QueueAsyncMessageBroker.cs

using System;
using System.Collections.Generic;
using System.Linq;
using UniRx;

namespace Misc
{
    public class QueueAsyncMessageBroker
    {
        private bool Running;
        private AsyncMessageBroker Broker { get; } = new AsyncMessageBroker();
        private List<Action> Queue { get; } = new List<Action>();

        public void Publish<T>(T message)
        {
            Queue.Add(() =>
            {
                Running = true;
                Broker.PublishAsync(message).Subscribe(_ =>
                {
                    Running = false;
                    Next();
                });
            });
            Next();
        }

        private void Next()
        {
            if (Running || Queue.Count == 0) return;

            var e = Queue.First();
            Queue.RemoveAt(0);
            e();
        }

        public IDisposable Subscribe<T>(Func<T, UniRx.IObservable<Unit>> asyncMessageReceiver)
        {
            return Broker.Subscribe(asyncMessageReceiver);
        }
    }
}