Jetpack Transformation复杂应用

背景：通过Transformations.switchMap 实现实时变换影响其他变量值的API,实现牵一发而动全身的操作，不过由于其代码过于简洁，有必要分解一下帮助阅读。
项目地址：android-architecture-components

用户手动输入subredditName 关键字，repoResult跟随变化，然后通过repoResult的变化动态改变posts ，networkState ，refreshState 的功能实现，简化写法后通过Transformations静态导入switchMap，map,代码看起来非常简洁

//导包
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.Transformations.map
import androidx.lifecycle.Transformations.switchMap
import androidx.lifecycle.ViewModel
import com.android.example.paging.pagingwithnetwork.reddit.repository.RedditPostRepository

class SubRedditViewModel(private val repository: RedditPostRepository) : ViewModel() {
    private val subredditName = MutableLiveData<String>()
    private val repoResult = map(subredditName) {
        repository.postsOfSubreddit(it, 30)
    }
    val posts = switchMap(repoResult, { it.pagedList })!!
    val networkState = switchMap(repoResult, { it.networkState })!!
    val refreshState = switchMap(repoResult, { it.refreshState })!!

}

还原代码：代码逻辑就很清晰了，当然也就相对不精简了

//导包
import androidx.arch.core.util.Function
import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.Transformations
import androidx.lifecycle.ViewModel
import androidx.paging.PagedList
import com.android.example.paging.pagingwithnetwork.reddit.repository.Listing
import com.android.example.paging.pagingwithnetwork.reddit.repository.NetworkState
import com.android.example.paging.pagingwithnetwork.reddit.repository.RedditPostRepository
import com.android.example.paging.pagingwithnetwork.reddit.vo.RedditPost
import kotlin.jvm.internal.Intrinsics

class SubRedditViewModel : ViewModel{
	val networkState: LiveData<NetworkState>
    val posts: LiveData<PagedList<RedditPost>>
    val refreshState: LiveData<NetworkState>
    val repository: RedditPostRepository
    val subredditName: MutableLiveData<String>
    val repoResult: LiveData<Listing<RedditPost>>

    constructor(repository: RedditPostRepository) : super() {
        this.repository = repository
        this.subredditName = MutableLiveData<String>()
        this.repoResult = Transformations.map(this.subredditName, object : Function<String, Listing<RedditPost>> {
            override fun apply(input: String): Listing<RedditPost> {
                return repository.postsOfSubreddit(input, 30)
            }
        })
        Intrinsics.checkParameterIsNotNull(repository, "repository")

        val switchMap = Transformations.switchMap(this.repoResult, object : Function<Listing<RedditPost>, LiveData<PagedList<RedditPost>>> {
            override fun apply(input: Listing<RedditPost>?): LiveData<PagedList<RedditPost>>? {
                return input?.pagedList
            }

        })
        if (switchMap == null) {
            Intrinsics.throwNpe()
        }
        this.posts = switchMap


        val switchMap2 = Transformations.switchMap(this.repoResult, object : Function<Listing<RedditPost>, LiveData<NetworkState>> {
            override fun apply(input: Listing<RedditPost>?): LiveData<NetworkState>? {
                return input?.networkState
            }
        })
        if (switchMap2 == null) {
            Intrinsics.throwNpe()
        }
        this.networkState = switchMap2


        val switchMap3 = Transformations.switchMap(this.repoResult, object : Function<Listing<RedditPost>, LiveData<NetworkState>> {
            override fun apply(input: Listing<RedditPost>?): LiveData<NetworkState>? {
                return input?.refreshState
            }

        })
        if (switchMap3 == null) {
            Intrinsics.throwNpe()
        }
        this.refreshState = switchMap3
    }
}