Filter an observable by an observable

Question

Let's consider the following simplified situation:

• We have an Observable apples of type Observable < Apple >
• Every Apple object has a method isRotten() which returns an observable of type Observable < Boolean > which is guaranteed to emit at least one boolean value.

I want to filter the apples observable such that the rotten apples don't pass the filter. More precisely, an apple A passes the filter if and only if the first item emitted by A.isRotten() is false. What is the best way to implement this filter?

After some thinking I could come up with this:

apples
    .concatMap(apple => 
        apple.isRotten()
            .first()
            .filter(bool => bool)
            .map(bool => apple))

Which is written in javascript. ( ... => ... is a function). This works, but I think it is rather lengthy and difficult to understand. Is there a better way to do this kind of thing?

Answer 1

What you've got is fine and, tbh, I can't think of a more concise way of doing it. I'd probably use flatMap rather than concatMap if out-of-order apples aren't an issue.

If readibility is an issue for you, just move the implementation into it's one function (eg. filterObservable that accepts a function that takes a value and returns an IObservable<bool>)