问题
I have two Observable
s, let's call them PeanutButter
and Jelly
. I'd like to combine them to a Sandwich
Observable
. I can do that using:
Observable<PeanutButter> peanutButterObservable = ...;
Observable<Jelly> jellyObservable = ...;
Observable<Sandwich> sandwichObservable = Observable.combineLatest(
peanutButterObservable,
jellyObservable,
(pb, j) -> makeSandwich(pb, j))
The problem is that RX waits for the first PeanutButter
and the first Jelly
to be emitted before emitting the first combined Sandwich
but Jelly
may never be emitted which means I never get the first Sandwich
.
I'd like to combine the two feeds such that a combined item is emitted as soon as the first item from either feed is emitted, regardless of whether the other feed has yet to emit anything, how do I do that in RxJava?
回答1:
one possible approach would be to use the startWith
operator to trigger an emission of a known value from each stream upon subscription. this way combineLatest()
will trigger if either stream emits a value. you'd just have to be mindful of looking out for the initial/signal values in the onNext
consumer.
something like this...:
@Test
public void sandwiches() {
final Observable<String> peanutButters = Observable.just("chunky", "smooth")
.startWith("--initial--");
final Observable<String> jellies = Observable.just("strawberry", "blackberry", "raspberry")
.startWith("--initial--");
Observable.combineLatest(peanutButters, jellies, (peanutButter, jelly) -> {
return new Pair<>(peanutButter, jelly);
})
.subscribe(
next -> {
final String peanutButter = next.getFirst();
final String jelly = next.getSecond();
if(peanutButter.equals("--initial--") && jelly.equals("--initial--")) {
// initial emissions
} else if(peanutButter.equals("--initial--")) {
// jelly emission
} else if(jelly.equals("--initial--")) {
// peanut butter emission
} else {
// peanut butter + jelly emissions
}
},
error -> {
System.err.println("## onError(" + error.getMessage() + ")");
},
() -> {
System.out.println("## onComplete()");
}
);
}
回答2:
I think this problem can be approached by using merge
and scan
operators:
public class RxJavaUnitTestJava {
public Observable<Sandwich> getSandwich(Observable<Jelly> jelly, Observable<PeanutButter> peanutButter) {
return Observable.merge(jelly, peanutButter)
.scan(new Sandwich(null, null), (BiFunction<Object, Object, Object>) (prevResult, newItem) -> {
Sandwich prevSandwich = (Sandwich) prevResult;
if (newItem instanceof Jelly) {
System.out.println("emitted: " + ((Jelly) newItem).tag);
return new Sandwich((Jelly) newItem, prevSandwich.peanutButter);
} else {
System.out.println("emitted: " + ((PeanutButter) newItem).tag);
return new Sandwich(prevSandwich.jelly, (PeanutButter) newItem);
}
})
.skip(1) // skip emitting scan's default item
.cast(Sandwich.class);
}
@Test
public void testGetSandwich() {
PublishSubject<Jelly> jelly = PublishSubject.create();
PublishSubject<PeanutButter> peanutButter = PublishSubject.create();
getSandwich(jelly, peanutButter).subscribe(new Observer<Sandwich>() {
@Override
public void onSubscribe(Disposable d) {
System.out.println("onSubscribe");
}
@Override
public void onNext(Sandwich sandwich) {
System.out.println("onNext: Sandwich: " + sandwich.toString());
}
@Override
public void onError(Throwable e) {
System.out.println("onError: " + e.toString());
}
@Override
public void onComplete() {
System.out.println("onComplete");
}
});
jelly.onNext(new Jelly("jelly1"));
jelly.onNext(new Jelly("jelly2"));
peanutButter.onNext(new PeanutButter("peanutButter1"));
jelly.onNext(new Jelly("jelly3"));
peanutButter.onNext(new PeanutButter("peanutButter2"));
}
class Jelly {
String tag;
public Jelly(String tag) {
this.tag = tag;
}
}
class PeanutButter {
String tag;
public PeanutButter(String tag) {
this.tag = tag;
}
}
class Sandwich {
Jelly jelly;
PeanutButter peanutButter;
public Sandwich(Jelly jelly, PeanutButter peanutButter) {
this.jelly = jelly;
this.peanutButter = peanutButter;
}
@Override
public String toString() {
String jellyResult = (jelly != null) ? jelly.tag : "no jelly";
String peanutButterResult = (peanutButter != null) ? peanutButter.tag : "no peanutButter";
return jellyResult + " | " + peanutButterResult;
}
}
}
Output:
onSubscribe
emitted: jelly1
onNext: Sandwich: jelly1 | no peanutButter
emitted: jelly2
onNext: Sandwich: jelly2 | no peanutButter
emitted: peanutButter1
onNext: Sandwich: jelly2 | peanutButter1
emitted: jelly3
onNext: Sandwich: jelly3 | peanutButter1
emitted: peanutButter2
onNext: Sandwich: jelly3 | peanutButter2
The fact that Jelly
, PeanutButter
and Sandwich
are all independent types makes it a bit more complex around casting and nullability in scan
. If you have control over these types, this solution can be further improved.
来源:https://stackoverflow.com/questions/47931923/rxjava-combine-two-optional-observables