RxSwift deferred

网上只有deferred的使用，鲜有其源码流程，因其十分简洁，故详细贴上步骤，如有误，请留言指出，谢谢

let obv_defer = Observable.deferred { () -> Observable in
            return Observable.just(1)
        }        
obv_defer.subscribe(onNext:{
            print($0)
            }).disposed(by: disposeBag)

贴出关键步骤：

deferred后返回return Deferred(observableFactory: observableFactory)，保存了工厂闭包，subscribe了AnonymousObserver后，来到deferred的run方法

override func run(_ observer: Observer, cancel: Cancelable) -> (sink: Disposable, subscription: Disposable)
             where Observer.Element == Source.Element {
        let sink = DeferredSink(observableFactory: self._observableFactory, observer: observer, cancel: cancel)
        let subscription = sink.run()
        return (sink: sink, subscription: subscription)
    }

生成一个继承于Sink的DeferredSink，然后sink走run

func run() -> Disposable {
        do {
            let result = try self._observableFactory()
            return result.subscribe(self)
        }
        catch let e {
            self.forwardOn(.error(e))
            self.dispose()
            return Disposables.create()
        }
    }

这里是核心，也就是工厂里生成新Observable的时机，执行deferred保存的工厂闭包，拿到闭包返回的Observable，这里是Just(1)，由工厂新生成的observable来subscribe自身，也就是deferredSink

override func subscribe(_ observer: Observer) -> Disposable where Observer.Element == Element {
        observer.on(.next(self._element))
        observer.on(.completed)
        return Disposables.create()
    }

来到Just的subscribe方法，on出next后，来到deferredSink的on方法

func on(_ event: Event) {
        self.forwardOn(event)
        
        switch event {
        case .next:
            break
        case .error:
            self.dispose()
        case .completed:
            self.dispose()
        }
    }

forwardOn来到父类Sink

final func forwardOn(_ event: Event) {
        #if DEBUG
            self._synchronizationTracker.register(synchronizationErrorMessage: .default)
            defer { self._synchronizationTracker.unregister() }
        #endif
        if isFlagSet(self._disposed, 1) {
            return
        }
        self._observer.on(event)
    }

self._observer就是之前的AnonymousObserver

override func onCore(_ event: Event) {
        return self._eventHandler(event)
    }

AnonymousObserver只保存了一个闭包，也只有一个onCore方法，走到父类observerBase

func on(_ event: Event) {
        switch event {
        case .next:
            if load(self._isStopped) == 0 {
                self.onCore(event)
            }
        case .error, .completed:
            if fetchOr(self._isStopped, 1) == 0 {
                self.onCore(event)
            }
        }
    }

onCore后，调用AnonymousObserver的闭包，然后事件闭包onNext调用，走完最开始的print。最后是方法一系列返回，走完Just发送的completed，再一系列返回，走完BinaryDisposable，走完dispose，结束。