zipWith for ReactiveCocoa

gistfile1.swift

/// Zips elements of two signals into pairs. The elements of any Nth pair
/// are the Nth elements of the two input signals.
public func zipWith<T, U, E>(otherSignal: Signal<U, E>)(signal: Signal<T, E>) -> Signal<(T, U), E> {
	return Signal { observer in
		let lock = NSRecursiveLock()
		lock.name = "org.reactivecocoa.ReactiveCocoa.zipWith"
		
		var signalValues: [T] = []
		var otherValues: [U] = []
		
		enum Either<T, U> {
			case Left(T)
			case Right(U)
		}
		
		let appendAndFlushValues: Either<T, U> -> () = { either in
			var tuple: (T, U)?
			
			lock.lock()
			switch either {
			case let .Left(val):
				signalValues.append(val)
			case let .Right(val):
				otherValues.append(val)
			}
			if !signalValues.isEmpty && !otherValues.isEmpty {
				tuple = (signalValues.removeAtIndex(0), otherValues.removeAtIndex(0))
			}
			lock.unlock()
			
			if let tuple = tuple {
				sendNext(observer, tuple)
			}
		}
		
		let onError = { sendError(observer, $0) }
		let onCompleted = { sendCompleted(observer) }
		
		let signalDisposable = signal.observe(next: { value in
			appendAndFlushValues(.Left(value))
		}, error: onError, completed: onCompleted)
		
		let otherDisposable = otherSignal.observe(next: { value in
			appendAndFlushValues(.Right(value))
		}, error: onError, completed: onCompleted)
		
		return CompositeDisposable([ signalDisposable, otherDisposable ])
	}
}