Reducing operators

While developing applications, you may face such a situation where you may need to accumulate and consolidate emissions. Note that nearly all the operators under this criteria will only work on a finite producer (Observable/Flowable) that calls onComplete() because typically, we can consolidate only finite datasets. We will explore this behavior as we cover these operators.

Here is a short list of reducing operators, which we will cover in this chapter:

• count
• reduce
• all
• any
• contains

Counting emissions (count operator)

The count operator subscribes to a producer, counts the emissions, and emits a Single, containing the count of emissions by the producer.

Here is an example:

    fun main(args: Array<String>) { 
      listOf(1,5,9,7,6,4,3,2,4,6,9).toObservable() 
      .count() 
      .subscribeBy { println("count $it") } 
    } 

The following is the output:

As we can see from the output, this operator counts the emissions from the producer, and emits the count once it receives...