once: Use NSRecursiveLock instead of deprecated OSAtomicOr32OrigBarrier (#209)

Author: M0rtyMerr

CHANGELOG.md

@@ -2,6 +2,7 @@ Changelog
 =========
 
 - added `reachedBottom(offset:)` for `UIScrollView`
+- `once` now uses a `NSRecursiveLock` instead of the deprecated `OSAtomicOr32OrigBarrier`
 
 5.0.0
 -----

Source/RxSwift/once.swift

@@ -25,12 +25,15 @@ extension Observable {
 	*/
 
 	public static func once(_ element: Element) -> Observable<Element> {
-		var delivered: UInt32 = 0
+        let lock = NSRecursiveLock()
+		var isDelivered = false
 		return create { observer in
-			let wasDelivered = OSAtomicOr32OrigBarrier(1, &delivered)
-			if wasDelivered == 0 {
+			lock.lock()
+			if !isDelivered {
 				observer.onNext(element)
 			}
+            isDelivered = true
+            lock.unlock()
 			observer.onCompleted()
             return Disposables.create()
 		}