Add design document for Swift AsyncSequence / AsyncStream support for Firebase's realtime APIs (#15350)

Author: peterfriese

docs/AsyncStreams/swift-async-sequence-api-design.md

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+# API Design for Firebase `AsyncSequence` Event Streams
+
+* **Authors**
+  * Peter Friese
+* **Contributors**
+  * Nick Cooke
+  * Paul Beusterien
+* **Status**: `In Review`
+* **Last Updated**: 2025-09-25
+
+## 1. Abstract
+
+This proposal outlines the integration of Swift's `AsyncStream` and `AsyncSequence` APIs into the Firebase Apple SDK. The goal is to provide a modern, developer-friendly way to consume real-time data streams from Firebase APIs, aligning the SDK with Swift's structured concurrency model and improving the overall developer experience.
+
+## 2. Background
+
+Many Firebase APIs produce a sequence of asynchronous events, such as authentication state changes, document and collection updates, and remote configuration updates. Currently, the SDK exposes these through completion-handler-based APIs (listeners).
+
+```swift
+// Current listener-based approach
+db.collection("cities").document("SF")
+  .addSnapshotListener { documentSnapshot, error in
+    guard let document = documentSnapshot else { /* ... */ }
+    guard let data = document.data() else { /* ... */ }
+    print("Current data: \(data)")
+  }
+```
+
+This approach breaks the otherwise linear control flow, requires manual management of listener lifecycles, and complicates error handling. Swift's `AsyncSequence` provides a modern, type-safe alternative that integrates seamlessly with structured concurrency, offering automatic resource management, simplified error handling, and a more intuitive, linear control flow.
+
+## 3. Motivation
+
+Adopting `AsyncSequence` will:
+
+*   **Modernize the SDK:** Align with Swift's modern concurrency approach, making Firebase feel more native to Swift developers.
+*   **Simplify Development:** Eliminate the need for manual listener management and reduce boilerplate code, especially when integrating with SwiftUI.
+*   **Improve Code Quality:** Provide official, high-quality implementations for streaming APIs, reducing ecosystem fragmentation caused by unofficial solutions.
+*   **Enhance Readability:** Leverage structured error handling (`throws`) and a linear `for try await` syntax to make asynchronous code easier to read and maintain.
+*   **Enable Composition:** Allow developers to use a rich set of sequence operators (like `map`, `filter`, `prefix`) to transform and combine streams declaratively.
+
+## 4. Goals
+
+*   To design and implement an idiomatic, `AsyncSequence`-based API surface for all relevant event-streaming Firebase APIs.
+*   To provide a clear and consistent naming convention that aligns with Apple's own Swift APIs.
+*   To ensure the new APIs automatically manage the lifecycle of underlying listeners, removing this burden from the developer.
+*   To improve the testability of asynchronous Firebase interactions.
+
+## 5. Non-Goals
+
+*   To deprecate or remove the existing listener-based APIs in the immediate future. The new APIs will be additive.
+*   To introduce `AsyncSequence` wrappers for one-shot asynchronous calls (which are better served by `async/await` functions). This proposal is focused exclusively on event streams.
+*   To provide a custom `AsyncSequence` implementation. We will use Swift's standard `Async(Throwing)Stream` types.
+
+## 6. API Naming Convention
+
+The guiding principle is to establish a clear, concise, and idiomatic naming convention that aligns with modern Swift practices and mirrors Apple's own frameworks.
+
+### Recommended Approach: Name the sequence based on its conceptual model.
+
+1.  **For sequences of discrete items, use a plural noun.**
+    *   This applies when the stream represents a series of distinct objects, like data snapshots.
+    *   **Guidance:** Use a computed property for parameter-less access and a method for cases that require parameters.
+    *   **Examples:** `url.lines`, `db.collection("users").snapshots`.
+
+2.  **For sequences observing a single entity, describe the event with a suffix.**
+    *   This applies when the stream represents the changing value of a single property or entity over time.
+    *   **Guidance:** Use the entity's name combined with a suffix like `Changes`, `Updates`, or `Events`.
+    *   **Example:** `auth.authStateChanges`.
+
+This approach was chosen over verb-based (`.streamSnapshots()`) or suffix-based (`.snapshotStream`) alternatives because it aligns most closely with Apple's API design guidelines, leading to a more idiomatic and less verbose call site.
+
+## 7. Proposed API Design
+
+### 7.1. Cloud Firestore
+
+Provides an async alternative to `addSnapshotListener`.
+
+#### API Design
+
+```swift
+// Collection snapshots
+extension CollectionReference {
+  var snapshots: AsyncThrowingStream<QuerySnapshot, Error> { get }
+  func snapshots(includeMetadataChanges: Bool = false) -> AsyncThrowingStream<QuerySnapshot, Error>
+}
+
+// Query snapshots
+extension Query {
+  var snapshots: AsyncThrowingStream<QuerySnapshot, Error> { get }
+  func snapshots(includeMetadataChanges: Bool = false) -> AsyncThrowingStream<QuerySnapshot, Error>
+}
+
+// Document snapshots
+extension DocumentReference {
+  var snapshots: AsyncThrowingStream<DocumentSnapshot, Error> { get }
+  func snapshots(includeMetadataChanges: Bool = false) -> AsyncThrowingStream<DocumentSnapshot, Error>
+}
+```
+
+#### Usage
+
+```swift
+// Streaming updates on a collection
+func observeUsers() async throws {
+  for try await snapshot in db.collection("users").snapshots {
+    // ...
+  }
+}
+```
+
+### 7.2. Realtime Database
+
+Provides an async alternative to the `observe(_:with:)` method.
+
+#### API Design
+
+```swift
+/// An enumeration of granular child-level events.
+public enum DatabaseEvent {
+    case childAdded(DataSnapshot, previousSiblingKey: String?)
+    case childChanged(DataSnapshot, previousSiblingKey: String?)
+    case childRemoved(DataSnapshot)
+    case childMoved(DataSnapshot, previousSiblingKey: String?)
+}
+
+extension DatabaseQuery {
+  /// An asynchronous stream of the entire contents at a location.
+  /// This stream emits a new `DataSnapshot` every time the data changes.
+  var value: AsyncThrowingStream<DataSnapshot, Error> { get }
+
+  /// An asynchronous stream of child-level events at a location.
+  func events() -> AsyncThrowingStream<DatabaseEvent, Error>
+}
+```
+
+#### Usage
+
+```swift
+// Streaming a single value
+let scoreRef = Database.database().reference(withPath: "game/score")
+for try await snapshot in scoreRef.value {
+  // ...
+}
+
+// Streaming child events
+let messagesRef = Database.database().reference(withPath: "chats/123/messages")
+for try await event in messagesRef.events() {
+  switch event {
+    case .childAdded(let snapshot, _):
+      // ...
+    // ...
+  }
+}
+```
+
+### 7.3. Authentication
+
+Provides an async alternative to `addStateDidChangeListener`.
+
+#### API Design
+
+```swift
+extension Auth {
+  /// An asynchronous stream of authentication state changes.
+  var authStateChanges: AsyncStream<User?> { get }
+}
+```
+
+#### Usage
+
+```swift
+// Monitoring authentication state
+for await user in Auth.auth().authStateChanges {
+  if let user = user {
+    // User is signed in
+  } else {
+    // User is signed out
+  }
+}
+```
+
+### 7.4. Cloud Storage
+
+Provides an async alternative to `observe(.progress, ...)`.
+
+#### API Design
+
+```swift
+extension StorageTask {
+  /// An asynchronous stream of progress updates for an ongoing task.
+  var progressUpdates: AsyncThrowingStream<StorageTaskSnapshot, Error> { get }
+}
+```
+
+#### Usage
+
+```swift
+// Monitoring an upload task
+let uploadTask = ref.putData(data, metadata: nil)
+do {
+  for try await progress in uploadTask.progress {
+    // Update progress bar
+  }
+  print("Upload complete")
+} catch {
+  // Handle error
+}
+```
+
+### 7.5. Remote Config
+
+Provides an async alternative to `addOnConfigUpdateListener`.
+
+#### API Design
+
+```swift
+extension RemoteConfig {
+  /// An asynchronous stream of configuration updates.
+  var updates: AsyncThrowingStream<RemoteConfigUpdate, Error> { get }
+}
+```
+
+#### Usage
+
+```swift
+// Listening for real-time config updates
+for try await update in RemoteConfig.remoteConfig().updates {
+  // Activate new config
+}
+```
+
+### 7.6. Cloud Messaging (FCM)
+
+Provides an async alternative to the delegate-based approach for token updates and foreground messages.
+
+#### API Design
+
+```swift
+extension Messaging {
+  /// An asynchronous stream of FCM registration token updates.
+  var tokenUpdates: AsyncStream<String> { get }
+
+  /// An asynchronous stream of remote messages received while the app is in the foreground.
+  var foregroundMessages: AsyncStream<MessagingRemoteMessage> { get }
+}
+```
+
+#### Usage
+
+```swift
+// Monitoring FCM token updates
+for await token in Messaging.messaging().tokenUpdates {
+  // Send token to server
+}
+```
+
+## 8. Testing Plan
+
+The quality and reliability of this new API surface will be ensured through a multi-layered testing strategy, covering unit, integration, and cancellation scenarios.
+
+### 8.1. Unit Tests
+
+The primary goal of unit tests is to verify the correctness of the `AsyncStream` wrapping logic in isolation from the network and backend services.
+
+*   **Mocking:** Each product's stream implementation will be tested against a mocked version of its underlying service (e.g., a mock `Firestore` client).
+*   **Behavior Verification:**
+    *   Tests will confirm that initiating a stream correctly registers a listener with the underlying service.
+    *   We will use the mock listeners to simulate events (e.g., new snapshots, auth state changes) and assert that the `AsyncStream` yields the corresponding values correctly.
+    *   Error conditions will be simulated to ensure that the stream correctly throws errors.
+*   **Teardown Logic:** We will verify that the underlying listener is removed when the stream is either cancelled or finishes naturally.
+
+### 8.2. Integration Tests
+
+Integration tests will validate the end-to-end functionality of the async sequences against a live backend environment using the **Firebase Emulator Suite**.
+
+*   **Environment:** A new integration test suite will be created that configures the Firebase SDK to connect to the local emulators (Firestore, Database, Auth, etc.).
+*   **Validation:** These tests will perform real operations (e.g., writing a document and then listening to its `snapshots` stream) to verify that real-time updates are correctly received and propagated through the `AsyncSequence` API.
+*   **Cross-Product Scenarios:** We will test scenarios that involve multiple Firebase products where applicable.
+
+### 8.3. Cancellation Behavior Tests
+
+A specific set of tests will be dedicated to ensuring that resource cleanup (i.e., listener removal) happens correctly and promptly when the consuming task is cancelled.
+
+*   **Test Scenario:**
+    1.  A stream will be consumed within a Swift `Task`.
+    2.  The `Task` will be cancelled immediately after the stream is initiated.
+    3.  Using a mock or a spy object, we will assert that the `remove()` method on the underlying listener registration is called.
+*   **Importance:** This is critical for preventing resource leaks and ensuring the new API behaves predictably within the Swift structured concurrency model, especially in SwiftUI contexts where tasks are automatically managed.
+
+## 9. Implementation Plan
+
+The implementation will be phased, with each product's API being added in a separate Pull Request to facilitate focused reviews.
+
+*   **Firestore:** [PR #14924: Support AsyncStream in realtime query](https://github.com/firebase/firebase-ios-sdk/pull/14924)
+*   **Authentication:** [Link to PR when available]
+*   **Realtime Database:** [Link to PR when available]
+*   ...and so on.
+
+## 10. Open Questions & Future Work
+
+*   Should we provide convenience wrappers for common `AsyncSequence` operators? (e.g., a method to directly stream decoded objects instead of snapshots). For now, this is considered a **Non-Goal** but could be revisited.