Fix crash in the Combine API (#39)

Sources/Defaults/Observation+Combine.swift

@@ -11,15 +11,16 @@ extension Defaults {
 	final class DefaultsSubscription<SubscriberType: Subscriber>: Subscription where SubscriberType.Input == BaseChange {
 		private var subscriber: SubscriberType?
 		private var observation: UserDefaultsKeyObservation?
+		private let options: NSKeyValueObservingOptions
 
 		init(subscriber: SubscriberType, suite: UserDefaults, key: String, options: NSKeyValueObservingOptions) {
 			self.subscriber = subscriber
+			self.options = options
 			self.observation = UserDefaultsKeyObservation(
 				object: suite,
 				key: key,
 				callback: observationCallback(_:)
 			)
-			self.observation?.start(options: options)
 		}
 
 		func request(_ demand: Subscribers.Demand) {
@@ -32,6 +33,10 @@ extension Defaults {
 			subscriber = nil
 		}
 
+		func start() {
+			observation?.start(options: options)
+		}
+
 		private func observationCallback(_ change: BaseChange) {
 			_ = subscriber?.receive(change)
 		}
@@ -64,6 +69,7 @@ extension Defaults {
 			)
 
 			subscriber.receive(subscription: subscription)
+			subscription.start()
 		}
 	}
 

Tests/DefaultsTests/DefaultsTests.swift

@@ -384,6 +384,27 @@ final class DefaultsTests: XCTestCase {
 		waitForExpectations(timeout: 10)
 	}
 
+	@available(iOS 13.0, macOS 10.15, tvOS 13.0, watchOS 6.0, iOSApplicationExtension 13.0, macOSApplicationExtension 10.15, tvOSApplicationExtension 13.0, watchOSApplicationExtension 6.0, *)
+	func testReceiveValueBeforeSubscriptionCombine() {
+		let key = Defaults.Key<String>("receiveValueBeforeSubscription", default: "hello")
+		let expect = expectation(description: "Observation closure being called")
+		
+		let publisher = Defaults
+			.publisher(key, options: [.initial, .new])
+			.compactMap { $0.newValue }
+			.eraseToAnyPublisher()
+			.collect(2)
+
+		let cancellable = publisher.sink { values in
+			XCTAssertEqual(["hello", "world"], values)
+			expect.fulfill()
+		}
+
+		Defaults[key] = "world";
+		cancellable.cancel()
+		waitForExpectations(timeout: 10)
+	}
+
 	func testObserveKey() {
 		let key = Defaults.Key<Bool>("observeKey", default: false)
 		let expect = expectation(description: "Observation closure being called")