Reactive Cocoa custom signal: diffDetect

diffDetect.swift

// Example of writing a custom signal:

// Starts false, sends true when the signal departs from 
// the initial value. Thereafter sends nothing, until the 
// signal gets back to initial value, whereupon sends false.

import ReactiveCocoa

extension SignalType where Value: Equatable {
  func diffDetect(i: Value) -> Signal<Bool, Error> {
    return Signal { observer in
      var lastDiff = true
      return self.observeNext { (v: Self.Value) in
        let diff = !(v == i)
        if lastDiff != diff {
          lastDiff = diff
          observer.sendNext(diff)
        }
      }
    }
  }
}

extension SignalProducerType where Value: Equatable {
  func diffDetect(i: Value) -> SignalProducer<Bool, Error> {
    return lift { $0.diffDetect(i) }
  }
}