Observable pattern on reactive extension

Question

I am trying to understand the below code trying to use reactive extensions

 IObservable<string> textChangedObservable =
            Observable.FromEventPattern<TextChangedEventArgs>(searchScrip, "TextChanged")
                .Select(evt => ((TextBox) sender).Text);

 textChangedObservable.Subscribe(OnNext, OnCompleted);

private void OnNext(string s)
    {
        System.Diagnostics.Debug.Print("OnNext " + s + "\n");
    }

    private void OnCompleted()
    {
        System.Diagnostics.Debug.Print("OnCompleted " + "\n");
    }

If I enter SEARC in the input box, the output looks

• OnNext SE
• OnNext SEA
• OnNext SEA
• OnNext SEAR
• OnNext SEAR
• OnNext SEAR
• OnNext SEARC
• OnNext SEARC
• OnNext SEARC

• OnNext SEARC

  1. Why is "S" not triggering an OnNext?
  2. Why is OnCompleted never called?
  3. Why is the OnNext called n-1 time on the nth character?

Answer 1

It appears that you are subscribing to the observable in your searchScrip_TextChanged handler.

This means that the first time that searchScrip_TextChanged gets called the S has already happened before you've wired up the observable. So of course it doesn't fire.

But now that the S is hit you have one subscription so when the E is typed you get a single SE. But since the searchScrip_TextChanged handler gets called for the E too you now have two subscriptions to your observable.

So when the A is typed you get two SEA because you have two observables. But again the searchScrip_TextChanged is called for the A so now you have three observables.

Etc, etc, etc.

Events don't complete automatically. You need to manually dispose of the subscriptions to have them end. This should make sense because that's what you have to do with normal event handlers that you want to stop.

You should create your observable when your form is loaded so that it is created once.

It should look like this:

IObservable<string> textChangedObservable =
        Observable.FromEventPattern<TextChangedEventArgs>(searchScrip, "TextChanged")
            .Select(evt => searchScrip.Text);

IDisposable subscription =
    textChangedObservable
        .Subscribe(
            s => Debug.Print("OnNext " + s + "\n"),
            s => Debug.Print("OnCompleted\n"));

Answer 2

The problem here really has nothing to do with Rx.

1: Why is "S" not triggering an OnNext?

Because the TextChanged event you have subscribed to did not fire on the first S.

2: Why is OnCompleted never called?

When you wrap a .NET event as an IObservable<T>, you never get OnError or OnCompleted notifications. There is no concept of error or completion with a .NET event.

If there are two events, one for values and one for completion, you can combine them like so:

var values = Observable.FromEvent(...);
var completion = Observable.FromEvent(...);
var query = values.TakeUntil(completion);

Now query will yield a proper OnCompleted notification.

3: Why is the OnNext called n-1 time on the nth character?

Because the TextChanged event you have subscribed to fired this way. As was pointed out by @Kari-Antti, this may be a side effect of using a "routed property" event.