仿照RAS实现SignalProducer

SignalProducer的作用：

1. 与Signal相比，Signal在调用send的时候，指定观察者动作，SignalProducer在构造时候，就指定了观察者动作；

SignalProducer的实现：

1. 在执行start方法时，构造了Signal，将观察者对象添加到Signal中，指定SignalProducer的startHandler，并传入内部的obs控制对象

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附代码：

struct aSignalProducer<Value, Error> {
    let core: aSignalProducerCore<Value, Error>
    
    init(_ startHandler: @escaping (aSignal<Value, Error>.aObserver) -> Void) {
        self.init(aSignalCore {
            let fakeDisposable = aDisposable({ print("fake Disposable") })
            let (signal, obs) = aSignal<Value, Error>.pipe()
            let observerDidSetup = { startHandler(obs) }
            
            return aSignalCore<Value, Error>.Instance(signal: signal, observerDidSetup: observerDidSetup, interruptedHandle: fakeDisposable)
        })
    }
    
    init(value: Value) {
        self.init(aGeneratorCore<Value, Error> { observer, _ in
            observer.send(value)
        })
    }
    
    init(_ core: aSignalProducerCore<Value, Error>) {
        self.core = core
    }
    
    @discardableResult
    func start(_ observer: aSignal<Value, Error>.aObserver) -> aDisposable {
        return core.start { _ in observer }
    }
}

internal class aSignalProducerCore<Value, Error> {
    struct Instance {
        let signal: aSignal<Value, Error>
        let observerDidSetup: () -> Void
        let interruptedHandle: aDisposable
    }
    
    func makeInstance() -> Instance {
        fatalError()
    }
    
    func start(_ generator: (_ disposable: aDisposable) -> aSignal<Value, Error>.aObserver) -> aDisposable {
        fatalError()
    }
}

private class aSignalCore<Value, Error>: aSignalProducerCore<Value, Error> {
    let _make: () -> Instance
    
    init(_ action: @escaping () -> Instance) {
        self._make = action
    }
    
    override func makeInstance() -> aSignalProducerCore<Value, Error>.Instance {
        return self._make()
    }
    
    override func start(_ generator: (aDisposable) -> aSignal<Value, Error>.aObserver) -> aDisposable {
        let instance = makeInstance()
        instance.signal.observe(generator(instance.interruptedHandle))
        instance.observerDidSetup()
        
        return instance.interruptedHandle
    }
}

private class aGeneratorCore<Value, Error>: aSignalProducerCore<Value, Error> {
    let _generator: (aSignal<Value, Error>.aObserver, aDisposable) -> Void
    
    init(_ generator: @escaping (aSignal<Value, Error>.aObserver, aDisposable) -> Void) {
        self._generator = generator
    }
    
    override func makeInstance() -> aSignalProducerCore<Value, Error>.Instance {
        let fakeDisposable = aDisposable({})
        let (signal, obs) = aSignal<Value, Error>.pipe()
    
        
        return Instance(signal: signal, observerDidSetup: { self._generator(obs, fakeDisposable) }, interruptedHandle: fakeDisposable)
    }
    
    override func start(_ generator: (aDisposable) -> aSignal<Value, Error>.aObserver) -> aDisposable {
        let instance = makeInstance()
        instance.signal.observe(generator(instance.interruptedHandle))
        instance.observerDidSetup()
        
        return instance.interruptedHandle
    }
}

        /**
         * aSignalProducer 使用方式
         */
        
        let asps = aSignalProducer<String, NoError> { observer in
            observer.send("22222")
        }
        
        let asubscriber = aSignal<String, NoError>.aObserver { event in
            if case let .value(v) = event {
                print(v)
            } else if case .interupt = event {
                print("interrupted")
            }
        }
        
        asps.start(asubscriber)