feature: Adds async/await-based EventStream

Author: NeedleInAJayStack

Sources/GraphQL/Subscription/EventStream.swift

@@ -6,3 +6,51 @@ open class EventStream<Element> {
         fatalError("This function should be overridden by implementing classes")
     }
 }
+
+#if compiler(>=5.5) && canImport(_Concurrency)
+
+@available(macOS 12, iOS 15, watchOS 8, tvOS 15, *)
+/// Event stream that wraps an `AsyncThrowingStream` from Swift's standard concurrency system.
+public class ConcurrentEventStream<Element>: EventStream<Element> {
+    public let stream: AsyncThrowingStream<Element, Error>
+
+    public init(_ stream: AsyncThrowingStream<Element, Error>) {
+        self.stream = stream
+    }
+    
+    /// Performs the closure on each event in the current stream and returns a stream of the results.
+    /// - Parameter closure: The closure to apply to each event in the stream
+    /// - Returns: A stream of the results
+    override open func map<To>(_ closure: @escaping (Element) throws -> To) -> ConcurrentEventStream<To> {
+        let newStream = self.stream.mapStream(closure)
+        return ConcurrentEventStream<To>.init(newStream)
+    }
+}
+
+@available(macOS 12, iOS 15, watchOS 8, tvOS 15, *)
+extension AsyncThrowingStream {
+    func mapStream<To>(_ closure: @escaping (Element) throws -> To) -> AsyncThrowingStream<To, Error> {
+        return AsyncThrowingStream<To, Error> { continuation in
+            Task {
+                for try await event in self {
+                    let newEvent = try closure(event)
+                    continuation.yield(newEvent)
+                }
+            }
+        }
+    }
+    
+    func filterStream(_ isIncluded: @escaping (Element) throws -> Bool) -> AsyncThrowingStream<Element, Error> {
+        return AsyncThrowingStream<Element, Error> { continuation in
+            Task {
+                for try await event in self {
+                    if try isIncluded(event) {
+                        continuation.yield(event)
+                    }
+                }
+            }
+        }
+    }
+}
+
+#endif