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StateFlow implementation (Kotlin#1974)
StateFlow is a Flow analogue to ConflatedBroadcastChannel. Since Flow API is simpler than channels APIs, the implementation of StateFlow is simpler. It consumes and allocates less memory, while still providing full deadlock-freedom (even though it is not lock-free internally). Fixes Kotlin#1973 Fixes Kotlin#395 Fixes Kotlin#1816 Co-authored-by: Vsevolod Tolstopyatov <qwwdfsad@gmail.com>
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| /* | ||
| * Copyright 2016-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license. | ||
| */ | ||
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| package kotlinx.coroutines.flow | ||
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| import kotlinx.atomicfu.* | ||
| import kotlinx.coroutines.* | ||
| import kotlinx.coroutines.channels.* | ||
| import kotlinx.coroutines.flow.internal.* | ||
| import kotlinx.coroutines.internal.* | ||
| import kotlin.coroutines.* | ||
| import kotlin.native.concurrent.* | ||
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| /** | ||
| * A [Flow] that represents a read-only state with a single updatable data [value] that emits updates | ||
| * to the value to its collectors. The current value can be retrieved via [value] property. | ||
| * The flow of future updates to the value can be observed by collecting values from this flow. | ||
| * | ||
| * A [mutable state flow][MutableStateFlow] is created using `MutableStateFlow(value)` constructor function with | ||
| * the initial value. The value of mutable state flow can be updated by setting its [value] property. | ||
| * Updates to the [value] are always [conflated][Flow.conflate]. So a slow collector skips fast updates, | ||
| * but always collects the most recently emitted value. | ||
| * | ||
| * [StateFlow] is useful as a data-model class to represent any kind of state. | ||
| * Derived values can be defined using various operators on the flows, with [combine] operator being especially | ||
| * useful to combine values from multiple state flows using arbitrary functions. | ||
| * | ||
| * For example, the following class encapsulates an integer state and increments its value on each call to `inc`: | ||
| * | ||
| * ``` | ||
| * class CounterModel { | ||
| * private val _counter = MutableStateFlow(0) // private mutable state flow | ||
| * val counter: StateFlow<Int> get() = _counter // publicly exposed as read-only state flow | ||
| * | ||
| * fun inc() { | ||
| * _counter.value++ | ||
| * } | ||
| * } | ||
| * ``` | ||
| * | ||
| * Having two instances of the above `CounterModel` class one can define the sum of their counters like this: | ||
| * | ||
| * ``` | ||
| * val aModel = CounterModel() | ||
| * val bModel = CounterModel() | ||
| * val sumFlow: Flow<Int> = aModel.counter.combine(bModel.counter) { a, b -> a + b } | ||
| * ``` | ||
| * | ||
| * ### Strong equality-based conflation | ||
| * | ||
| * Values in state flow are conflated using [Any.equals] comparison in a similar way to | ||
| * [distinctUntilChanged] operator. It is used to conflate incoming updates | ||
| * to [value][MutableStateFlow.value] in [MutableStateFlow] and to suppress emission of the values to collectors | ||
| * when new value is equal to the previously emitted one. State flow behavior with classes that violate | ||
| * the contract for [Any.equals] is unspecified. | ||
| * | ||
| * ### StateFlow vs ConflatedBroadcastChannel | ||
| * | ||
| * Conceptually state flow is similar to | ||
| * [ConflatedBroadcastChannel][kotlinx.coroutines.channels.ConflatedBroadcastChannel] | ||
| * and is designed to completely replace `ConflatedBroadcastChannel` in the future. | ||
| * It has the following important difference: | ||
| * | ||
| * * `StateFlow` is simpler, because it does not have to implement all the [Channel] APIs, which allows | ||
| * for faster, garbage-free implementation, unlike `ConflatedBroadcastChannel` implementation that | ||
| * allocates objects on each emitted value. | ||
| * * `StateFlow` always has a value which can be safely read at any time via [value] property. | ||
| * Unlike `ConflatedBroadcastChannel`, there is no way to create a state flow without a value. | ||
| * * `StateFlow` has a clear separation into a read-only `StateFlow` interface and a [MutableStateFlow]. | ||
| * * `StateFlow` conflation is based on equality like [distinctUntilChanged] operator, | ||
| * unlike conflation in `ConflatedBroadcastChannel` that is based on reference identity. | ||
| * * `StateFlow` cannot be currently closed like `ConflatedBroadcastChannel` and can never represent a failure. | ||
| * This feature might be added in the future if enough compelling use-cases are found. | ||
| * | ||
| * `StateFlow` is designed to better cover typical use-cases of keeping track of state changes in time, taking | ||
| * more pragmatic design choices for the sake of convenience. | ||
| * | ||
| * ### Concurrency | ||
| * | ||
| * All methods of data flow are **thread-safe** and can be safely invoked from concurrent coroutines without | ||
| * external synchronization. | ||
| * | ||
| * ### Operator fusion | ||
| * | ||
| * Application of [flowOn][Flow.flowOn], [conflate][Flow.conflate], | ||
| * [buffer] with [CONFLATED][Channel.CONFLATED] or [RENDEZVOUS][Channel.RENDEZVOUS] capacity, | ||
| * or a [distinctUntilChanged][Flow.distinctUntilChanged] operator has no effect on the state flow. | ||
| * | ||
| * ### Implementation notes | ||
| * | ||
| * State flow implementation is optimized for memory consumption and allocation-freedom. It uses a lock to ensure | ||
| * thread-safety, but suspending collector coroutines are resumed outside of this lock to avoid dead-locks when | ||
| * using unconfined coroutines. Adding new collectors has `O(1)` amortized cost, but updating a [value] has `O(N)` | ||
| * cost, where `N` is the number of active collectors. | ||
| * | ||
| * ### Not stable for inheritance | ||
| * | ||
| * **`StateFlow` interface is not stable for inheritance in 3rd party libraries**, as new methods | ||
| * might be added to this interface in the future, but is stable for use. | ||
| * Use `MutableStateFlow()` constructor function to create an implementation. | ||
| */ | ||
| @ExperimentalCoroutinesApi | ||
| public interface StateFlow<out T> : Flow<T> { | ||
| /** | ||
| * The current value of this state flow. | ||
| */ | ||
| public val value: T | ||
| } | ||
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| /** | ||
| * A mutable [StateFlow] that provides a setter for [value] and a method to [close] the flow. | ||
| * | ||
| * See [StateFlow] documentation for details. | ||
| * | ||
| * ### Not stable for inheritance | ||
| * | ||
| * **`MutableStateFlow` interface is not stable for inheritance in 3rd party libraries**, as new methods | ||
| * might be added to this interface in the future, but is stable for use. | ||
| * Use `MutableStateFlow()` constructor function to create an implementation. | ||
| */ | ||
| @ExperimentalCoroutinesApi | ||
| public interface MutableStateFlow<T> : StateFlow<T> { | ||
| /** | ||
| * The current value of this state flow. | ||
| * | ||
| * Setting a value that is [equal][Any.equals] to the previous one does nothing. | ||
| */ | ||
| public override var value: T | ||
| } | ||
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| /** | ||
| * Creates a [MutableStateFlow] with the given initial [value]. | ||
| */ | ||
| @Suppress("FunctionName") | ||
| @ExperimentalCoroutinesApi | ||
| public fun <T> MutableStateFlow(value: T): MutableStateFlow<T> = StateFlowImpl(value ?: NULL) | ||
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| // ------------------------------------ Implementation ------------------------------------ | ||
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| @SharedImmutable | ||
| private val NONE = Symbol("NONE") | ||
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| @SharedImmutable | ||
| private val PENDING = Symbol("PENDING") | ||
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| private const val INITIAL_SIZE = 2 // optimized for just a few collectors | ||
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| // StateFlow slots are allocated for its collectors | ||
| private class StateFlowSlot { | ||
| /** | ||
| * Each slot can have one of the following states: | ||
| * | ||
| * * `null` -- it is not used right now. Can [allocate] to new collector. | ||
| * * `NONE` -- used by a collector, but neither suspended nor has pending value. | ||
| * * `PENDING` -- pending to process new value. | ||
| * * `CancellableContinuationImpl<Unit>` -- suspended waiting for new value. | ||
| * | ||
| * It is important that default `null` value is used, because there can be a race between allocation | ||
| * of a new slot and trying to do [makePending] on this slot. | ||
| */ | ||
| private val _state = atomic<Any?>(null) | ||
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| fun allocate(): Boolean { | ||
| // No need for atomic check & update here, since allocated happens under StateFlow lock | ||
| if (_state.value != null) return false // not free | ||
| _state.value = NONE // allocated | ||
| return true | ||
| } | ||
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| fun free() { | ||
| _state.value = null // free now | ||
| } | ||
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| @Suppress("UNCHECKED_CAST") | ||
| fun makePending() { | ||
| _state.loop { state -> | ||
| when { | ||
| state == null -> return // this slot is free - skip it | ||
| state === PENDING -> return // already pending, nothing to do | ||
| state === NONE -> { // mark as pending | ||
| if (_state.compareAndSet(state, PENDING)) return | ||
| } | ||
| else -> { // must be a suspend continuation state | ||
| // we must still use CAS here since continuation may get cancelled and free the slot at any time | ||
| if (_state.compareAndSet(state, NONE)) { | ||
| (state as CancellableContinuationImpl<Unit>).resume(Unit) | ||
| return | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
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| fun takePending(): Boolean = _state.getAndSet(NONE)!!.let { state -> | ||
| assert { state !is CancellableContinuationImpl<*> } | ||
| return state === PENDING | ||
| } | ||
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| @Suppress("UNCHECKED_CAST") | ||
| suspend fun awaitPending(): Unit = suspendCancellableCoroutine sc@ { cont -> | ||
| assert { _state.value !is CancellableContinuationImpl<*> } // can be NONE or PENDING | ||
| if (_state.compareAndSet(NONE, cont)) return@sc // installed continuation, waiting for pending | ||
| // CAS failed -- the only possible reason is that it is already in pending state now | ||
| assert { _state.value === PENDING } | ||
| cont.resume(Unit) | ||
| } | ||
| } | ||
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| private class StateFlowImpl<T>(initialValue: Any) : SynchronizedObject(), MutableStateFlow<T>, FusibleFlow<T> { | ||
| private val _state = atomic(initialValue) // T | NULL | ||
| private var sequence = 0 // serializes updates, value update is in process when sequence is odd | ||
| private var slots = arrayOfNulls<StateFlowSlot?>(INITIAL_SIZE) | ||
| private var nSlots = 0 // number of allocated (!free) slots | ||
| private var nextIndex = 0 // oracle for the next free slot index | ||
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| @Suppress("UNCHECKED_CAST") | ||
| public override var value: T | ||
| get() = NULL.unbox(_state.value) | ||
| set(value) { | ||
| var curSequence = 0 | ||
| var curSlots: Array<StateFlowSlot?> = this.slots // benign race, we will not use it | ||
| val newState = value ?: NULL | ||
| synchronized(this) { | ||
| val oldState = _state.value | ||
| if (oldState == newState) return // Don't do anything if value is not changing | ||
| _state.value = newState | ||
| curSequence = sequence | ||
| if (curSequence and 1 == 0) { // even sequence means quiescent state flow (no ongoing update) | ||
| curSequence++ // make it odd | ||
| sequence = curSequence | ||
| } else { | ||
| // update is already in process, notify it, and return | ||
| sequence = curSequence + 2 // change sequence to notify, keep it odd | ||
| return | ||
| } | ||
| curSlots = slots // read current reference to collectors under lock | ||
| } | ||
| /* | ||
| Fire value updates outside of the lock to avoid deadlocks with unconfined coroutines | ||
| Loop until we're done firing all the changes. This is sort of simple flat combining that | ||
| ensures sequential firing of concurrent updates and avoids the storm of collector resumes | ||
| when updates happen concurrently from many threads. | ||
| */ | ||
| while (true) { | ||
| // Benign race on element read from array | ||
| for (col in curSlots) { | ||
| col?.makePending() | ||
| } | ||
| // check if the value was updated again while we were updating the old one | ||
| synchronized(this) { | ||
| if (sequence == curSequence) { // nothing changed, we are done | ||
| sequence = curSequence + 1 // make sequence even again | ||
| return // done | ||
| } | ||
| // reread everything for the next loop under the lock | ||
| curSequence = sequence | ||
| curSlots = slots | ||
| } | ||
| } | ||
| } | ||
|
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| override suspend fun collect(collector: FlowCollector<T>) { | ||
| val slot = allocateSlot() | ||
| var prevState: Any? = null // previously emitted T!! | NULL (null -- nothing emitted yet) | ||
| try { | ||
| // The loop is arranged so that it starts delivering current value without waiting first | ||
| while (true) { | ||
| // Here the coroutine could have waited for a while to be dispatched, | ||
| // so we use the most recent state here to ensure the best possible conflation of stale values | ||
| val newState = _state.value | ||
| // Conflate value emissions using equality | ||
| if (prevState == null || newState != prevState) { | ||
| collector.emit(NULL.unbox(newState)) | ||
| prevState = newState | ||
| } | ||
| // Note: if awaitPending is cancelled, then it bails out of this loop and calls freeSlot | ||
| if (!slot.takePending()) { // try fast-path without suspending first | ||
| slot.awaitPending() // only suspend for new values when needed | ||
| } | ||
| } | ||
| } finally { | ||
| freeSlot(slot) | ||
| } | ||
| } | ||
|
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| private fun allocateSlot(): StateFlowSlot = synchronized(this) { | ||
| val size = slots.size | ||
| if (nSlots >= size) slots = slots.copyOf(2 * size) | ||
| var index = nextIndex | ||
| var slot: StateFlowSlot | ||
| while (true) { | ||
| slot = slots[index] ?: StateFlowSlot().also { slots[index] = it } | ||
| index++ | ||
| if (index >= slots.size) index = 0 | ||
| if (slot.allocate()) break // break when found and allocated free slot | ||
| } | ||
| nextIndex = index | ||
| nSlots++ | ||
| slot | ||
| } | ||
|
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| private fun freeSlot(slot: StateFlowSlot): Unit = synchronized(this) { | ||
| slot.free() | ||
| nSlots-- | ||
| } | ||
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| override fun fuse(context: CoroutineContext, capacity: Int): FusibleFlow<T> { | ||
| // context is irrelevant for state flow and it is always conflated | ||
| // so it should not do anything unless buffering is requested | ||
| return when (capacity) { | ||
| Channel.CONFLATED, Channel.RENDEZVOUS -> this | ||
| else -> ChannelFlowOperatorImpl(this, context, capacity) | ||
| } | ||
| } | ||
| } |
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