Add synchronized access to EventEngine's state variable

fix(subscribe): fix the crash issue caused by multiple accesses to the state property in multithreaded scenarios

Sources/PubNub/EventEngine/Core/EventEngine.swift

@@ -17,8 +17,10 @@ struct EventEngineDependencies<Dependencies> {
 class EventEngine<State, Event, Invocation: AnyEffectInvocation, Input> {
   private let transition: any TransitionProtocol<State, Event, Invocation>
   private let dispatcher: any Dispatcher<Invocation, Event, Input>
-  private(set) var state: State
+  private let stateContainer: Atomic<State>
+  private let recursiveLock = NSRecursiveLock()
 
+  var state: State { stateContainer.lockedRead { $0 } }
   var dependencies: EventEngineDependencies<Input>
   var onStateUpdated: ((State) -> Void)?
 
@@ -29,30 +31,33 @@ class EventEngine<State, Event, Invocation: AnyEffectInvocation, Input> {
     dispatcher: some Dispatcher<Invocation, Event, Input>,
     dependencies: EventEngineDependencies<Input>
   ) {
-    self.state = state
+    self.stateContainer = Atomic(state)
     self.onStateUpdated = onStateUpdated
     self.transition = transition
     self.dispatcher = dispatcher
     self.dependencies = dependencies
   }
 
   func send(event: Event) {
-    objc_sync_enter(self)
+    recursiveLock.lock()
 
     defer {
-      objc_sync_exit(self)
+      recursiveLock.unlock()
     }
+
+    let currentState = state
+
     guard transition.canTransition(
-      from: state,
+      from: currentState,
       dueTo: event
     ) else {
       return
     }
 
-    let transitionResult = transition.transition(from: state, event: event)
+    let transitionResult = transition.transition(from: currentState, event: event)
     let invocations = transitionResult.invocations
 
-    state = transitionResult.state
+    stateContainer.lockedWrite { $0 = transitionResult.state }
     onStateUpdated?(state)
 
     let listener = DispatcherListener<Event>(