Question

所以我正在尝试使用Rx.NET组建一个小消息总线。

public class Bus {
    private readonly Subject<BaseCommand> _commands = new Subject<BaseComand>();

    public void RegisterHandler<TCommand>(Action<TCommand> handler) where TCommand: BaseCommand {
        _commands
            .OfType&lt;TCommand&gt;()
            .Publish()
            .RefCount()
            .Subscribe(handler);
    }

    public void SendCommand<TCommand>(TCommand command) where TCommand: BaseCommand {
        _commands.OnNext(command);
    }
}

所以这是代码的要点。我想限制订阅，以便单个消息类型只能存在一个订阅。在添加新订阅之前，有没有从OfType<T>检查Observable的任何现有订阅？

Answer 1

我会建议这样的事情（我已经将RegisterHandler更改为具有IDisposable返回类型，因此您实际上可以再次取消订阅）：

public class Bus
{
    private readonly Subject<BaseCommand> _commands = new Subject<BaseCommand>();

    private class Counter<TCommand> where TCommand : BaseCommand
    {
        public static int Count;
    }

    public IDisposable RegisterHandler<TCommand>(Action<TCommand> handler, Action<Exception> OnError = null) where TCommand : BaseCommand
    {
        OnError = OnError ?? (Action<Exception>)((ex) => Dispatcher.CurrentDispatcher.Invoke(() => {throw ex; })); // alternative case of course only works if dispatcher is available

        return Observable.Create<TCommand>(o =>
        {
            if (Interlocked.Increment(ref Counter<TCommand>.Count) > 1)
            {
                Interlocked.Decrement(ref Counter<TCommand>.Count);
                o.OnError(new InvalidOperationException("Too many subscribers!"));
                return Disposable.Empty;
            }

            var subscription = _commands
                .OfType<TCommand>()
                      .Publish()
                      .RefCount()
                      .Subscribe(o);

            var decrement = Disposable.Create(() =>
            {
                Interlocked.Decrement(ref Counter<TCommand>.Count);
            });

            return new CompositeDisposable(subscription, decrement);
        })
        .Subscribe(handler, OnError);

    }

    public void SendCommand<TCommand>(TCommand command) where TCommand : BaseCommand
    {
        _commands.OnNext(command);
    }
}

编辑：我可能会将RegisterHandler函数的签名更改为

 public IObservable<TCommand> RegisterHandler<TCommand>() where TCommand : BaseCommand

虽然;通过错误管理节省了一些麻烦（订阅者必须自己处理），并且您的消费者在他们想要订阅这些活动的时间和方式方面更加自由。

如何判断Subject.OfType <t>是否有任何订阅者

1 个答案: