Proposal: filterBy(Class) API

[ZacSweers]

When dealing with polymorphic types, I've often had cases where I needed to filter by a type and then immediately have to follow that with a seemingly-redundant cast to that type that I've already filtered on. This issue proposes an operator that would add a convenience API for this.

Example:

myMammalObservable
    .filter(m -> m instanceof Dog)
    .cast(Dog.class)
    .subscribe(dog -> dog.playFetch());

Proposal would probably encapsulate that exact logic, though we could combine it into a single operator if there's a perf gain to be had by doing so. Would look like this in use:

myMammalObservable
    .filterBy(Dog.class)
    .subscribe(dog -> dog.playFetch());

Up for better naming suggestions, just found myself doing this several times in the last week. Totally understand if it's trivial enough not to warrant something on the first-party API. Happy to submit a PR if you'd want it.

[akarnokd]

We have such an operator from very early on:

    /**
     * Filters the items emitted by an Observable, only emitting those of the specified type.
     * <p>
     * <img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/ofClass.png" alt="">
     * <dl>
     *  <dt><b>Scheduler:</b></dt>
     *  <dd>{@code ofType} does not operate by default on a particular {@link Scheduler}.</dd>
     * </dl>
     * 
     * @param <R> the output type
     * @param klass
     *            the class type to filter the items emitted by the source Observable
     * @return an Observable that emits items from the source Observable of type {@code klass}
     * @see <a href="http://reactivex.io/documentation/operators/filter.html">ReactiveX operators documentation: Filter</a>
     */
    public final <R> Observable<R> ofType(final Class<R> klass) {

[ZacSweers]

ಠ_ಠ brb going to go buy reading glasses because I'm apparently blind.

[davidmoten]

@hzsweers the alias Observable.filterBy(...) for Observable.ofType would be a useful PR though I think. Discoverability improvement (I'd forgotten it existed and had been using filter!)

[DavidMGross]

A good example of where the website documentation of operators improves on
the javadocs: filter and ofType are documented side-by-side there:
http://reactivex.io/documentation/operators/filter.html

On Thu, Jun 23, 2016 at 5:32 AM, Dave Moten notifications@github.com
wrote:

@hzsweers https://github.com/hzsweers the alias Observable.filterBy(...)
for Observable.ofType would be a useful PR though I think.
Discoverability improvement (I'd forgotten it existed and had been using
filter!)

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David M. Gross
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