Kotlin sealed class subclass needs to be casted to base class if provided as RxJava Observable

问题

I'm trying to avoid terminal states of RxJava chains in my app written in Kotlin, so I found out that it is right thing to transform Observable<T> to Observable<Result<T>> where Result is sealed class.

sealed class Result<T>
data class Success<T>(val data: T) : Result<T>()
data class Failure<T>(val throwable: Throwable) : Result<T>()

And let's say I have this network request observable.

fun getOrganization(): Observable<Result<Boolean>> {
    return api.getOrganization("google")
            .map { Success(true) }
            .onErrorReturn { Failure(RuntimeException("throwable")) }
}

So what I want to do in the end is to handle result like this.

fun main(args: Array<String>) {
    getOrganization().subscribe({
        when (it) {
            is Success -> print("success = ${it.data}")
            is Failure -> print("failure = ${it.throwable}")
        }
    })
}

Everything should work great, but I have this error in IDE.

Isn't it just simple using of subclass instead of base class?

I found out that if I explicitly cast Success(true) to Result<Boolean> everything works, but shows this warning.

Why it even happens and what am I doing wrong here?

UPD. It looks like the main issue here is near rxjava's onErrorReturn operator. If I remove it completely, then it works even without out keyword like answers suggest.

If I return Success from both map and onErrorReturn then out keyword helps to get rid of errors.

But if I return Failure from onErrorReturn there's still error. Compiler has to know that Failure Result typed with the same Boolean as Success. What should I do to avoid it or to satisfy it's requirements?

回答1:

maybe you misunderstand the generic variance in kotlin. it works fine since Success<Boolean> is a subtype of Result<Boolean>. so the "No need cast" warning reported and the code below works fine:

val ok:Success<Boolean>  = Success(true);
val result:Result<Boolean>  = ok;

But you can't assign a Success<Boolean> to a Result<Any> since their type parameters are different, so this why compiler reports the "Type mismatch" error , for example:

val ok:Success<Boolean>  = Success(true);
val result1:Result<Any>  = ok;// error
val result2:Result<out Any>  = ok;// ok

to fix the error and warnings you can try the code below:

fun getOrganization(): Observable<out Result<Boolean>> {
    return api.getOrganization("google")
            .map<Result<Boolean>> { Success(true) }
            .onErrorReturn { Failure(RuntimeException("throwable")) }
}

for more details, you can see java generic subtypes & kotlin type projections.

java generic subtype

回答2:

An Observable<Success<Boolean>> is not a subtype Observable<Result<Boolean>>, just as List<String> is not a subtype of List<Object>. See the Generics documentation.

To solve this, either return an Observable<out Result<Boolean>>, or explicitly add a type to your map function:

.map<Result<Boolean>> { Success(true) }

来源：https://stackoverflow.com/questions/44564836/kotlin-sealed-class-subclass-needs-to-be-casted-to-base-class-if-provided-as-rxj

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android

rx-java

kotlin