CachedLiveData with SavedStateHandle extension

CachedLiveData.kt

// 
// Title: CachedLiveData
// Author: quwac (https://github.com/quwac)
// License: CC0 (https://creativecommons.org/publicdomain/zero/1.0/deed.en)
// 

package io.github.quwac.cachedlivedata

import androidx.lifecycle.*

fun <T : Any> SavedStateHandle.getCachedLiveData(
    key: String,
    source: LiveData<T>,
    initialValue: T
): LiveData<T> {
    val cache = getLiveData(key, initialValue)
    return CachedLiveData(source, cache)
}

fun <T : Any> SavedStateHandle.getCachedLiveData(key: String, source: LiveData<T>): LiveData<T> {
    val cache = getLiveData<T>(key)
    return CachedLiveData(source, cache)
}

private class CachedLiveData<T>(
    source: LiveData<T>,
    private val cache: MutableLiveData<T>
) : MediatorLiveData<T>() {

    private val cacheObserver = Observer<T> {
        // nop
    }
    private var isCacheInSource = true
    private var isCacheObserved = false

    init {
        addSource(source) {
            if (isCacheInSource) {
                removeSource(cache)
                isCacheInSource = false

                if (hasActiveObservers()) {
                    setCacheObserver()
                }
            }
            value = it
            cache.value = it
        }
        addSource(cache) {
            value = it
        }
    }

    override fun onActive() {
        super.onActive()
        setCacheObserver()
    }

    override fun onInactive() {
        super.onInactive()
        unsetCacheObserver()
    }

    private fun setCacheObserver() {
        if (!isCacheInSource && !isCacheObserved) {
            cache.observeForever(cacheObserver)
            isCacheObserved = true
        }
    }

    private fun unsetCacheObserver() {
        if (!isCacheInSource && isCacheObserved) {
            cache.removeObserver(cacheObserver)
            isCacheObserved = false
        }
    }

}