSource
    /// object and an <see cref="ItemsRepeater"/> control.
    /// </summary>
    /// <remarks>
    /// Components written to work with ItemsRepeater should consume the
    /// <see cref="ItemsRepeater.Items"/> via ItemsSourceView since this provides a normalized
    /// view of the Items. That way, each component does not need to know if the source is an
    /// IEnumerable, an IList, or something else.
    /// </remarks>
    public class ItemsSourceView : INotifyCollectionChanged, IDisposable
    {
        private readonly IList _inner;
        private INotifyCollectionChanged _notifyCollectionChanged;
        private int _cachedSize = -1;

        /// <summary>
        /// Initializes a new instance of the ItemsSourceView class for the specified data source.
        /// </summary>
        /// <param name="source">The data source.</param>
        public ItemsSourceView(IEnumerable source)
        {
            Contract.Requires<ArgumentNullException>(source != null);

            if (source is IList list)
            {
                _inner = list;
            }
            else if (source is IEnumerable<object> objectEnumerable)
            {
                _inner = new List<object>(objectEnumerable);
            }
            else
            {
                _inner = new List<object>(source.Cast<object>());
            }

            ListenToCollectionChanges();
        }

        /// <summary>
        /// Gets the number of items in the collection.
        /// </summary>
        public int Count
        {
            get
            {
                if (_cachedSize == -1)
                {
                    _cachedSize = _inner.Count;
                }

                return _cachedSize;
            }
        }

        /// <summary>
        /// Gets a value that indicates whether the items source can provide a unique key for each item.
        /// </summary>
        /// <remarks>
        /// TODO: Not yet implemented in Avalonia.
        /// </remarks>
        public bool HasKeyIndexMapping => false;

        /// <summary>
        /// Occurs when the collection has changed to indicate the reason for the change and which items changed.
        /// </summary>
        public event NotifyCollectionChangedEventHandler CollectionChanged;

        /// <inheritdoc/>
        public void Dispose()
        {
            if (_notifyCollectionChanged != null)
            {
                _notifyCollectionChanged.CollectionChanged -= OnCollectionChanged;
            }
        }

        /// <summary>
        /// Retrieves the item at the specified index.
        /// </summary>
        /// <param name="index">The index.</param>
        /// <returns>the item.</returns>
        public object GetAt(int index) => _inner[index];

        public int IndexOf(object item) => _inner.IndexOf(item);

        /// <summary>
        /// Retrieves the index of the item that has the specified unique identifier (key).
        /// </summary>
        /// <param name="index">The index.</param>
        /// <returns>The key</returns>
        /// <remarks>
        /// TODO: Not yet implemented in Avalonia.
        /// </remarks>
        public string KeyFromIndex(int index)
        {
            throw new NotImplementedException();
        }

        /// <summary>
        /// Retrieves the unique identifier (key) for the item at the specified index.
        /// </summary>
        /// <param name="key">The key.</param>
        /// <returns>The index.</returns>
        /// <remarks>
        /// TODO: Not yet implemented in Avalonia.
        /// </remarks>
        public int IndexFromKey(string key)
        {
            throw new NotImplementedException();
        }

        protected void OnItemsSourceChanged(NotifyCollectionChangedEventArgs args)
        {
            _cachedSize = _inner.Count;
            CollectionChanged?.Invoke(this, args);
        }

        private void ListenToCollectionChanges()
        {
            if (_inner is INotifyCollectionChanged incc)
            {
                incc.CollectionChanged += OnCollectionChanged;
                _notifyCollectionChanged = incc;
            }
        }

        private void OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
        {
            OnItemsSourceChanged(e);
        }
    }
}