Add skipUntil operator

ReactiveCocoa/Swift/Signal.swift

@@ -1016,6 +1016,27 @@ extension SignalType {
 			return disposable
 		}
 	}
+
+	/// Does not forward any values from `self` until `trigger` sends a Next or
+	/// Completed, at which point the returned signal behaves exactly like `signal`.
+	@warn_unused_result(message="Did you forget to call `observe` on the signal?")
+	public func skipUntil(trigger: Signal<(), NoError>) -> Signal<Value, Error> {
+		return Signal { observer in
+			let disposable = SerialDisposable()
+
+			disposable.innerDisposable = trigger.observe { event in
+				switch event {
+				case .Next, .Completed:
+					disposable.innerDisposable = self.observe(observer)
+
+				case .Failed, .Interrupted:
+					break
+				}
+			}
+
+			return disposable
+		}
+	}
 
 	/// Forwards events from `self` with history: values of the returned signal
 	/// are a tuple whose first member is the previous value and whose second member

ReactiveCocoa/Swift/SignalProducer.swift

@@ -588,6 +588,20 @@ extension SignalProducerType {
 		return lift(Signal.takeUntil)(trigger)
 	}
 
+	/// Does not forward any values from `self` until `trigger` sends a Next or
+	/// Completed, at which point the returned signal behaves exactly like `signal`.
+	@warn_unused_result(message="Did you forget to call `start` on the producer?")
+	public func skipUntil(trigger: SignalProducer<(), NoError>) -> SignalProducer<Value, Error> {
+		return liftRight(Signal.skipUntil)(trigger)
+	}
+
+	/// Does not forward any values from `self` until `trigger` sends a Next or
+	/// Completed, at which point the returned signal behaves exactly like `signal`.
+	@warn_unused_result(message="Did you forget to call `start` on the producer?")
+	public func skipUntil(trigger: Signal<(), NoError>) -> SignalProducer<Value, Error> {
+		return lift(Signal.skipUntil)(trigger)
+	}
+
 	/// Forwards events from `self` with history: values of the returned producer
 	/// are a tuple whose first member is the previous value and whose second member
 	/// is the current value. `initial` is supplied as the first member when `self`

ReactiveCocoaTests/Swift/SignalProducerLiftingSpec.swift

@@ -298,6 +298,48 @@ class SignalProducerLiftingSpec: QuickSpec {
 				expect(lastValue).to(equal(1))
 			}
 		}
+
+		describe("skipUntil") {
+			var producer: SignalProducer<Int, NoError>!
+			var observer: Signal<Int, NoError>.Observer!
+			var triggerObserver: Signal<(), NoError>.Observer!
+
+			var lastValue: Int? = nil
+
+			beforeEach {
+				let (baseProducer, baseIncomingObserver) = SignalProducer<Int, NoError>.buffer()
+				let (triggerSignal, incomingTriggerObserver) = SignalProducer<(), NoError>.buffer()
+
+				producer = baseProducer.skipUntil(triggerSignal)
+				observer = baseIncomingObserver
+				triggerObserver = incomingTriggerObserver
+
+				lastValue = nil
+
+				producer.start { event in
+					switch event {
+					case let .Next(value):
+						lastValue = value
+					default:
+						break
+					}
+				}
+			}
+
+			it("should skip values until the trigger fires") {
+				expect(lastValue).to(beNil())
+
+				observer.sendNext(1)
+				expect(lastValue).to(beNil())
+
+				observer.sendNext(2)
+				expect(lastValue).to(beNil())
+
+				triggerObserver.sendNext(())
+				observer.sendNext(0)
+				expect(lastValue).to(equal(0))
+			}
+		}
 
 		describe("take") {
 			it("should take initial values") {

ReactiveCocoaTests/Swift/SignalSpec.swift

@@ -784,6 +784,48 @@ class SignalSpec: QuickSpec {
 				expect(lastValue).to(equal(1))
 			}
 		}
+
+		describe("skipUntil") {
+			var signal: Signal<Int, NoError>!
+			var observer: Signal<Int, NoError>.Observer!
+			var triggerObserver: Signal<(), NoError>.Observer!
+
+			var lastValue: Int? = nil
+
+			beforeEach {
+				let (baseSignal, incomingObserver) = Signal<Int, NoError>.pipe()
+				let (triggerSignal, incomingTriggerObserver) = Signal<(), NoError>.pipe()
+
+				signal = baseSignal.skipUntil(triggerSignal)
+				observer = incomingObserver
+				triggerObserver = incomingTriggerObserver
+
+				lastValue = nil
+
+				signal.observe { event in
+					switch event {
+					case let .Next(value):
+						lastValue = value
+					default:
+						break
+					}
+				}
+			}
+
+			it("should skip values until the trigger fires") {
+				expect(lastValue).to(beNil())
+
+				observer.sendNext(1)
+				expect(lastValue).to(beNil())
+
+				observer.sendNext(2)
+				expect(lastValue).to(beNil())
+
+				triggerObserver.sendNext(())
+				observer.sendNext(0)
+				expect(lastValue).to(equal(0))
+			}
+		}		
 
 		describe("take") {
 			it("should take initial values") {