Initial implementation of AsyncStream for RC + basic test

Author: peterfriese

FirebaseRemoteConfig/Swift/RemoteConfig+Async.swift

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+// Copyright 2024 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import Foundation
+
+@available(iOS 13.0.0, macOS 10.15.0, macCatalyst 13.0.0, tvOS 13.0.0, watchOS 7.0.0, *)
+extension RemoteConfig {
+  /// Returns an `AsyncThrowingStream` that provides real-time updates to the configuration.
+  ///
+  /// You can listen for updates by iterating over the stream using a `for try await` loop.
+  /// The stream will yield a `RemoteConfigUpdate` whenever a change is pushed from the
+  /// Remote Config backend. After receiving an update, you must call `activate()` to make the
+  /// new configuration available to your app.
+  ///
+  /// The underlying listener is automatically added when you begin iterating and is removed when
+  /// the iteration is cancelled or finishes.
+  ///
+  /// - Throws: `RemoteConfigUpdateError` if the listener encounters a server-side error or another
+  ///           issue, causing the stream to terminate.
+  ///
+  /// ### Example Usage
+  ///
+  /// ```swift
+  /// func listenForRealtimeUpdates() {
+  ///   Task {
+  ///     do {
+  ///       for try await configUpdate in remoteConfig.updates {
+  ///         print("Updated keys: \(configUpdate.updatedKeys)")
+  ///         // Activate the new config to make it available
+  ///         let status = try await remoteConfig.activate()
+  ///         print("Config activated with status: \(status)")
+  ///       }
+  ///     } catch {
+  ///       print("Error listening for remote config updates: \(error)")
+  ///     }
+  ///   }
+  /// }
+  /// ```
+  public var updates: AsyncThrowingStream<RemoteConfigUpdate, Error> {
+    return AsyncThrowingStream { continuation in
+      let listener = addOnConfigUpdateListener { update, error in
+        switch (update, error) {
+        case (let update?, _):
+          // If there's an update, yield it. We prioritize the update over a potential error.
+          continuation.yield(update)
+        case (_, let error?):
+          // If there's no update but there is an error, terminate the stream with the error.
+          continuation.finish(throwing: error)
+        case (nil, nil):
+          // If both are nil (the "should not happen" case), gracefully finish the stream.
+          continuation.finish()
+        }
+      }
+
+      continuation.onTermination = { @Sendable _ in
+        listener.remove()
+      }
+    }
+  }
+}

FirebaseRemoteConfig/Tests/Swift/SwiftAPI/AsyncStreamTests.swift

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+// Copyright 2024 Google LLC
+//
+// Licensed under the Apache-Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing-software
+// distributed under the License is distributed on an "AS IS" BASIS-
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND-either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import FirebaseCore
+@testable import FirebaseRemoteConfig
+
+import XCTest
+
+@available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
+class AsyncStreamTests: APITestBase {
+  func testConfigUpdateStreamReceivesUpdates() async throws {
+    guard APITests.useFakeConfig else { return }
+    
+    let expectation = self.expectation(description: #function)
+
+    Task {
+      for try await update in config.updates {
+        expectation.fulfill()
+      }
+    }
+
+    fakeConsole.config[Constants.key1] = Constants.value1
+    await fulfillment(of: [expectation], timeout: 5)
+  }
+}