Add utility method to convert a Transformer to an Operator

[davidmoten]

I have a documented API for rxjava-jdbc that suggests certain special operations to be performed using the Observable.lift method with custom Operators. Works fine but if internally I decide to use a composition of existing operators instead of a custom operator then I need the ability to convert a Transformer into an Operator so that the documented use of the API does not need to change. I'd suggest the addition of a Transformers utility class and supporting classes:

public final class Transformers {

    public static <T,R> Operator<R,T> toOperator(Transformer<T,R> transformer) {
        return TransformerOperator.toOperator(transformer);
    }

}

I have the code for this and tests resident in rxjava-jdbc and can contribute it if there is support. Are people happy for me to proceed with a PR?

Incidentally, Operator -> Transformer is a trivial conversion, it is just the Transformer -> Operator conversion that the suggested PR could facilitate.

[adipascu]

I'd also like this enhancement

[akarnokd]

If we are talking about internal API changes, you should be able to replace the operator without any transformations:

public Observable<T> someOperation() {
    return lift(new MyOperator());
}
// ...
public Observable<T> someOperation() {
    return this.subscribeOn(Schedulers.io()).observeOn(Schedulers.computation()).takeLast(1);
}

[davidmoten]

That's not what I'm after. A client has been told to process an Observable o like this:

o.lift(thing.createOperator())......

but I now want to change the implementation of thing.createOperator() so that it still returns an Operator but is based on a Transformer like

o -> o.buffer(2).blah().blah()

without the client having to change their usage.

This is how I implemented it for my own usage and I wondered if it should be part of the RxJava API:

https://github.com/davidmoten/rxjava-extras/blob/master/src/main/java/com/github/davidmoten/rx/operators/OperatorFromTransformer.java

[akarnokd]

I'm still not convinced this is transformation should be part of the public API.

[davidmoten]

Righto, feature is in rxjava-extras (Transformers.toOperator) if anyone wants it.