#nullable disable
using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using Microsoft.Maui.Dispatching;
 
namespace Microsoft.Maui.Controls
{
	// Wraps a List which implements INotifyCollectionChanged (usually an ObservableCollection)
	// and marshals all of the list modifications to the main thread. Modifications to the underlying 
	// collection which are made off of the main thread remain invisible to consumers on the main thread
	// until they have been processed by the main thread.
 
	/// <include file="../../../docs/Microsoft.Maui.Controls/MarshalingObservableCollection.xml" path="Type[@FullName='Microsoft.Maui.Controls.MarshalingObservableCollection']/Docs/*" />
	public class MarshalingObservableCollection : List<object>, INotifyCollectionChanged
	{
		readonly IList _internalCollection;
		readonly IDispatcher _dispatcher;
 
		/// <include file="../../../docs/Microsoft.Maui.Controls/MarshalingObservableCollection.xml" path="//Member[@MemberName='.ctor']/Docs/*" />
		public MarshalingObservableCollection(IList list)
		{
			if (!(list is INotifyCollectionChanged incc))
				throw new ArgumentException($"{nameof(list)} must implement {nameof(INotifyCollectionChanged)}");
 
			_internalCollection = list;
			_dispatcher = Dispatcher.GetForCurrentThread();
 
			incc.CollectionChanged += InternalCollectionChanged;
 
			foreach (var item in _internalCollection)
			{
				Add(item);
			}
		}
 
		class ResetNotifyCollectionChangedEventArgs : NotifyCollectionChangedEventArgs
		{
			public IList Items { get; }
			public ResetNotifyCollectionChangedEventArgs(IList items)
				: base(NotifyCollectionChangedAction.Reset) => Items = items;
		}
 
		public event NotifyCollectionChangedEventHandler CollectionChanged;
 
		void OnCollectionChanged(NotifyCollectionChangedEventArgs args)
		{
			CollectionChanged?.Invoke(this, args);
		}
 
		void InternalCollectionChanged(object sender, NotifyCollectionChangedEventArgs args)
		{
			if (args.Action == NotifyCollectionChangedAction.Reset)
			{
				var items = new List<object>();
				for (int n = 0; n < _internalCollection.Count; n++)
				{
					items.Add(_internalCollection[n]);
				}
 
				args = new ResetNotifyCollectionChangedEventArgs(items);
			}
 
			_dispatcher.DispatchIfRequired(() => HandleCollectionChange(args));
		}
 
		void HandleCollectionChange(NotifyCollectionChangedEventArgs args)
		{
			switch (args.Action)
			{
				case NotifyCollectionChangedAction.Add:
					Add(args);
					break;
				case NotifyCollectionChangedAction.Move:
					Move(args);
					break;
				case NotifyCollectionChangedAction.Remove:
					Remove(args);
					break;
				case NotifyCollectionChangedAction.Replace:
					Replace(args);
					break;
				case NotifyCollectionChangedAction.Reset:
					Reset(args);
					break;
			}
		}
 
		void Move(NotifyCollectionChangedEventArgs args)
		{
			var count = args.OldItems.Count;
 
			for (int n = 0; n < count; n++)
			{
				var toMove = this[args.OldStartingIndex];
				RemoveAt(args.OldStartingIndex);
				Insert(args.NewStartingIndex, toMove);
			}
 
			OnCollectionChanged(args);
		}
 
		void Remove(NotifyCollectionChangedEventArgs args)
		{
			var startIndex = args.OldStartingIndex + args.OldItems.Count - 1;
			for (int n = startIndex; n >= args.OldStartingIndex; n--)
			{
				RemoveAt(n);
			}
 
			OnCollectionChanged(args);
		}
 
		void Replace(NotifyCollectionChangedEventArgs args)
		{
			var startIndex = args.NewStartingIndex;
			foreach (var item in args.NewItems)
			{
				this[startIndex] = item;
				startIndex += 1;
			}
 
			OnCollectionChanged(args);
		}
 
		void Add(NotifyCollectionChangedEventArgs args)
		{
			var startIndex = args.NewStartingIndex;
			foreach (var item in args.NewItems)
			{
				Insert(startIndex, item);
				startIndex += 1;
			}
 
			OnCollectionChanged(args);
		}
 
		void Reset(NotifyCollectionChangedEventArgs args)
		{
			if (!(args is ResetNotifyCollectionChangedEventArgs resetArgs))
			{
				throw new InvalidOperationException($"Cannot guarantee collection accuracy for Resets which do not use {nameof(ResetNotifyCollectionChangedEventArgs)}");
			}
 
			Clear();
			foreach (var item in resetArgs.Items)
			{
				Add(item);
			}
 
			OnCollectionChanged(args);
		}
	}
}