/// <summary>
/// Adds an asynchronous event handler.
/// </summary>
/// <typeparam name="T">The message type.</typeparam>
/// <param name="box">A Postal.NET implementation.</param>
/// <param name="handler">The handler.</param>
/// <param name="channel">An optional channel.</param>
/// <param name="topic">An optional topic.</param>
/// <returns>A subscription.</returns>
public static IDisposable AddAsyncHandler <T>(this IBox box, IAsyncHandler <T> handler, string channel = null, string topic = null)
{
if (channel == string.Empty)
{
channel = null;
}
if (topic == string.Empty)
{
topic = null;
}
return(box
.Channel(channel ?? Postal.All)
.Topic(topic ?? Postal.All)
.SubscribeWhen(async(env) => await handler.HandleAsync((T)env.Data), env => env.Data is T));
}