kotlin中的冷流和热流示例详解

fun coldStream(): Flow<Int> = flow {
    println("开始发射")
    emit(1)
    emit(2)
    emit(3)
}
// 使用
suspend fun main() {
    val cold = coldStream()
    println("第一次收集:")
    cold.collect { println(it) } // 会触发完整的发射流程
    println("第二次收集:")
    cold.collect { println(it) } // 会再次触发完整的发射流程
}

suspend fun hotStreamExample() {
    val channel = Channel<Int>() // 热流
    launch {
        println("开始发射")
        channel.send(1)
        channel.send(2)
        channel.send(3)
        channel.close()
    }
    delay(100) // 确保发射已经开始
    println("第一次收集:")
    channel.consumeEach { println(it) } // 只能收到剩余数据
    // 第二次收集会失败，因为Channel已经被关闭
}

fun fetchUserData(): Flow<User> = flow {
    // 只有收集时才会真正查询数据库
    val data = database.queryUsers()
    emitAll(data.asFlow())
}

// 使用StateFlow管理UI状态
class ViewModel {
    private val _uiState = MutableStateFlow<UiState>(Loading)
    val uiState: StateFlow<UiState> = _uiState.asStateFlow()
    fun loadData() {
        viewModelScope.launch {
            _uiState.value = Loading
            try {
                val data = repository.fetchData()
                _uiState.value = Success(data)
            } catch (e: Exception) {
                _uiState.value = Error(e.message)
            }
        }
    }
}

val coldFlow = flow { /*...*/ }
// 转为SharedFlow（热流）
val sharedFlow = coldFlow.shareIn(
    scope = viewModelScope,
    started = SharingStarted.WhileSubscribed(),
    replay = 1
)
// 转为StateFlow（热流）
val stateFlow = coldFlow.stateIn(
    scope = viewModelScope,
    started = SharingStarted.WhileSubscribed(),
    initialValue = null
)

val hotChannel = Channel<Int>()
// 转为Flow（冷流）
val coldFlow = hotChannel.consumeAsFlow()