[Proposal] Introduce ObservableSystem make system chainable. about rxfeedback.swift HOT 3 OPEN

let events = [
   "tm" : Event.throwToMachine,
   "th" : .throwToHuman,
]

let states = [
   "h" : State.humanHasIt,
   "m" : .machineHasIt
]

// 1. create `TestScheduler`
let scheduler = TestScheduler(initialClock: 0, resolution: resolution, simulateProcessingDelay: false)

//  2. create mocked effects
let mockedEffects: (PitchRequest) -> Observable<Event> = scheduler.mock(values: events) { _ -> String in
   return "----th"
};


// 3. create mocked events
let (
   inputEvents,
   expectedStates
   ) = (
   scheduler.parseEventsAndTimes(timeline: "------tm------tm------tm-------", values: events).first!,
   scheduler.parseEventsAndTimes(timeline: "h-----m---h---m---h---m---h----", values: states).first!
)

// 4. inject mocked effects and events to system
let observableSystem = ObservableSystem.create(
   initialState: State.humanHasIt,
   reduce: { (state: State, event: Event) -> State in
       switch event {
       case .throwToMachine:
           return .machineHasIt
       case .throwToHuman:
           return .humanHasIt
       }
},
   scheduler: scheduler
   )
   .reacted(request: { $0.machinePitching }, effects: mockedEffects)

let state = observableSystem.system([{ _ in scheduler.createHotObservable(inputEvents).asObservable() }])

let recordedState = scheduler.record(source: state)

scheduler.start()

// 5. assert output states
XCTAssertEqual(recordedState.events, expectedStates)