I have an IObservable<Item> inside a class and I want to expose a read-only property that provides the last item pushed to the observable at a given time. So it will provide a single value of Item.
If no value has been pushed, then it will have to return a default value.
How can I do this without having to subscribe to the observable and having a "backing field"?
Just to supplement @Asti's answer a bit here, and perhaps help you with your frustration:
An observable isn't a physical 'thing', it's more a logical concept. Rx is often compared to LINQ, and it's a fair comparison much of the time. It breaks down though when you start talking data structures: LINQ's enumerables are similar enough to Lists for learning purposes.
However, on the Rx side, there's simply no good equivalent to List. An observable is a transient data structure, all operators deal with this transient state. If you're looking for a permanent state, you're leaving Rx.
Having said that, converting an observable to some sort of state is a common problem, and there are some packages that may help you: ReactiveUI is perhaps the most known. ReactiveProperty is another. Both of these packages are flawed, but may help you.
If you're simply looking for an easier way to get a backing field, without boiler-plating out a backing field, this will work:
public static class ReactivePropertyExtensions { public static ReactiveProperty<T> ToReactiveProperty<T>(this IObservable<T> source) { return new ReactiveProperty<T>(source); } public static ReactiveProperty<T> ToReactiveProperty<T>(this IObservable<T> source, T defaultValue) { return new ReactiveProperty<T>(source, defaultValue); } } public class ReactiveProperty<T> : IDisposable { private IObservable<T> Source { get; } private IDisposable Subscription { get; } public T Value { get; private set; } public ReactiveProperty(IObservable<T> source) : this(source, default(T)) { } public ReactiveProperty(IObservable<T> source, T defaultValue) { Value = defaultValue; Source = source; Subscription = source.Subscribe(t => Value = t); } public void Dispose() { Subscription.Dispose(); } } Example use:
var ticker = Observable.Interval(TimeSpan.FromSeconds(1)) .Publish().RefCount(); var latestTickerValue = ticker.ToReactiveProperty(); Console.WriteLine(latestTickerValue.Value); await Task.Delay(TimeSpan.FromSeconds(1)); Console.WriteLine(latestTickerValue.Value); await Task.Delay(TimeSpan.FromSeconds(3)); Console.WriteLine(latestTickerValue.Value); 
Assuming a hot observable.
For observable = source.Replay(1); observable.Connect();
Provide the value with:
public int Value => observable.Take(1).Amb(Observable.Return(defaultValue)).Wait();
This will return a default value in case no values have been pushed.
You want a transition from Reactive to state, so a backing field isn't a terrible option. You mentioned that you don't want to subscribe, but to observe anything: something, somewhere has to subscribe.
Return method or the replay method activates on subscribe.Here is another way to define the Value property, in the spirit of Asti's solution.
private readonly IObservable<Item> _source; private readonly IObservable<Item> _lastValue; public SomeClass() // Constructor { _source = /* Initialize the source observable (hot) */ _lastValue = _source .Catch(Observable.Never<Item>()) .Concat(Observable.Never<Item>()) .Publish(default) .AutoConnect(0) .FirstAsync(); } public Item Value => _lastValue.Wait(); The Publish operator that accepts an initialValue parameter...
Returns a connectable observable sequence that shares a single subscription to the underlying sequence and starts with
initialValue. This operator is a specialization ofMulticastusing aBehaviorSubject<T>.
The BehaviorSubject<T> is a specialized ISubject<T> that...
Represents a value that changes over time. Observers can subscribe to the subject to receive the last (or initial) value and all subsequent notifications.
The Catch and Concat operators have been added in order to preserve the last value, even in case that the source sequence completes normally or exceptionally.
Personally I would be hesitant to use this solution, since a volatile field updated in the Do operator would accomplish the same thing more naturally. I am posting it mainly as a demonstration of the Rx capabilities.
