D3.js-like enter and exit functions for ReactiveSwift

SignalProducer+D3.swift

//
//  SignalProducer+D3.swift
//
//  Created by Cristián Arenas Ulloa on 3/23/17.
//  WTFPLv2
//
//  This overcomplicated tool fits my use case.
//  It will probably make a lot of sense to you too if you've ever used D3.js.
//  There was probably an easier way to do it,
//  but I'm just learning ReactiveSwift so don't hate me .
//

import ReactiveSwift

extension SignalProducer {
    /// Observe entering and exiting values (see D3.js)
    ///
    /// - parameters:
    ///   - onEnter: A closure that is invoked when a new value is emitted.
    ///   - onExit: A closure that is invoked after the last value is gone.
    public func observeValues(
        onEnter: ((Value) -> Void)? = nil,
        onExit: ((Value) -> Void)? = nil
    ) {
        take(first: 1).on(value: onEnter).start()
        take(last: 1).on(value: onExit).start()
        transitions().on(value: tupleFunctions(onExit, onEnter)).start()
    }
    
    /// Zip elements of the same producer into pairs. The elements of any Nth pair
    /// are the (N-1)th element and the Nth element of the producer.
    /// The tuples represent the transition from the last value to the current value.
    ///
    /// - returns: A producer that sends tuples of the last value and the current value.
    public func transitions() -> SignalProducer<(Value, Value), Error> {
        return zip(with: skip(first: 1))
    }
}

/// Mix two single parameter functions to get one that acts on tuples
public func tupleFunctions<A, B>(
    _ a: ((A) -> Void)?,
    _ b: ((B) -> Void)?
) -> ((A, B) -> Void) {
    return { va, vb in
        a?(va)
        b?(vb)
    }
}