如何使用RxJava和Kotlin进行groupBy和收集？

Question

我有,Observable<Rates>而Rate只是一个简单的对象:

Rate(val value:String){}
Rates(val rates: List<Rate>)

我想改变Observable<Rates>成Observable<HashMap<String,Long>.

所以例如对于Rates(arrayOf(Rate("1"),Rate("2"), Rate("3"),Rate("3"), Rate("2"),Rate("2")))我期望结果的费率:

(1 -> 1)
(2 -> 3)
(3 -> 2)
(4 -> 0)
(5 -> 0)

我开始创建类似的东西:

service.getRates()
        .flatMap {it-> Observable.from(it.rates) }
        .filter { !it.value.isNullOrEmpty() }
        .groupBy {it -> it.value}
        .collect({ HashMap<String,Long>()}, { b, t -> b.put(t.key, t.count???)}

但我被困在这里,我不知道所有的价值？如果没有5中的4,我不知道如何添加空值(0).有没有办法用rx做到这一点？

Answer 1

请查看代码中的注释以获取问题的答案.

import rx.Observable

fun main(args: Array<String>) {
    val service = Service()

    // This adds all keys with each key mapped to zero
    val referenceKeyCounts = Observable
        .just("1", "2", "3", "4", "5")
        .map { it to 0 }

    val keyCountsFromService = service.getRates()
        .flatMap { Observable.from(it.rates) }
        .filter { !it.value.isNullOrEmpty() }
        .map { it.value to 1 } // map each occurrence of key to 1

    Observable.concat(referenceKeyCounts, keyCountsFromService)
        .groupBy { it.first }
        .flatMap { group ->  // this converts GroupedObservable to final values
            group.reduce(0, { acc, pair -> acc + pair.second }) // add instead of counting
                .map { group.key to it }
        }
        .subscribe(::println)

}

class Service {
    fun getRates(): Observable<Rates> = Observable.just(Rates(listOf(
        Rate("1"), Rate("2"), Rate("3"), Rate("3"), Rate("2"), Rate("2")
    )))
}

class Rate(val value: String)

class Rates(val rates: List<Rate>)

Answer 2

技巧是使用counton，GroupedObservable因为当源可观察对象完成时它只发出单个值：

从那里开始如下：

rates
  .flatMap { Observable.from(it.rates) }
  .filter { !it.value.isNullOrEmpty() }
  .groupBy { it.value }
  .flatMap { group -> group.count().map { group.key to it } } // list "1"->1, "2"->3, ...
  .mergeWith(Observable.from((1..5).map { it.toString() to 0 })) // defaults "4"->0
  .reduce(mutableMapOf<String, Int>()) { acc, cur ->
      acc.apply {
        val (key, count) = cur  
        this[key] = (this[key] ?: 0) + count // add counts
      }
  }.subscribe { countedRates -> 
    println(countedRates)
}