[ECO-5577] Get rid of existential type usage in the public API

CONTRIBUTING.md

@@ -40,6 +40,8 @@ To check formatting and code quality, run `swift run BuildTool lint`. Run with `
   - When defining a `struct` that is emitted by the public API of the library, make sure to define a public memberwise initializer so that users can create one to be emitted by their mocks. (There is no way to make Swift’s autogenerated memberwise initializer public, so you will need to write one yourself. In Xcode, you can do this by clicking at the start of the type declaration and doing Editor → Refactor → Generate Memberwise Initializer.)
 - When writing code that implements behaviour specified by the Chat SDK features spec, add a comment that references the identifier of the relevant spec item.
 - The SDK isolates all of its mutable state to the main actor. Stateful objects should be marked as `@MainActor`.
+- Avoid the usage of existential types (`any FooProtocol`) in the public API; favour protocol associated types combined with opaque types (`some FooProtocol`).
+  - There are exceptions; for example, sometimes you will have to choose between making a concrete type generic or using an existential type, and in some of these situations the existential type will be more appropriate. One such example is `ChatClientOptions`'s `logHandler` property.
 
 ### Throwing errors
 

Example/AblyChatExample/Mocks/Misc.swift

@@ -30,11 +30,11 @@ final class MockMessagesPaginatedResult: PaginatedResult {
 
     var isLast: Bool { fatalError("Not implemented") }
 
-    var next: (any PaginatedResult<Item>)? { fatalError("Not implemented") }
+    var next: Self? { fatalError("Not implemented") }
 
-    var first: any PaginatedResult<Item> { fatalError("Not implemented") }
+    var first: Self { fatalError("Not implemented") }
 
-    var current: any PaginatedResult<Item> { fatalError("Not implemented") }
+    var current: Self { fatalError("Not implemented") }
 
     init(clientID: String, roomName: String, numberOfMockMessages: Int = 3) {
         self.clientID = clientID

Example/AblyChatExample/Mocks/MockClients.swift

@@ -9,7 +9,7 @@ class MockChatClient: ChatClient {
     let realtime = Realtime()
     nonisolated let clientOptions: ChatClientOptions
     nonisolated let rooms: MockRooms
-    nonisolated let connection: Connection
+    nonisolated let connection: MockConnection
 
     init(clientOptions: ChatClientOptions?) {
         self.clientOptions = clientOptions ?? .init()
@@ -53,11 +53,11 @@ class MockRoom: Room {
 
     nonisolated let name: String
     nonisolated let options: RoomOptions
-    nonisolated let messages: any Messages
-    nonisolated let presence: any Presence
-    nonisolated let reactions: any RoomReactions
-    nonisolated let typing: any Typing
-    nonisolated let occupancy: any Occupancy
+    nonisolated let messages: MockMessages
+    nonisolated let presence: MockPresence
+    nonisolated let reactions: MockRoomReactions
+    nonisolated let typing: MockTyping
+    nonisolated let occupancy: MockOccupancy
 
     let channel = Channel()
 
@@ -88,7 +88,7 @@ class MockRoom: Room {
     }
 
     @discardableResult
-    func onStatusChange(_ callback: @escaping @MainActor (RoomStatusChange) -> Void) -> StatusSubscriptionProtocol {
+    func onStatusChange(_ callback: @escaping @MainActor (RoomStatusChange) -> Void) -> DefaultStatusSubscription {
         var needNext = true
         periodic(with: randomStatusInterval) { [weak self] in
             guard let self else {
@@ -99,13 +99,13 @@ class MockRoom: Room {
             }
             return needNext
         }
-        return StatusSubscription {
+        return DefaultStatusSubscription {
             needNext = false
         }
     }
 
     @discardableResult
-    func onDiscontinuity(_: @escaping @MainActor (DiscontinuityEvent) -> Void) -> StatusSubscriptionProtocol {
+    func onDiscontinuity(_: @escaping @MainActor (DiscontinuityEvent) -> Void) -> DefaultStatusSubscription {
         fatalError("Not yet implemented")
     }
 }
@@ -114,22 +114,17 @@ class MockMessages: Messages {
     let clientID: String
     let roomName: String
 
-    var reactions: any MessageReactions
+    var reactions: MockMessageReactions
 
-    var mockReactions: MockMessageReactions {
-        // swiftlint:disable:next force_cast
-        reactions as! MockMessageReactions
-    }
-
-    private let mockSubscriptions = MockMessageSubscriptionStorage<ChatMessageEvent>()
+    private let mockSubscriptions = MockMessageSubscriptionStorage<ChatMessageEvent, MockMessagesPaginatedResult>()
 
     init(clientID: String, roomName: String) {
         self.clientID = clientID
         self.roomName = roomName
         reactions = MockMessageReactions(clientID: clientID, roomName: roomName)
     }
 
-    func subscribe(_ callback: @escaping @MainActor (ChatMessageEvent) -> Void) -> MessageSubscriptionResponseProtocol {
+    func subscribe(_ callback: @escaping @MainActor (ChatMessageEvent) -> Void) -> some MessageSubscriptionResponseProtocol {
         mockSubscriptions.create(
             randomElement: {
                 let message = Message(
@@ -146,9 +141,9 @@ class MockMessages: Messages {
                     timestamp: Date(),
                 )
                 if byChance(30) { /* 30% of the messages will get the reaction */
-                    self.mockReactions.messageSerials.append(message.serial)
+                    self.reactions.messageSerials.append(message.serial)
                 }
-                self.mockReactions.clientIDs.insert(message.clientID)
+                self.reactions.clientIDs.insert(message.clientID)
                 return ChatMessageEvent(message: message)
             },
             previousMessages: { _ in
@@ -159,7 +154,7 @@ class MockMessages: Messages {
         )
     }
 
-    func history(options _: QueryOptions) async throws(ARTErrorInfo) -> any PaginatedResult<Message> {
+    func history(options _: QueryOptions) async throws(ARTErrorInfo) -> some PaginatedResult<Message> {
         MockMessagesPaginatedResult(clientID: clientID, roomName: roomName)
     }
 
@@ -282,7 +277,7 @@ class MockMessageReactions: MessageReactions {
         )
     }
 
-    func subscribe(_ callback: @escaping @MainActor @Sendable (MessageReactionSummaryEvent) -> Void) -> SubscriptionProtocol {
+    func subscribe(_ callback: @escaping @MainActor @Sendable (MessageReactionSummaryEvent) -> Void) -> MockSubscription {
         mockSubscriptions.create(
             randomElement: {
                 guard let senderClientID = self.clientIDs.randomElement(), let messageSerial = self.messageSerials.randomElement() else {
@@ -308,7 +303,7 @@ class MockMessageReactions: MessageReactions {
         )
     }
 
-    func subscribeRaw(_: @escaping @MainActor @Sendable (MessageReactionRawEvent) -> Void) -> SubscriptionProtocol {
+    func subscribeRaw(_: @escaping @MainActor @Sendable (MessageReactionRawEvent) -> Void) -> MockSubscription {
         fatalError("Not implemented")
     }
 }
@@ -338,7 +333,7 @@ class MockRoomReactions: RoomReactions {
     }
 
     @discardableResult
-    func subscribe(_ callback: @escaping @MainActor (RoomReactionEvent) -> Void) -> SubscriptionProtocol {
+    func subscribe(_ callback: @escaping @MainActor (RoomReactionEvent) -> Void) -> some SubscriptionProtocol {
         mockSubscriptions.create(
             randomElement: {
                 let reaction = RoomReaction(
@@ -369,7 +364,7 @@ class MockTyping: Typing {
     }
 
     @discardableResult
-    func subscribe(_ callback: @escaping @MainActor (TypingSetEvent) -> Void) -> SubscriptionProtocol {
+    func subscribe(_ callback: @escaping @MainActor (TypingSetEvent) -> Void) -> some SubscriptionProtocol {
         mockSubscriptions.create(
             randomElement: {
                 TypingSetEvent(
@@ -422,7 +417,7 @@ class MockPresence: Presence {
         self.roomName = roomName
     }
 
-    private func createSubscription(callback: @escaping @MainActor (PresenceEvent) -> Void) -> SubscriptionProtocol {
+    private func createSubscription(callback: @escaping @MainActor (PresenceEvent) -> Void) -> MockSubscription {
         mockSubscriptions.create(
             randomElement: {
                 let member = PresenceMember(
@@ -536,11 +531,11 @@ class MockPresence: Presence {
         )
     }
 
-    func subscribe(event _: PresenceEventType, _ callback: @escaping @MainActor (PresenceEvent) -> Void) -> SubscriptionProtocol {
+    func subscribe(event _: PresenceEventType, _ callback: @escaping @MainActor (PresenceEvent) -> Void) -> MockSubscription {
         createSubscription(callback: callback)
     }
 
-    func subscribe(events _: [PresenceEventType], _ callback: @escaping @MainActor (PresenceEvent) -> Void) -> SubscriptionProtocol {
+    func subscribe(events _: [PresenceEventType], _ callback: @escaping @MainActor (PresenceEvent) -> Void) -> MockSubscription {
         createSubscription(callback: callback)
     }
 }
@@ -557,7 +552,7 @@ class MockOccupancy: Occupancy {
     }
 
     @discardableResult
-    func subscribe(_ callback: @escaping @MainActor (OccupancyEvent) -> Void) -> SubscriptionProtocol {
+    func subscribe(_ callback: @escaping @MainActor (OccupancyEvent) -> Void) -> MockSubscription {
         mockSubscriptions.create(
             randomElement: {
                 let random = Int.random(in: 1 ... 10)
@@ -590,7 +585,7 @@ class MockConnection: Connection {
     }
 
     @discardableResult
-    func onStatusChange(_ callback: @escaping @MainActor (ConnectionStatusChange) -> Void) -> StatusSubscriptionProtocol {
+    func onStatusChange(_ callback: @escaping @MainActor (ConnectionStatusChange) -> Void) -> some StatusSubscriptionProtocol {
         mockSubscriptions.create(
             randomElement: {
                 ConnectionStatusChange(

Example/AblyChatExample/Mocks/MockSubscriptionStorage.swift

@@ -6,7 +6,7 @@ import AblyChat
 class MockSubscriptionStorage<Element: Sendable> {
     @MainActor
     private struct SubscriptionItem {
-        let subscription: SubscriptionProtocol
+        let subscription: MockSubscription
         let callback: (Element) -> Void
 
         init(
@@ -39,7 +39,7 @@ class MockSubscriptionStorage<Element: Sendable> {
         randomElement: @escaping @MainActor @Sendable () -> Element?,
         interval: @autoclosure @escaping @MainActor @Sendable () -> Double,
         callback: @escaping @MainActor (Element) -> Void,
-    ) -> SubscriptionProtocol {
+    ) -> MockSubscription {
         let id = UUID()
         let subscriptionItem = SubscriptionItem(randomElement: randomElement, interval: interval, callback: callback) { [weak self] in
             self?.subscriptionDidTerminate(id: id)
@@ -64,7 +64,7 @@ class MockSubscriptionStorage<Element: Sendable> {
 class MockStatusSubscriptionStorage<Element: Sendable> {
     @MainActor
     private struct SubscriptionItem {
-        let subscription: StatusSubscriptionProtocol
+        let subscription: MockStatusSubscription
         let callback: (Element) -> Void
 
         init(
@@ -97,7 +97,7 @@ class MockStatusSubscriptionStorage<Element: Sendable> {
         randomElement: @escaping @MainActor @Sendable () -> Element?,
         interval: @autoclosure @escaping @MainActor @Sendable () -> Double,
         callback: @escaping @MainActor (Element) -> Void,
-    ) -> StatusSubscriptionProtocol {
+    ) -> MockStatusSubscription {
         let id = UUID()
         let subscriptionItem = SubscriptionItem(randomElement: randomElement, interval: interval, callback: callback) { [weak self] in
             self?.subscriptionDidTerminate(id: id)
@@ -119,15 +119,15 @@ class MockStatusSubscriptionStorage<Element: Sendable> {
 
 // This is copied from `MockSubscriptionStorage`, but for `MessageSubscriptionResponse`.
 @MainActor
-class MockMessageSubscriptionStorage<Element: Sendable> {
+class MockMessageSubscriptionStorage<Element: Sendable, PaginatedResult: AblyChat.PaginatedResult<Message>> {
     @MainActor
     private struct SubscriptionItem {
-        let subscription: MessageSubscriptionResponseProtocol
+        let subscription: MockMessageSubscriptionResponse<PaginatedResult>
         let callback: (Element) -> Void
 
         init(
             randomElement: @escaping @MainActor @Sendable () -> Element,
-            previousMessages: @escaping @MainActor @Sendable (QueryOptions) async throws(ARTErrorInfo) -> any PaginatedResult<Message>,
+            previousMessages: @escaping @MainActor @Sendable (QueryOptions) async throws(ARTErrorInfo) -> PaginatedResult,
             interval: @escaping @MainActor @Sendable () -> Double,
             callback: @escaping @MainActor (Element) -> Void,
             onTerminate: @escaping () -> Void,
@@ -152,10 +152,10 @@ class MockMessageSubscriptionStorage<Element: Sendable> {
 
     func create(
         randomElement: @escaping @MainActor @Sendable () -> Element,
-        previousMessages: @escaping @MainActor @Sendable (QueryOptions) async throws(ARTErrorInfo) -> any PaginatedResult<Message>,
+        previousMessages: @escaping @MainActor @Sendable (QueryOptions) async throws(ARTErrorInfo) -> PaginatedResult,
         interval: @autoclosure @escaping @MainActor @Sendable () -> Double,
         callback: @escaping @MainActor (Element) -> Void,
-    ) -> MessageSubscriptionResponseProtocol {
+    ) -> some MessageSubscriptionResponseProtocol {
         let id = UUID()
         let subscriptionItem = SubscriptionItem(
             randomElement: randomElement,
@@ -204,11 +204,11 @@ struct MockStatusSubscription: StatusSubscriptionProtocol {
     }
 }
 
-struct MockMessageSubscriptionResponse: MessageSubscriptionResponseProtocol {
+struct MockMessageSubscriptionResponse<PaginatedResult: AblyChat.PaginatedResult<Message>>: MessageSubscriptionResponseProtocol {
     private let _unsubscribe: () -> Void
-    private let previousMessages: @MainActor @Sendable (QueryOptions) async throws(ARTErrorInfo) -> any PaginatedResult<Message>
+    private let previousMessages: @MainActor @Sendable (QueryOptions) async throws(ARTErrorInfo) -> PaginatedResult
 
-    func historyBeforeSubscribe(_ params: QueryOptions) async throws(ARTErrorInfo) -> any PaginatedResult<Message> {
+    func historyBeforeSubscribe(_ params: QueryOptions) async throws(ARTErrorInfo) -> PaginatedResult {
         try await previousMessages(params)
     }
 
@@ -217,7 +217,7 @@ struct MockMessageSubscriptionResponse: MessageSubscriptionResponseProtocol {
     }
 
     init(
-        previousMessages: @escaping @MainActor @Sendable (QueryOptions) async throws(ARTErrorInfo) -> any PaginatedResult<Message>,
+        previousMessages: @escaping @MainActor @Sendable (QueryOptions) async throws(ARTErrorInfo) -> PaginatedResult,
         unsubscribe: @MainActor @Sendable @escaping () -> Void,
     ) {
         self.previousMessages = previousMessages

Sources/AblyChat/ChatAPI.swift

@@ -16,7 +16,7 @@ internal final class ChatAPI {
     }
 
     // (CHA-M6) Messages should be queryable from a paginated REST API.
-    internal func getMessages(roomName: String, params: QueryOptions) async throws(InternalError) -> any PaginatedResult<Message> {
+    internal func getMessages(roomName: String, params: QueryOptions) async throws(InternalError) -> some PaginatedResult<Message> {
         let endpoint = "\(apiVersionV4)/rooms/\(roomName)/messages"
         return try await makePaginatedRequest(endpoint, params: params.asQueryItems())
     }
@@ -242,7 +242,7 @@ internal final class ChatAPI {
     private func makePaginatedRequest<Response: JSONDecodable & Sendable & Equatable>(
         _ url: String,
         params: [String: String]? = nil,
-    ) async throws(InternalError) -> any PaginatedResult<Response> {
+    ) async throws(InternalError) -> some PaginatedResult<Response> {
         let paginatedResponse = try await realtime.request("GET", path: url, params: params, body: nil, headers: [:])
         let jsonValues = paginatedResponse.items.map { JSONValue(ablyCocoaData: $0) }
         let items = try jsonValues.map { jsonValue throws(InternalError) in

Sources/AblyChat/ChatClient.swift

@@ -3,6 +3,7 @@ import Ably
 @MainActor
 public protocol ChatClient: AnyObject, Sendable {
     associatedtype Realtime
+    associatedtype Connection: AblyChat.Connection
     associatedtype Rooms: AblyChat.Rooms
 
     /**
@@ -18,7 +19,7 @@ public protocol ChatClient: AnyObject, Sendable {
      *
      * - Returns: The connection object.
      */
-    nonisolated var connection: any Connection { get }
+    nonisolated var connection: Connection { get }
 
     /**
      * Returns the clientId of the current client.
@@ -70,7 +71,10 @@ public class DefaultChatClient: ChatClient {
 
     // (CHA-CS1) Every chat client has a status, which describes the current status of the connection.
     // (CHA-CS4) The chat client must allow its connection status to be observed by clients.
-    public let connection: any Connection
+    private let _connection: DefaultConnection
+    public var connection: some Connection {
+        _connection
+    }
 
     /**
      * Constructor for Chat
@@ -101,7 +105,7 @@ public class DefaultChatClient: ChatClient {
         logger = DefaultInternalLogger(logHandler: self.clientOptions.logHandler, logLevel: self.clientOptions.logLevel)
         let roomFactory = DefaultRoomFactory<InternalRealtimeClientAdapter<ARTWrapperSDKProxyRealtime>>()
         _rooms = DefaultRooms(realtime: internalRealtime, clientOptions: self.clientOptions, logger: logger, roomFactory: roomFactory)
-        connection = DefaultConnection(realtime: internalRealtime)
+        _connection = DefaultConnection(realtime: internalRealtime)
     }
 
     public nonisolated var clientID: String {

Sources/AblyChat/Connection.swift

@@ -5,6 +5,8 @@ import Ably
  */
 @MainActor
 public protocol Connection: AnyObject, Sendable {
+    associatedtype StatusSubscription: StatusSubscriptionProtocol
+
     /**
      * The current status of the connection.
      */
@@ -25,7 +27,7 @@ public protocol Connection: AnyObject, Sendable {
      * - Returns: A subscription that can be used to unsubscribe from ``ConnectionStatusChange`` events.
      */
     @discardableResult
-    func onStatusChange(_ callback: @escaping @MainActor (ConnectionStatusChange) -> Void) -> any StatusSubscriptionProtocol
+    func onStatusChange(_ callback: @escaping @MainActor (ConnectionStatusChange) -> Void) -> StatusSubscription
 }
 
 /// `AsyncSequence` variant of `Connection` status changes.

Sources/AblyChat/DefaultConnection.swift

@@ -21,7 +21,7 @@ internal final class DefaultConnection: Connection {
 
     // (CHA-CS4d) Clients must be able to register a listener for connection status events and receive such events.
     @discardableResult
-    internal func onStatusChange(_ callback: @escaping @MainActor (ConnectionStatusChange) -> Void) -> any StatusSubscriptionProtocol {
+    internal func onStatusChange(_ callback: @escaping @MainActor (ConnectionStatusChange) -> Void) -> some StatusSubscriptionProtocol {
         // (CHA-CS5) The chat client must monitor the underlying realtime connection for connection status changes.
         let eventListener = realtime.connection.on { [weak self] stateChange in
             guard let self else {
@@ -72,7 +72,7 @@ internal final class DefaultConnection: Connection {
             callback(statusChange)
         }
 
-        return StatusSubscription { [weak self] in
+        return DefaultStatusSubscription { [weak self] in
             guard let self else {
                 return
             }