[TIL] 🌼24/04/30🌼#StateFlow #SharedFlow

[TIL] 🌼24/04/30🌼#StateFlow #SharedFlow

• [LeetCode] 2095. Delete the Middle Node of a Linked List(Kotlin)

StateFlow, SharedFlow

📌참고자료

• StateFlow and SharedFlow | Android Developers

📌참고하면 좋을 지난 포스팅

• [TIL] 🌼24/04/18🌼#Kotlin Flow

• StateFlow, SharedFlow:
  Flow APIs that enable flows to optimally emit state updates and emit values to multiple consumers

StateFlow

• state-holder observable
• requires initial state to construct
• emits current value & new state updates to its collectors
• unlike Flow, StateFlow is hot
  -> collecting from the flow doesn't trigger any producer code
• always active in memory (모든 reference가 없어지면 GC 대상이 됨)

SharedFlow

• SharedFlow is hot, like StateFlow
• emits values to all comsumers that collect from it

LiveData에서 Flow로 마이그레이션

📌참고자료

• Migrating from LiveData to Flows: An Engineer’s Guide | Kerry Bisset
• [Kotlin] Flow를 StateFlow로 변환 | th.k

val myStateFlow = myFlow.stateIn(
	scope = lifecycleScope,
	started = SharingStarted.Lazily,
	initialValue = 초기값
)

📌참고하면 좋을 지난 포스팅

• [TIL] 🌼24/04/14🌼 #MediatorLiveData

LiveData -> StateFlow

• LiveData에서 StateFlow로 마이그레이션할 때 가장 direct한 방법:
  MutableLiveData를 MutableStateFlow로 바꾸기!

• MutableLiveData의 특징:
  • part of AAC(Android Architecture Components)
  • Default로, new value와 old value가 distinct한지 체크하지 X
    -> new value와 old value가 같더라도, observer에 update notification 전달
  • main thread에서 수행됨

• MutableStateFlow의 특징:
  • part of Kotlin's coroutines library
  • value에 distinct change가 발생한 경우에만 update 트리거 함
    (distinct한지 아닌지는 equality check(==)로 판별)
  • integrated with Kotlin's coroutine framework -> 비동기 작업 수행에 특화
    • value를 set할 때 coroutine context 내에서 수행됨
    • updates are queued and delivered in an orderly & non-blockable manner
      -> high-frequency data update 처리에 유리

• MutableLiveData와 MutableStateFlow 사용 syntax
  • 둘다 value 프로퍼티로 value get & set
• observingMutableLiveData
  • Lifecycle Awareness (Activity/Fragment의 생명주기에 묶임)
  • UI-중심적

    // Observing LiveData in an Activity or Fragment
    personNameLiveData.observe(this, Observer { name ->
       // Update UI with the new name
    })

• collecting MutableStateFlow
  • No inherit lifecycle awareness
    -> 개발자가 생명주기 관리해야
    (ex. lifecycleScope 지정, Lifecycle.repeatOnLifecycle 블록 사용)
  • UI & UI 관련 data stream 관리에 적합
  • data stream 별로 별도의 coroutine launch 필요

    // Collecting StateFlow in a Coroutine
    lifecycleScope.launch {
       personNameStateFlow.collect { name ->
           // Update UI with the new name
       }
    }

MediatorLiveData -> Flow

• MediatorLiveData 마이그레이션 방법:
  여러 MutableStateFlow를 combine/zip하여 Flow로 합치기!

• ImageSearch 프로젝트 SearchViewModel.kt 마이그레이션 전

   private val _searchItems = MutableLiveData<List<Item>>(emptyList())
    val searchItems: LiveData<List<Item>> get() = _searchItems
    
    private val _savedItems = MutableLiveData<List<Item>>(emptyList())
    val savedItems: LiveData<List<Item>> get() = _savedItems
  
    private val _resultItems = MediatorLiveData<List<Item>>().apply {
        val onChange = { _: List<Item> ->
            value = searchItems.value!!.map { item ->
                savedItems.value!!.find { it.id == item.id }?.let {
                    item.copy(folderId = it.folderId)
                } ?: item.copy(folderId = FolderId.NO_FOLDER.id)
            }
        }
        addSource(searchItems, onChange)
        addSource(savedItems, onChange)
    }
    val resultItems: LiveData<List<Item>> get() = _resultItems

• ImageSearch 프로젝트 SearchViewModel.kt 마이그레이션 후

   private val _searchItems = MutableStateFlow<List<Item>>(emptyList())
    private val searchItems: StateFlow<List<Item>> get() = _searchItems
  
    private val _savedItems = MutableStateFlow<List<Item>>(emptyList())
    val savedItems: StateFlow<List<Item>> get() = _savedItems
  
    private val _resultItems =
        searchItems.combine(savedItems) { searchItems, savedItems ->
            searchItems.map { item ->
                savedItems.find { it.id == item.id }?.let {
                    item.copy(folderId = it.folderId)
                } ?: item.copy(folderId = FolderId.NO_FOLDER.id)
            }
        }
    val resultItems: Flow<List<Item>> get() = _resultItems