CustomFlow.kt

flowOf("Bojack Horseman!")
      .map { value -> "$value Modified1" }
      .map { value -> "$value Modified2" }
      .map { value -> "$value Modified3" }
      .collect(object : Collector<String> {
          override fun emit(value: String) {
             //value will be: Bojack Horseman Modified1 Modified2 Modified3
          }
       })

      

private inline fun <T, R> Flow<T>.map(crossinline transform: (value: T) -> R): Flow<R> {
    val upstream = this
    return object : Flow<R> {
        override fun collect(collector: Collector<R>) {
            upstream.collect(object : Collector<T> {
                override fun emit(value: T) {
                    val newValue = transform.invoke(value)
                    collector.emit(newValue)
                }
            })
        }
    }
}

fun flowOf(value: String): Flow<String> = object : Flow<String> {
    override fun collect(collector: Collector<String>) {
        collector.emit(value)
    }
}


interface Collector<in T> {
    fun emit(value: T)
}

interface Flow<out T> {
    fun collect(collector: Collector<T>)
}