Add ConnectionCollection and Swift test (flutter#167962)

This rewrites the ConnectionCollection C++ class and its test to Swift.
This ensures we're successfully able to write/run XCTests written in
Swift.

It also adds new tests for ConnectionCollection:
* Verifies connection IDs are > 0.
* Verifies connection IDs are unique.
* Verifies the IDs of error connections are as specified.

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [X] I signed the [CLA].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [X] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Author: cbracken

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -52755,6 +52755,8 @@ ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Headers/FlutterPlat
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Headers/FlutterPlugin.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Headers/FlutterPluginAppLifeCycleDelegate.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Headers/FlutterViewController.h + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/ConnectionCollection.swift + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/ConnectionCollectionTest.swift + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/FakeUIPressProxy.swift + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterAppDelegate.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterAppDelegateTest.mm + ../../../flutter/LICENSE
@@ -52855,9 +52857,6 @@ ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/accessibilit
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/accessibility_bridge_ios.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/accessibility_bridge_test.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/availability_version_check_test.mm + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/connection_collection.h + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/connection_collection.mm + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/connection_collection_test.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/overlay_layer_pool.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/overlay_layer_pool.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/platform_message_response_darwin.h + ../../../flutter/LICENSE
@@ -55766,6 +55765,8 @@ FILE: ../../../flutter/shell/platform/darwin/ios/framework/Headers/FlutterPlugin
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Headers/FlutterViewController.h
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Info.plist
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/PrivacyInfo.xcprivacy
+FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/ConnectionCollection.swift
+FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/ConnectionCollectionTest.swift
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FakeUIPressProxy.swift
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterAppDelegate.mm
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterAppDelegateTest.mm
@@ -55866,9 +55867,6 @@ FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/accessibility_
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/accessibility_bridge_ios.h
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/accessibility_bridge_test.mm
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/availability_version_check_test.mm
-FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/connection_collection.h
-FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/connection_collection.mm
-FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/connection_collection_test.mm
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/overlay_layer_pool.h
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/overlay_layer_pool.mm
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/platform_message_response_darwin.h

engine/src/flutter/shell/platform/darwin/ios/BUILD.gn

@@ -68,7 +68,10 @@ source_set("InternalFlutterSwift") {
   visibility = [ ":*" ]
   configs += [ ":config_ios" ]
 
-  sources = [ "framework/Source/UIPressProxy.swift" ]
+  sources = [
+    "framework/Source/ConnectionCollection.swift",
+    "framework/Source/UIPressProxy.swift",
+  ]
 }
 
 source_set("flutter_framework_source") {
@@ -149,8 +152,6 @@ source_set("flutter_framework_source") {
     "framework/Source/UIViewController+FlutterScreenAndSceneIfLoaded.mm",
     "framework/Source/accessibility_bridge.h",
     "framework/Source/accessibility_bridge.mm",
-    "framework/Source/connection_collection.h",
-    "framework/Source/connection_collection.mm",
     "framework/Source/overlay_layer_pool.h",
     "framework/Source/overlay_layer_pool.mm",
     "framework/Source/platform_message_response_darwin.h",
@@ -224,7 +225,10 @@ if (enable_ios_unittests) {
     testonly = true
     visibility = [ ":*" ]
     configs += [ ":config_ios_test" ]
-    sources = [ "framework/Source/FakeUIPressProxy.swift" ]
+    sources = [
+      "framework/Source/ConnectionCollectionTest.swift",
+      "framework/Source/FakeUIPressProxy.swift",
+    ]
     frameworks = [ "XCTest.framework" ]
     deps = [ ":InternalFlutterSwift" ]
   }
@@ -278,7 +282,6 @@ if (enable_ios_unittests) {
       "framework/Source/VsyncWaiterIosTest.mm",
       "framework/Source/accessibility_bridge_test.mm",
       "framework/Source/availability_version_check_test.mm",
-      "framework/Source/connection_collection_test.mm",
       "ios_context_noop_unittests.mm",
       "ios_surface_noop_unittests.mm",
       "platform_message_handler_ios_test.mm",

engine/src/flutter/shell/platform/darwin/ios/framework/Source/ConnectionCollection.swift

@@ -0,0 +1,48 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import Foundation
+
+/// A collection of active channel connections, tracked by unique IDs.
+///
+/// ConnectionIDs are guaranteed to be positive Int64s.
+///
+/// This class is not thread-safe. All accesses should happen from the
+/// platform thread.
+@objc(FlutterConnectionCollection)
+public class ConnectionCollection: NSObject {
+  public typealias ConnectionID = Int64
+
+  // The connection ID of the most recently used connection, or 0 if none.
+  private var counter: ConnectionID = 0
+
+  // Active connections map of channel name to connection ID.
+  private var connections: [String: ConnectionID] = [:]
+
+  /// Acquires a new connection for the specified channel.
+  @objc public func acquireConnection(forChannel channel: String) -> ConnectionID {
+    counter += 1
+    connections[channel] = counter
+    return counter
+  }
+
+  /// Cleans up an active connection.
+  ///
+  /// Returns the name of the associated channel if successful, otherwise the
+  /// empty string.
+  @objc public func cleanupConnection(withID connectionID: ConnectionID) -> String {
+    guard
+      connectionID > 0,
+      let entry = connections.first(where: { $0.value == connectionID })
+    else { return "" }
+
+    connections[entry.key] = nil
+    return entry.key
+  }
+
+  /// Creates an error connection from an error code.
+  @objc public static func makeErrorConnection(errorCode: Int64) -> ConnectionID {
+    return abs(errorCode)
+  }
+}

engine/src/flutter/shell/platform/darwin/ios/framework/Source/ConnectionCollectionTest.swift

@@ -0,0 +1,30 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import InternalFlutterSwift
+import XCTest
+
+class ConnectionCollectionTest: XCTestCase {
+  func testAcquireAndRelease() {
+    let connections = ConnectionCollection()
+    let connectionID = connections.acquireConnection(forChannel: "foo")
+    XCTAssertGreaterThan(connectionID, 0)
+    XCTAssertEqual("foo", connections.cleanupConnection(withID: connectionID))
+    XCTAssertEqual("", connections.cleanupConnection(withID: connectionID))
+  }
+
+  func testUniqueIDs() {
+    let connections = ConnectionCollection()
+    let firstConnectionID = connections.acquireConnection(forChannel: "foo")
+    let secondConnectionID = connections.acquireConnection(forChannel: "bar")
+    XCTAssertNotEqual(firstConnectionID, secondConnectionID)
+    XCTAssertEqual("foo", connections.cleanupConnection(withID: firstConnectionID))
+    XCTAssertEqual("bar", connections.cleanupConnection(withID: secondConnectionID))
+  }
+
+  func testErrorConnectionWithNegativeCode() {
+    XCTAssertEqual(55, ConnectionCollection.makeErrorConnection(errorCode: 55))
+    XCTAssertEqual(55, ConnectionCollection.makeErrorConnection(errorCode: -55))
+  }
+}

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterEngine.mm

@@ -22,6 +22,7 @@
 #include "flutter/shell/common/variable_refresh_rate_display.h"
 #import "flutter/shell/platform/darwin/common/command_line.h"
 #import "flutter/shell/platform/darwin/common/framework/Source/FlutterBinaryMessengerRelay.h"
+#import "flutter/shell/platform/darwin/ios/InternalFlutterSwift/InternalFlutterSwift.h"
 #import "flutter/shell/platform/darwin/ios/framework/Source/FlutterDartProject_Internal.h"
 #import "flutter/shell/platform/darwin/ios/framework/Source/FlutterDartVMServicePublisher.h"
 #import "flutter/shell/platform/darwin/ios/framework/Source/FlutterIndirectScribbleDelegate.h"
@@ -34,7 +35,6 @@
 #import "flutter/shell/platform/darwin/ios/framework/Source/FlutterUndoManagerPlugin.h"
 #import "flutter/shell/platform/darwin/ios/framework/Source/FlutterViewController_Internal.h"
 #import "flutter/shell/platform/darwin/ios/framework/Source/UIViewController+FlutterScreenAndSceneIfLoaded.h"
-#import "flutter/shell/platform/darwin/ios/framework/Source/connection_collection.h"
 #import "flutter/shell/platform/darwin/ios/framework/Source/platform_message_response_darwin.h"
 #import "flutter/shell/platform/darwin/ios/framework/Source/profiler_metrics_ios.h"
 #import "flutter/shell/platform/darwin/ios/framework/Source/vsync_waiter_ios.h"
@@ -117,6 +117,7 @@ @interface FlutterEngine () <FlutterIndirectScribbleDelegate,
 @property(nonatomic, copy) NSString* initialRoute;
 @property(nonatomic, strong) id<NSObject> flutterViewControllerWillDeallocObserver;
 @property(nonatomic, strong) FlutterDartVMServicePublisher* publisher;
+@property(nonatomic, strong) FlutterConnectionCollection* connections;
 @property(nonatomic, assign) int64_t nextTextureId;
 
 #pragma mark - Channel properties
@@ -158,7 +159,6 @@ @implementation FlutterEngine {
 
   FlutterBinaryMessengerRelay* _binaryMessenger;
   FlutterTextureRegistryRelay* _textureRegistry;
-  std::unique_ptr<flutter::ConnectionCollection> _connections;
 }
 
 - (int64_t)engineIdentifier {
@@ -220,7 +220,7 @@ - (instancetype)initWithName:(NSString*)labelPrefix
   [self recreatePlatformViewsController];
   _binaryMessenger = [[FlutterBinaryMessengerRelay alloc] initWithParent:self];
   _textureRegistry = [[FlutterTextureRegistryRelay alloc] initWithParent:self];
-  _connections.reset(new flutter::ConnectionCollection());
+  _connections = [[FlutterConnectionCollection alloc] init];
 
   NSNotificationCenter* center = [NSNotificationCenter defaultCenter];
   [center addObserver:self
@@ -1256,19 +1256,19 @@ - (FlutterBinaryMessengerConnection)setMessageHandlerOnChannel:(NSString*)channe
   if (_shell && _shell->IsSetup()) {
     self.platformView->GetPlatformMessageHandlerIos()->SetMessageHandler(channel.UTF8String,
                                                                          handler, taskQueue);
-    return _connections->AquireConnection(channel.UTF8String);
+    return [self.connections acquireConnectionForChannel:channel];
   } else {
     NSAssert(!handler, @"Setting a message handler before the FlutterEngine has been run.");
     // Setting a handler to nil for a channel that has not yet been set up is a no-op.
-    return flutter::ConnectionCollection::MakeErrorConnection(-1);
+    return [FlutterConnectionCollection makeErrorConnectionWithErrorCode:-1L];
   }
 }
 
 - (void)cleanUpConnection:(FlutterBinaryMessengerConnection)connection {
   if (_shell && _shell->IsSetup()) {
-    std::string channel = _connections->CleanupConnection(connection);
-    if (!channel.empty()) {
-      self.platformView->GetPlatformMessageHandlerIos()->SetMessageHandler(channel.c_str(), nil,
+    NSString* channel = [self.connections cleanupConnectionWithID:connection];
+    if (channel.length > 0) {
+      self.platformView->GetPlatformMessageHandlerIos()->SetMessageHandler(channel.UTF8String, nil,
                                                                            nil);
     }
   }