Question

Usually when you subscribe to the changes of a value you are also interested in knowing the initial value. I want my IObservable to cache the latest (or initial) value and push that value on subscription.

When using plain events I often end up with code that looks like

x.SomeEvent += SomeEventHandler;
SomeEventHandler(x, EventArgs.Empty);

Using IObservable I was hoping to wrap the event with something that pushes the initial value. If I have multiple subscribers they should receive the newest value upon subscription
I have some code that works right if I subscribe right after creating the IObservable but not if the event fires before subscribing:

class Program
{
    static void Main()
    {
        var s = new Source { Value = 1 };
        var values = Observable.Return(s.Value).Concat(
            Observable.FromEvent(
                h => s.ValueChanged += h,
                h => s.ValueChanged -= h)
            .Select(_ => s.Value));
        using (values.Subscribe(Console.WriteLine))
        {
            s.Value = 2; // prints 1,2 as expected
        }
        using (values.Subscribe(Console.WriteLine))
        {
            s.Value = 3; // prints 1,3 - expected 2,3
        }
    }
}

class Source
{
    private int _value;
    public int Value
    {
        get { return _value; }
        set
        {
            if (_value == value)
                return;
            _value = value;
            if (ValueChanged != null)
                ValueChanged(this, EventArgs.Empty);
        }
    }

    public event EventHandler ValueChanged;
}

How do I create an IObservable that works as expected?

Answer 1

The solution is to subscribe a BehaviorSubject to the observable, and to subscribe all observers to the BehaviorSubject. The BehaviorSubject will remember the last notification and notify new observers of it upon subscription.

Have a look at the Observable.Publish extension method that has a initialValue parameter. This creates an IConnectableObservable that internally uses an BehaviorSubject.

var s = new Source { Value = 1 };

var values = Observable.FromEvent(h => s.ValueChanged += h,
                                  h => s.ValueChanged -= h)
                       .Select(e => e.NewValue)
                       .Publish(s.Value);

using (values.Connect())                         // subscribes subject to event
{
    using (values.Subscribe(Console.WriteLine))  // subscribes to subject
    {
        s.Value = 2;
    }                                            // unsubscribes from subject

    using (values.Subscribe(Console.WriteLine))  // subscribes to subject
    {
        s.Value = 3;
    }                                            // unsubscribes from subject

}                                            // unsubscribes subject from event

(untested)