Joining empty list of producers fix (#774)

* joining an empty list of producers returns a single event of the collected results, which is the empty array - this becomes relevant, when the list of producers is calculated and you are merging the resulting joined signal with some other signal. the end result will stall, if no event is sent from the joined producers.

* updated changelog

* no upstream sentinal is optional

* updated changelog

* typo

Co-authored-by: Nikolas Mayr <nikolas.mayr@rocket-internet.de>

CHANGELOG.md

@@ -1,7 +1,10 @@
 # master
 *Please add new entries at the top.*
 
+1. Joining an empty sequence of producers can now send an event on the joined signal producer by providing the `noUpstreamSentinel` parameter. This becomes relevant, when the sequence of producers is calculated from some other Signal and the signal resulting from the joined producers is observed. If no event is sent only when the producers sequence is empty, then the observer gets stalled and e.g. the ui won't update.
+
 # 6.2.1
+
 1. Improved performance of joining signals by a factor of around 5. This enables joining of 1000 and more signals in a reasonable amount of time.
 1. Fixed `SignalProducer.debounce` operator that, when started more than once, would not deliver values on producers started after the first time. (#772, kudos to @gpambrozio)
 1. `FlattenStrategy.throttle` is introduced. (#713, kudos to @inamiy)
@@ -20,6 +23,7 @@
 1. Add `<~` binding operator to `Signal.Observer` (#635, kudos to @Marcocanc)
 
 # 6.0.0
+
 1. Dropped support for Swift 4.2 (Xcode 9)
 2. Removed dependency on https://github.com/antitypical/Result (#702, kudos to @NachoSoto and @mdiep)
 
@@ -30,6 +34,7 @@
 * Replace all cases where `AnyError` was used in a `Signal` or `SignalProducer` with `Swift.Error`
 
 # 5.0.1
+
 1. Fix warnings in Xcode 10.2
 
 # 5.0.0

Sources/Property.swift

@@ -318,13 +318,13 @@ extension PropertyProtocol {
 
 	/// Combines the values of all the given producers, in the manner described by
 	/// `combineLatest(with:)`. Returns nil if the sequence is empty.
-	public static func combineLatest<S: Sequence>(_ properties: S) -> Property<[S.Iterator.Element.Value]>? where S.Iterator.Element: PropertyProtocol {
+	public static func combineLatest<S: Sequence>(_ properties: S, noUpstreamSentinel: [S.Iterator.Element.Value]? = nil) -> Property<[S.Iterator.Element.Value]>? where S.Iterator.Element: PropertyProtocol {
 		let producers = properties.map { $0.producer }
 		guard !producers.isEmpty else {
 			return nil
 		}
 
-		return Property(unsafeProducer: SignalProducer.combineLatest(producers))
+		return Property(unsafeProducer: SignalProducer.combineLatest(producers, noUpstreamSentinel: noUpstreamSentinel))
 	}
 
 	/// Zips the values of all the given properties, in the manner described by
@@ -383,13 +383,13 @@ extension PropertyProtocol {
 
 	/// Zips the values of all the given properties, in the manner described by
 	/// `zip(with:)`. Returns nil if the sequence is empty.
-	public static func zip<S: Sequence>(_ properties: S) -> Property<[S.Iterator.Element.Value]>? where S.Iterator.Element: PropertyProtocol {
+	public static func zip<S: Sequence>(_ properties: S, noUpstreamSentinel: [S.Iterator.Element.Value]? = nil) -> Property<[S.Iterator.Element.Value]>? where S.Iterator.Element: PropertyProtocol {
 		let producers = properties.map { $0.producer }
 		guard !producers.isEmpty else {
 			return nil
 		}
 
-		return Property(unsafeProducer: SignalProducer.zip(producers))
+		return Property(unsafeProducer: SignalProducer.zip(producers, noUpstreamSentinel: noUpstreamSentinel))
 	}
 }
 

Sources/Signal.swift

@@ -1680,7 +1680,7 @@ extension Signal {
 					return true
 				}
 
-				_haveAllSentInitial = values.allSatisfy{ !($0 is Placeholder) }
+				_haveAllSentInitial = values.allSatisfy { !($0 is Placeholder) }
 				return _haveAllSentInitial
 			}
 		}

Sources/SignalProducer.swift

@@ -2098,8 +2098,8 @@ extension SignalProducer {
 
 	/// Combines the values of all the given producers, in the manner described by
 	/// `combineLatest(with:)`. Will return an empty `SignalProducer` if the sequence is empty.
-	public static func combineLatest<S: Sequence>(_ producers: S) -> SignalProducer<[Value], Error> where S.Iterator.Element: SignalProducerConvertible, S.Iterator.Element.Value == Value, S.Iterator.Element.Error == Error {
-		return start(producers, Signal.combineLatest)
+    public static func combineLatest<S: Sequence>(_ producers: S, noUpstreamSentinel: [S.Iterator.Element.Value]? = nil) -> SignalProducer<[Value], Error> where S.Iterator.Element: SignalProducerConvertible, S.Iterator.Element.Value == Value, S.Iterator.Element.Error == Error {
+        return start(producers, noUpstreamSentinel: noUpstreamSentinel, Signal.combineLatest)
 	}
 
 	/// Zips the values of all the given producers, in the manner described by
@@ -2176,18 +2176,21 @@ extension SignalProducer {
 
 	/// Zips the values of all the given producers, in the manner described by
 	/// `zipWith`. Will return an empty `SignalProducer` if the sequence is empty.
-	public static func zip<S: Sequence>(_ producers: S) -> SignalProducer<[Value], Error> where S.Iterator.Element: SignalProducerConvertible, S.Iterator.Element.Value == Value, S.Iterator.Element.Error == Error {
-		return start(producers, Signal.zip)
+	public static func zip<S: Sequence>(_ producers: S, noUpstreamSentinel: [S.Iterator.Element.Value]? = nil) -> SignalProducer<[Value], Error> where S.Iterator.Element: SignalProducerConvertible, S.Iterator.Element.Value == Value, S.Iterator.Element.Error == Error {
+		return start(producers, noUpstreamSentinel: noUpstreamSentinel, Signal.zip)
 	}
 
-	private static func start<S: Sequence>(_ producers: S, _ transform: @escaping (AnySequence<Signal<Value, Error>>) -> Signal<[Value], Error>) -> SignalProducer<[Value], Error> where S.Iterator.Element: SignalProducerConvertible, S.Iterator.Element.Value == Value, S.Iterator.Element.Error == Error
+	private static func start<S: Sequence>(_ producers: S, noUpstreamSentinel: [S.Iterator.Element.Value]?, _ transform: @escaping (AnySequence<Signal<Value, Error>>) -> Signal<[Value], Error>) -> SignalProducer<[Value], Error> where S.Iterator.Element: SignalProducerConvertible, S.Iterator.Element.Value == Value, S.Iterator.Element.Error == Error
 	{
 		return SignalProducer<[Value], Error> { observer, lifetime in
 			let setup = producers.map {
 				(producer: $0.producer, pipe: Signal<Value, Error>.pipe())
 			}
 
 			guard !setup.isEmpty else {
+                if let noUpstreamSentinel = noUpstreamSentinel {
+                    observer.send(value: noUpstreamSentinel)
+                }
 				observer.sendCompleted()
 				return
 			}
@@ -2717,7 +2720,7 @@ extension SignalProducer where Value == Bool {
 	///
 	/// - returns: A producer that emits the logical AND results.
 	public static func all<BooleansCollection: Collection>(_ booleans: BooleansCollection) -> SignalProducer<Value, Error> where BooleansCollection.Element == SignalProducer<Value, Error> {
-		return combineLatest(booleans).map { $0.reduce(true) { $0 && $1 } }
+		return combineLatest(booleans, noUpstreamSentinel: []).map { $0.reduce(true) { $0 && $1 } }
 	}
 
 	/// Create a producer that computes a logical AND between the latest values of `booleans`.
@@ -2759,7 +2762,7 @@ extension SignalProducer where Value == Bool {
 	///
 	/// - returns: A producer that emits the logical OR results.
 	public static func any<BooleansCollection: Collection>(_ booleans: BooleansCollection) -> SignalProducer<Value, Error> where BooleansCollection.Element == SignalProducer<Value, Error> {
-		return combineLatest(booleans).map { $0.reduce(false) { $0 || $1 } }
+		return combineLatest(booleans, noUpstreamSentinel: []).map { $0.reduce(false) { $0 || $1 } }
 	}
 
 	/// Create a producer that computes a logical OR between the latest values of `booleans`.