// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

namespace CommunityToolkit.Common.Deferred;

/// <summary>
/// Extensions to <see cref="EventHandler{TEventArgs}"/> for Deferred Events.
/// </summary>
public static class EventHandlerExtensions
{
    /// <summary>
    /// Use to invoke an async <see cref="EventHandler{TEventArgs}"/> using <see cref="DeferredEventArgs"/>.
    /// </summary>
    /// <typeparam name="T"><see cref="EventArgs"/> type.</typeparam>
    /// <param name="eventHandler"><see cref="EventHandler{TEventArgs}"/> to be invoked.</param>
    /// <param name="sender">Sender of the event.</param>
    /// <param name="eventArgs"><see cref="EventArgs"/> instance.</param>
    /// <returns><see cref="Task"/> to wait on deferred event handler.</returns>
    public static Task InvokeAsync<T>(this EventHandler<T>? eventHandler, object sender, T eventArgs)
        where T : DeferredEventArgs
    {
        return InvokeAsync(eventHandler, sender, eventArgs, CancellationToken.None);
    }

    /// <summary>
    /// Use to invoke an async <see cref="EventHandler{TEventArgs}"/> using <see cref="DeferredEventArgs"/> with a <see cref="CancellationToken"/>.
    /// </summary>
    /// <typeparam name="T"><see cref="EventArgs"/> type.</typeparam>
    /// <param name="eventHandler"><see cref="EventHandler{TEventArgs}"/> to be invoked.</param>
    /// <param name="sender">Sender of the event.</param>
    /// <param name="eventArgs"><see cref="EventArgs"/> instance.</param>
    /// <param name="cancellationToken"><see cref="CancellationToken"/> option.</param>
    /// <returns><see cref="Task"/> to wait on deferred event handler.</returns>
    public static Task InvokeAsync<T>(this EventHandler<T>? eventHandler, object sender, T eventArgs, CancellationToken cancellationToken)
        where T : DeferredEventArgs
    {
        if (eventHandler == null)
        {
            return Task.CompletedTask;
        }

        Task[]? tasks = eventHandler.GetInvocationList()
            .OfType<EventHandler<T>>()
            .Select(invocationDelegate =>
            {
                cancellationToken.ThrowIfCancellationRequested();

                invocationDelegate(sender, eventArgs);

#pragma warning disable CS0618 // Type or member is obsolete
                    EventDeferral? deferral = eventArgs.GetCurrentDeferralAndReset();

                return deferral?.WaitForCompletion(cancellationToken) ?? Task.CompletedTask;
#pragma warning restore CS0618 // Type or member is obsolete
                })
            .ToArray();

        return Task.WhenAll(tasks);
    }
}