Memory Management in RxSwift
The Question
What happens when an Observer subscribes to an Observable in RxSwift?
func someFunction() {
// Setup a local observable
let observable: Observable<Int> = Observable.create { observer in
// Send value 5 to the observer
observer.onNext(5)
return Disposables.create()
}
// Setup a local observer
let observer: AnyObserver<Int> = AnyObserver { event in
switch event {
case .next(let value):
// We expect the value to be 5 here
print(value)
case .completed, .error:
break
}
}
// The observer subscribes to the observable
observable.subscribe(observer)
// Question:
// Do the observable/observer remain in memory after the function returns?
}
Code-Level Decomposition
AnonymousObservable<Element>
A concrete observable to store the subscribe handler passed into Observable.create call.
extension ObservableType {
public static func create(_ subscribe: @escaping (AnyObserver<Element>) -> Disposable) -> Observable<Element> {
AnonymousObservable(subscribe)
}
}
final private class AnonymousObservable<Element>: Producer<Element> {
typealias SubscribeHandler = (AnyObserver<Element>) -> Disposable
let subscribeHandler: SubscribeHandler
init(_ subscribeHandler: @escaping SubscribeHandler) {
self.subscribeHandler = subscribeHandler
}
override func run<Observer: ObserverType>(_ observer: Observer, cancel: Cancelable) -> (sink: Disposable, subscription: Disposable) where Observer.Element == Element {
// An AnonymousObservableSink is created with references to:
// * The Observer
// * The SinkDisposer
let sink = AnonymousObservableSink(observer: observer, cancel: cancel)
let subscription = sink.run(self)
// Return:
// * AnonymousObservableSink as the sink
// * The disposable created & returned from the Observable.create closure as the subscription
return (sink: sink, subscription: subscription)
}
}
Producer<Element>
A concrete Observable type that inherits from the abstract type Observable<Element>.
class Producer<Element>: Observable<Element> {
override init() {
super.init()
}
override func subscribe<Observer: ObserverType>(_ observer: Observer) -> Disposable where Observer.Element == Element {
if !CurrentThreadScheduler.isScheduleRequired {
// The following happens when we call subscribe on an Observable:
// 1. A SinkDisposer is created
// 2. The Observable calls its implementation of the run function, and returns a Sink & Subscription
// 3. Assign the Sink & Subscription to the SinkDisposer
// 4. Return the SinkDisposer
let disposer = SinkDisposer()
let sinkAndSubscription = self.run(observer, cancel: disposer)
disposer.setSinkAndSubscription(sink: sinkAndSubscription.sink, subscription: sinkAndSubscription.subscription)
return disposer
}
else {
return CurrentThreadScheduler.instance.schedule(()) { _ in
let disposer = SinkDisposer()
let sinkAndSubscription = self.run(observer, cancel: disposer)
disposer.setSinkAndSubscription(sink: sinkAndSubscription.sink, subscription: sinkAndSubscription.subscription)
return disposer
}
}
}
func run<Observer: ObserverType>(_ observer: Observer, cancel: Cancelable) -> (sink: Disposable, subscription: Disposable) where Observer.Element == Element {
rxAbstractMethod()
}
}
Sink<Observer>
In the logical sense, a Sink serves as a bridge between the Observable and the Observer.</br> In practice, a Sink is used by RxSwift to store the logic used to designate the Producer, and maintains references to:
- Its observer, and
- A
SinkDisposer
class Sink<Observer: ObserverType>: Disposable {
// This property keeps a reference to the observer
fileprivate let observer: Observer
// This property keeps a reference to the SinkDisposer
fileprivate let cancel: Cancelable
// This flag tracks whether the sink is disposed
private let disposed = AtomicInt(0)
init(observer: Observer, cancel: Cancelable) {
self.observer = observer
self.cancel = cancel
}
final func forwardOn(_ event: Event<Observer.Element>) {
// Send an event to the observer only if the Sink has not yet been disposed
if isFlagSet(self.disposed, 1) {
return
}
self.observer.on(event)
}
final var isDisposed: Bool {
// Check if the sink has been disposed
isFlagSet(self.disposed, 1)
}
func dispose() {
// Update the disposed flag on the Sink, and call dispose on the SinkDisposer
fetchOr(self.disposed, 1)
self.cancel.dispose()
}
}
AnonymousObservableSink<Observer>
A sink to the AnonymousObservable type.
final private class AnonymousObservableSink<Observer: ObserverType>: Sink<Observer>, ObserverType {
typealias Element = Observer.Element
typealias Parent = AnonymousObservable<Element>
// state
private let isStopped = AtomicInt(0)
override init(observer: Observer, cancel: Cancelable) {
super.init(observer: observer, cancel: cancel)
}
func on(_ event: Event<Element>) {
switch event {
case .next:
if load(self.isStopped) == 1 {
return
}
self.forwardOn(event)
case .error, .completed:
if fetchOr(self.isStopped, 1) == 0 {
self.forwardOn(event)
self.dispose()
}
}
}
func run(_ parent: Parent) -> Disposable {
parent.subscribeHandler(AnyObserver(self))
}
}
SinkDisposer
A type responsible for disposing of both the sink & the subscription.
private final class SinkDisposer: Cancelable {
private enum DisposeState: Int32 {
case disposed = 1
case sinkAndSubscriptionSet = 2
}
// The SinkDisposer can be in one of the following states:
// * Initial - Sink & Subscription have not been set
// * SinkAndSubscriptionSet - Sink & Subscription have not been set
// * Disposed - SinkDisposer has been disposed
private let state = AtomicInt(0)
// This property keeps a reference to the Sink
private var sink: Disposable?
// This property keeps a reference to the SinkDisposer associated with
// the previous subscription
private var subscription: Disposable?
var isDisposed: Bool {
// Check if the sink has been disposed
isFlagSet(self.state, DisposeState.disposed.rawValue)
}
func setSinkAndSubscription(sink: Disposable, subscription: Disposable) {
// Assign the Sink & Subscription to the SinkDisposer
self.sink = sink
self.subscription = subscription
// Update the current state of the SinkDisposer to SinkAndSubscriptionSet
let previousState = fetchOr(self.state, DisposeState.sinkAndSubscriptionSet.rawValue)
// The Sink & Subscription are expected to be assigned only once.
// Thus if the Sink & Subscription have been previously assigned, throw an exception
if (previousState & DisposeState.sinkAndSubscriptionSet.rawValue) != 0 {
rxFatalError("Sink and subscription were already set")
}
// If the SinkDisposer is already disposed of, also dispose of its Sink & Subscription
if (previousState & DisposeState.disposed.rawValue) != 0 {
sink.dispose()
subscription.dispose()
self.sink = nil
self.subscription = nil
}
}
func dispose() {
// If the SinkDisposer is already disposed of, repeatedly calling
// this function does nothing
let previousState = fetchOr(self.state, DisposeState.disposed.rawValue)
if (previousState & DisposeState.disposed.rawValue) != 0 {
return
}
if (previousState & DisposeState.sinkAndSubscriptionSet.rawValue) != 0 {
// If either of the following:
// * Sink
// * Subscription
// is not set when this function is called, an exception is thrown
guard let sink = self.sink else {
rxFatalError("Sink not set")
}
guard let subscription = self.subscription else {
rxFatalError("Subscription not set")
}
// Otherwise dispose of the Sink & Subscription
sink.dispose()
subscription.dispose()
self.sink = nil
self.subscription = nil
}
}
}
Visual Depiction
Bird’s Eye View
Image from 8 Mistakes to Avoid while Using RxSwift — Part 1
Image from