/// <summary>
/// Handles an interaction and asynchronously returns the result.
/// </summary>
/// <remarks>
/// <para>
/// This method passes the interaction in turn to its registered handlers in reverse order of registration
/// until one of them handles the interaction. If the interaction remains unhandled after all
/// its registered handlers have executed, an <see cref="UnhandledInteractionException{TInput, TOutput}"/> is thrown.
/// </para>
/// </remarks>
/// <param name="input">
/// The input for the interaction.
/// </param>
/// <returns>
/// An observable that ticks when the interaction completes.
/// </returns>
/// <exception cref="UnhandledInteractionException{TInput, TOutput}">Thrown when no handler handles the interaction.</exception>
public virtual IObservable <TOutput> Handle(TInput input)
{
var context = new InteractionContext <TInput, TOutput>(input);
return(GetHandlers()
.Reverse()
.ToObservable()
.ObserveOn(_handlerScheduler)
.Select(handler => Observable.Defer(() => handler(context)))
.Concat()
.TakeWhile(_ => !context.IsHandled)
.IgnoreElements()
.Select(_ => default(TOutput) !)
.Concat(
Observable.Defer(
() => context.IsHandled
? Observable.Return(context.GetOutput())
: Observable.Throw <TOutput>(new UnhandledInteractionException <TInput, TOutput>(this, input)))));
}