Tweaked the composed property event delivery.

Sources/Property.swift

@@ -522,10 +522,7 @@ public final class Property<Value>: PropertyProtocol {
 	///
 	/// - parameters:
 	///   - unsafeProducer: The composed producer for creating the property.
-	fileprivate init(
-		unsafeProducer: SignalProducer<Value, NoError>,
-	    transform: ((Signal<Value, NoError>.Observer) -> Signal<Value, NoError>.Observer)? = nil
-	) {
+	fileprivate init(unsafeProducer: SignalProducer<Value, NoError>) {
 		// The ownership graph:
 		//
 		// ------------     weak  -----------    strong ------------------
@@ -537,33 +534,30 @@ public final class Property<Value>: PropertyProtocol {
 		// strong ------------       -----------                       strong ------------
 
 		let box = PropertyBox<Value?>(nil)
-		var relay: Signal<Value, NoError>!
 
-		unsafeProducer.startWithSignal { upstream, interruptHandle in
-			// A composed property tracks its active consumers through its relay signal, and
-			// interrupts `unsafeProducer` if the relay signal terminates.
-			let (signal, _observer) = Signal<Value, NoError>.pipe(disposable: interruptHandle)
-			let observer = transform?(_observer) ?? _observer
-			relay = signal
+		// A composed property tracks its active consumers through its relay signal, and
+		// interrupts `unsafeProducer` if the relay signal terminates.
+		let disposable = SerialDisposable()
+		let (relay, observer) = Signal<Value, NoError>.pipe(disposable: disposable)
 
+		disposable.inner = unsafeProducer.start { [weak box] event in
 			// `observer` receives `interrupted` only as a result of the termination of
 			// `signal`, and would not be delivered anyway. So transforming
 			// `interrupted` to `completed` is unnecessary here.
-			upstream.observe { [weak box] event in
-				guard let box = box else {
-					// Just forward the event, since no one owns the box or IOW no demand
-					// for a cached latest value.
-					return observer.send(event)
-				}
 
-				box.begin { storage in
-					storage.modify { value in
-						if let newValue = event.value {
-							value = newValue
-						}
+			guard let box = box else {
+				// Just forward the event, since no one owns the box or IOW no demand
+				// for a cached latest value.
+				return observer.send(event)
+			}
+
+			box.begin { storage in
+				storage.modify { value in
+					if let newValue = event.value {
+						value = newValue
 					}
-					observer.send(event)
 				}
+				observer.send(event)
 			}
 		}
 
@@ -576,10 +570,10 @@ public final class Property<Value>: PropertyProtocol {
 		_value = { box.value! }
 		signal = relay
 
-		producer = SignalProducer { [box, signal = relay!] observer, lifetime in
+		producer = SignalProducer { [box, relay] observer, lifetime in
 			box.withValue { value in
 				observer.send(value: value!)
-				lifetime += signal.observe(Signal.Observer(mappingInterruptedToCompleted: observer))
+				lifetime += relay.observe(Signal.Observer(mappingInterruptedToCompleted: observer))
 			}
 		}
 	}