RxSwift-dispose销毁者源码分析

• 当序列发出error 或者 completed 事件，观察者就不能继续响应，内部做了什么？
• 观察者返回的Disposable，手动调用，又做了什么？
  这一切的一切，都是RxSwift的销毁者dispose在作怪
  接下来lldb调试对源码进行分析，运行以下demo

       // 创建序列
        let ob = Observable<Any>.create { (observer) -> Disposable in
            observer.onNext("销毁者")
            return Disposables.create { print("销毁释放了")} // dispose.dispose()
        }
        // 序列订阅
        let dispose = ob.subscribe(onNext: { (anything) in
            print("订阅到了:\(anything)")
        }, onError: { (error) in
            print("订阅到了:\(error)")
        }, onCompleted: {
            print("完成了")
        }) {
            print("销毁回调")
        }
        // 对象 是无法销毁的
        print("执行完毕")
        dispose.dispose()

输出日志

订阅到了:销毁者
执行完毕
销毁释放了
销毁回调

• 点击create，创建了一个匿名销毁者对象

 public static func create(with dispose: @escaping () -> Void) -> Cancelable {
        return AnonymousDisposable(disposeAction: dispose)
    }

• 点击AnonymousObservable，初始化保存了响应回调闭包

fileprivate final class AnonymousDisposable : DisposeBase, Cancelable {
    public typealias DisposeAction = () -> Void
    private let _isDisposed = AtomicInt(0)
    private var _disposeAction: DisposeAction?
    public var isDisposed: Bool {
        return isFlagSet(self._isDisposed, 1)
    }
    fileprivate init(_ disposeAction: @escaping DisposeAction) {
        self._disposeAction = disposeAction
        super.init()
    }
    fileprivate init(disposeAction: @escaping DisposeAction) {
        self._disposeAction = disposeAction
        super.init()
    }
    fileprivate func dispose() {
        if fetchOr(self._isDisposed, 1) == 0 {
            if let action = self._disposeAction {
                self._disposeAction = nil
                action()
            }
        }
    }
}

• 通过上面代码可以看出，self._disposeAction是在调用dispose()的时候销毁的，然后执行开始保存的action()闭包，那么问题来了，什么时候开始调用的？

销毁调用

• 订阅序列时通过临时变量disposable保存外界的销毁闭包，当收到error和completed信号时，调用disposable.dispose()，销毁临时销毁者变量，具体分析看注释

public func subscribe(onNext: ((Element) -> Void)? = nil, onError: ((Swift.Error) -> Void)? = nil, onCompleted: (() -> Void)? = nil, onDisposed: (() -> Void)? = nil)
        -> Disposable {
            let disposable: Disposable
            ======保存外界销毁闭包======
            if let disposed = onDisposed {
                disposable = Disposables.create(with: disposed)
            }
            else {
                disposable = Disposables.create()
            }
            ======实际上保存的就是 print("销毁回调")======
            #if DEBUG
                let synchronizationTracker = SynchronizationTracker()
            #endif
            
            let callStack = Hooks.recordCallStackOnError ? Hooks.customCaptureSubscriptionCallstack() : []
            
            let observer = AnonymousObserver<Element> { event in
                
                #if DEBUG
                    synchronizationTracker.register(synchronizationErrorMessage: .default)
                    defer { synchronizationTracker.unregister() }
                #endif
                //当收到序列发送的.error和.completed信号时，会触发disposable.dispose()
                switch event {
                case .next(let value):
                    onNext?(value)
                case .error(let error):
                    if let onError = onError {
                        onError(error)
                    }
                    else {
                        Hooks.defaultErrorHandler(callStack, error)
                    }
                    disposable.dispose()
                case .completed:
                    onCompleted?()
                    disposable.dispose()
                }
            }
            
            return Disposables.create(
                self.asObservable().subscribe(observer),
                disposable
            )

• 通过上面代码可知，调用subscribe返回return Disposables.create( self.asObservable().subscribe(observer), disposable )，继续跟踪得知，实际上返回一个BinaryDisposable实例，入参为self.asObservable().subscribe(observer)和临时销毁者disposable，如下：

extension Disposables {
    /// Creates a disposable with the given disposables.
    public static func create(_ disposable1: Disposable, _ disposable2: Disposable) -> Cancelable {
        return BinaryDisposable(disposable1, disposable2)
    }
}

• 跟踪进入BinaryDisposable源码如下：实际上就是用两个销毁者构建一个二元销毁者

private final class BinaryDisposable : DisposeBase, Cancelable {
    private let _isDisposed = AtomicInt(0)
    private var _disposable1: Disposable?
    private var _disposable2: Disposable?
    /// - returns: Was resource disposed.
    var isDisposed: Bool {
        return isFlagSet(self._isDisposed, 1)
    }
    //通过两个销毁者得到一个元销毁者
    init(_ disposable1: Disposable, _ disposable2: Disposable) {
        self._disposable1 = disposable1
        self._disposable2 = disposable2
        super.init()
    }
    /// Calls the disposal action if and only if the current instance hasn't been disposed yet.
    ///
    /// After invoking disposal action, disposal action will be dereferenced.
    func dispose() {
        if fetchOr(self._isDisposed, 1) == 0 {
            self._disposable1?.dispose()
            self._disposable2?.dispose()
            self._disposable1 = nil
            self._disposable2 = nil
        }
    }
}

• 通过上面代码可以知道，在subscribe中返回的变量，具有直接调用dispose()功能，也就是在BinaryDisposable中的dispose(），而他的功能，既销毁了订阅中的临时销毁者，也销毁了。。。。。。到底销毁了啥？
• 关键点就是self.asObservable().subscribe(observer)做了什么，LLDB调试一下，分析进入Producer的subscribe源码如下：

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 returned disposable needs to release all references once it was disposed.
            //返回一个销毁者
            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()
    }
}

• subscribe就是返回了一个销毁者，这也验证了，BinaryDisposable(disposable1, disposable2)传入是的两个销毁者，一个是SinkDisposer(),另一个是保存外界销毁闭包的局部销毁者let disposable: Disposable
• 首先看看入参都是什么disposer.setSinkAndSubscription传入了一个元祖，一个AnonymousObservableSink和parent._subscribeHandler(AnyObserver(self))即（外界创建序列的闭包的返回销毁者）

override func run<Observer: ObserverType>(_ observer: Observer, cancel: Cancelable) -> (sink: Disposable, subscription: Disposable) where Observer.Element == Element {
        let sink = AnonymousObservableSink(observer: observer, cancel: cancel)
        let subscription = sink.run(self)
        return (sink: sink, subscription: subscription)
    }

• 接下来看看SinkDisposer做了什么,当状态满足的时候，就直接调用
  sink.dispose()
subscription.dispose()
self._sink = nil
self._subscription = nil
  执行销毁和释放，源码如下：

 func setSinkAndSubscription(sink: Disposable, subscription: Disposable) {
        self._sink = sink
        self._subscription = subscription
        //获取状态
        let previousState = fetchOr(self._state, DisposeState.sinkAndSubscriptionSet.rawValue)
        if (previousState & DisposeState.sinkAndSubscriptionSet.rawValue) != 0 {
            rxFatalError("Sink and subscription were already set")
        }
        //如果状态满足就销毁
        if (previousState & DisposeState.disposed.rawValue) != 0 {
            sink.dispose()
            subscription.dispose()
            self._sink = nil
            self._subscription = nil
        }
    }

• 当收到完成或者错误信号，传入的AnonymousObservableSink:Sink:Disposable就直接调用dispose()销毁

func on(_ event: Event<Element>) {
    #if DEBUG
        self._synchronizationTracker.register(synchronizationErrorMessage: .default)
        defer { self._synchronizationTracker.unregister() }
    #endif
    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()
        }
    }
}

• 进入dispose()查看如下:

    init(observer: Observer, cancel: Cancelable) {
#if TRACE_RESOURCES
        _ = Resources.incrementTotal()
#endif
        self._observer = observer
        self._cancel = cancel
    }
    func dispose() {
        fetchOr(self._disposed, 1)
        self._cancel.dispose()
    }

• self._cancel就是当时初始化传进来的销毁者SinkDisposer,

     let disposer = SinkDisposer()
     let sinkAndSubscription = self.run(observer, cancel: disposer)
     disposer.setSinkAndSubscription(sink: sinkAndSubscription.sink, subscription: sinkAndSubscription.subscription)

• 因此可以得知，如果序列发出complete或error信号，以后就无法再接收响应。

整个流程分析下来，可知：

• sink将序列、观察者、调度者、销毁者关联了起来，一旦将sink = nil，他们之间就失去联系，内部创建的临时序列和观察者,也会随着对外的观察者和序列的生命周期而销毁释放，因此就无法继续响应;
• 而外部创建的let dispose = ob.subscribe()局部变量，即使不主动调用dispose.dispose(),也会随着作用域空间释放而销毁。