Implement lazyMap as an event transformation.

Sources/Event.swift

@@ -699,6 +699,53 @@ extension Signal.Event {
 		}
 	}
 
+	internal static func lazyMap<U>(on scheduler: Scheduler, transform: @escaping (Value) -> U) -> Transformation<U, Error> {
+		return { action, lifetime in
+			let box = Atomic<Value?>(nil)
+			let completionDisposable = SerialDisposable()
+			let valueDisposable = SerialDisposable()
+
+			lifetime += valueDisposable
+			lifetime += completionDisposable
+
+			lifetime.observeEnded {
+				scheduler.schedule {
+					action(.interrupted)
+				}
+			}
+
+			return { event in
+				switch event {
+				case let .value(value):
+					// Schedule only when there is no prior outstanding value.
+					if box.swap(value) == nil {
+						valueDisposable.inner = scheduler.schedule {
+							if let value = box.swap(nil) {
+								action(.value(transform(value)))
+							}
+						}
+					}
+
+				case .completed, .failed:
+					// Completion and failure should not discard the outstanding
+					// value.
+					completionDisposable.inner = scheduler.schedule {
+						action(event.map(transform))
+					}
+
+				case .interrupted:
+					// `interrupted` overrides any outstanding value and any
+					// scheduled completion/failure.
+					valueDisposable.dispose()
+					completionDisposable.dispose()
+					scheduler.schedule {
+						action(.interrupted)
+					}
+				}
+			}
+		}
+	}
+
 	internal static func delay(_ interval: TimeInterval, on scheduler: DateScheduler) -> Transformation<Value, Error> {
 		precondition(interval >= 0)
 

Sources/Signal.swift

@@ -595,10 +595,7 @@ extension Signal {
 	/// - returns: A signal that sends values obtained using `transform` as this 
 	///            signal sends values.
 	public func lazyMap<U>(on scheduler: Scheduler, transform: @escaping (Value) -> U) -> Signal<U, Error> {
-		return flatMap(.latest) { value in
-			return SignalProducer({ transform(value) })
-				.start(on: scheduler)
-		}
+		return flatMapEvent(Signal.Event.lazyMap(on: scheduler, transform: transform))
 	}
 
 	/// Preserve only values which pass the given closure.

Sources/SignalProducer.swift

@@ -895,7 +895,7 @@ extension SignalProducer {
 	/// - returns: A producer that, when started, sends values obtained using 
 	///            `transform` as this producer sends values.
 	public func lazyMap<U>(on scheduler: Scheduler, transform: @escaping (Value) -> U) -> SignalProducer<U, Error> {
-		return lift { $0.lazyMap(on: scheduler, transform: transform) }
+		return core.flatMapEvent(Signal.Event.lazyMap(on: scheduler, transform: transform))
 	}
 
 	/// Preserve only values which pass the given closure.

Tests/ReactiveSwiftTests/SignalProducerLiftingSpec.swift

@@ -313,7 +313,11 @@ class SignalProducerLiftingSpec: QuickSpec {
 			}
 
 			it("should interrupt ASAP and discard outstanding events") {
-				testAsyncASAPInterruption { $0.lazyMap(on: $1) { $0 } }
+				testAsyncASAPInterruption(op: "lazyMap") { $0.lazyMap(on: $1) { $0 } }
+			}
+
+			it("should interrupt on the given scheduler") {
+				testAsyncInterruptionScheduler(op: "lazyMap") { $0.lazyMap(on: $1) { $0 } }
 			}
 		}
 
@@ -1133,7 +1137,11 @@ class SignalProducerLiftingSpec: QuickSpec {
 			}
 
 			it("should interrupt ASAP and discard outstanding events") {
-				testAsyncASAPInterruption { $0.observe(on: $1) }
+				testAsyncASAPInterruption(op: "observe(on:)") { $0.observe(on: $1) }
+			}
+
+			it("should interrupt on the given scheduler") {
+				testAsyncInterruptionScheduler(op: "observe(on:)") { $0.observe(on: $1) }
 			}
 		}
 
@@ -1200,7 +1208,11 @@ class SignalProducerLiftingSpec: QuickSpec {
 			}
 
 			it("should interrupt ASAP and discard outstanding events") {
-				testAsyncASAPInterruption { $0.delay(10.0, on: $1) }
+				testAsyncASAPInterruption(op: "delay") { $0.delay(10.0, on: $1) }
+			}
+
+			it("should interrupt on the given scheduler") {
+				testAsyncInterruptionScheduler(op: "delay") { $0.delay(10.0, on: $1) }
 			}
 		}
 
@@ -1298,13 +1310,21 @@ class SignalProducerLiftingSpec: QuickSpec {
 			}
 
 			it("should interrupt ASAP and discard outstanding events") {
-				testAsyncASAPInterruption { $0.throttle(10.0, on: $1) }
+				testAsyncASAPInterruption(op: "throttle") { $0.throttle(10.0, on: $1) }
+			}
+
+			it("should interrupt on the given scheduler") {
+				testAsyncInterruptionScheduler(op: "throttle") { $0.throttle(10.0, on: $1) }
 			}
 		}
 
 		describe("debounce") {
 			it("should interrupt ASAP and discard outstanding events") {
-				testAsyncASAPInterruption { $0.delay(10.0, on: $1) }
+				testAsyncASAPInterruption(op: "debounce") { $0.debounce(10.0, on: $1) }
+			}
+
+			it("should interrupt on the given scheduler") {
+				testAsyncInterruptionScheduler(op: "debounce") { $0.debounce(10.0, on: $1) }
 			}
 		}
 
@@ -2036,7 +2056,36 @@ class SignalProducerLiftingSpec: QuickSpec {
 	}
 }
 
+private func testAsyncInterruptionScheduler(
+	op: String,
+	file: FileString = #file,
+	line: UInt = #line,
+	transform: (SignalProducer<Int, NoError>, TestScheduler) -> SignalProducer<Int, NoError>
+) {
+	var isInterrupted = false
+
+	let scheduler = TestScheduler()
+	let producer = transform(SignalProducer(0 ..< 128), scheduler)
+
+	let failedExpectations = gatherFailingExpectations {
+		let disposable = producer.startWithInterrupted { isInterrupted = true }
+		expect(isInterrupted) == false
+
+		disposable.dispose()
+		expect(isInterrupted) == false
+
+		scheduler.run()
+		expect(isInterrupted) == true
+	}
+
+	if !failedExpectations.isEmpty {
+		fail("The async operator `\(op)` does not interrupt on the appropriate scheduler.",
+			 location: SourceLocation(file: file, line: line))
+	}
+}
+
 private func testAsyncASAPInterruption(
+	op: String,
 	file: FileString = #file,
 	line: UInt = #line,
 	transform: (SignalProducer<Int, NoError>, TestScheduler) -> SignalProducer<Int, NoError>
@@ -2073,7 +2122,7 @@ private func testAsyncASAPInterruption(
 	}
 
 	if !failedExpectations.isEmpty {
-		fail("The ASAP interruption test of the async operator has failed.",
+		fail("The ASAP interruption test of the async operator `\(op)` has failed.",
 			 location: SourceLocation(file: file, line: line))
 	}
 }