Idiomatic filtering of values.

[andrask]

Is there a predicate to filter out null values or any values from a signal? Currently a new function has to be created to do this but it would be easy to create a general function.

// Filter out null values
signal.filter(new Func1<Date, Boolean>() {
    @Override
    public Boolean call(Date t1) {
        return (t1 != null);
    }
}).map(...);

[benjchristensen]

We do not have a collection of common predicates. The closest to this idea we have currently is the rx.util.functions.Not predicate that negates whatever predicate it is given.

[andrask]

Guava has lots of predicates that could be reused here but that would be another dependency. As a simplified solution ReactiveCocoa has the ignore: operation that ignores elements equalling to the given parameter.

At the moment what I need is a simple method to ignore the default value of a BehaviorSubject. Filtering out nulls works in this case. But without predicates the code will be ugly.

Obviously skip does not work as it would skip the stored value for all new subscribers instead of just the default one. buffer with 1 is an alternative but that needs an additional map to flatten the one item lists. Do you have a better idea for how this could be done?

[akarnokd]

That sounds you need PublishSubject instead of BehaviorSubject as the former doesn't have a default value.

[andrask]

But I need the 1 long buffer. That's why publishsubject + buffer + map
would solve the issue but it is just ugly and unnecessarily complex for a
general purpose like this. In other RX implementations there are operations
that support this behavior.

2014/1/21 akarnokd [email protected]

That sounds you need PublishSubject instead of BehaviorSubject as the
former doesn't have a default value.

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[akarnokd]

So you need the latest value when someone subscribes but not an initial value? This can be achieved via replay(int).

[andrask]

There is a fundamental difference between behavior subject and publish
subject. Behavior subject is a connected signal: whenever a send happens,
that is stored. Publish subject starts working only after it is subscribed
to. (Error and Completed may be registered without a subscription, I
haven't tried those.) As a result, an explicit subscription is needed to
capture all events before a useful subscription is made.

Now the question is whether there is a connect method in the library that
just sits at the end of the line and ignores every send. Its only purpose
is to keep the signal hot to record all events in the replay.

2014/1/21 akarnokd [email protected]

So you need the latest value when someone subscribes but not an initial
value? This can be achieved via the replay(int)http://netflix.github.io/RxJava/javadoc/rx/Observable.html#replay(int)
.

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[benjchristensen]

Here is how filtering can be done:

public class TestFilter {

    public static void main(String[] args) {
        Observable.range(0, 20).filter(i -> i % 2 == 0).subscribe(System.out::println);
        Observable.range(0, 20).filter(TestFilter::filterEven).subscribe(System.out::println);
    }

    public static boolean filterEven(int i) {
        return i % 2 != 0;
    }
}

If a library of common filters is desired, that could be a good contrib module, but does not belong in the core.