The actual implementation of the reduce function for the ForYouScreen (from the Now In Android application)

MVIForYouReduceFunction.kt

override fun reduce(
    previousState: ForYouState,
    event: ForYouEvent
): Pair<ForYouState, ForYouEffect?> {
    return when (event) {
        // An Event that has NO associated Effect
        is ForYouEvent.UpdateTopicsLoading -> {
            previousState.copy(
                topicsLoading = event.isLoading
            ) to null
        }

        // An Event that has an associated Effect
        is ForYouEvent.UpdateNewsIsViewed -> {
            val updatedNews = previousState.news.map { news ->
                if (news.id == event.newsId) {
                    news.copy(hasBeenViewed = event.isViewed)
                } else {
                    news
                }
            }
            previousState.copy(
                news = updatedNews
            ) to ForYouEffect.NavigateToNews(updatedNews.first { it.id == event.newsId }.url)
        }
        
        // All other Events go here
    }
}