RXSwift源码解析4 ( 操作符 take(takeCount) )

这个比较简单，一贯的套路，生成中间的 TakeCountSink 作为源序列的实际订阅者，收到 next 事件就将计数 -1 ，计数为0时，调用 dispose 释放资源。（sink 将信号处理后转发给订阅者）

extension ObservableType {

    /**
     Returns a specified number of contiguous elements from the start of an observable sequence.

     - seealso: [take operator on reactivex.io](http://reactivex.io/documentation/operators/take.html)

     - parameter count: The number of elements to return.
     - returns: An observable sequence that contains the specified number of elements from the start of the input sequence.
     */
    public func take(_ count: Int)
        -> Observable<Element> {
        if count == 0 {
            return Observable.empty()
        }
        else {
            return TakeCount(source: self.asObservable(), count: count)
        }
    }
}

final private class TakeCountSink<Observer: ObserverType>: Sink<Observer>, ObserverType {
    typealias Element = Observer.Element 
    typealias Parent = TakeCount<Element>
    
    private let parent: Parent
    
    private var remaining: Int
    
    init(parent: Parent, observer: Observer, cancel: Cancelable) {
        self.parent = parent
        self.remaining = parent.count
        super.init(observer: observer, cancel: cancel)
    }
    
    func on(_ event: Event<Element>) {
        switch event {
        case .next(let value):
            
            if self.remaining > 0 {
                self.remaining -= 1
                
                self.forwardOn(.next(value))
            
                if self.remaining == 0 {
                    self.forwardOn(.completed)
                    self.dispose()
                }
            }
        case .error:
            self.forwardOn(event)
            self.dispose()
        case .completed:
            self.forwardOn(event)
            self.dispose()
        }
    }
    
}

final private class TakeCount<Element>: Producer<Element> {
    private let source: Observable<Element>
    fileprivate let count: Int
    
    init(source: Observable<Element>, count: Int) {
        if count < 0 {
            rxFatalError("count can't be negative")
        }
        self.source = source
        self.count = count
    }
    
    override func run<Observer: ObserverType>(_ observer: Observer, cancel: Cancelable) -> (sink: Disposable, subscription: Disposable) where Observer.Element == Element {
        let sink = TakeCountSink(parent: self, observer: observer, cancel: cancel)
        let subscription = self.source.subscribe(sink)
        return (sink: sink, subscription: subscription)
    }
}
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