Rewrite AppCheckCore in Swift (#111)

Signed-off-by: Nick Cooke <36927374+ncooke3@users.noreply.github.com>
Co-authored-by: Nick Cooke <36927374+ncooke3@users.noreply.github.com>
Co-authored-by: Nick Cooke <nickcooke@google.com>

AppCheckCore/Sources/DeviceCheckProvider/AppCheckCoreDeviceCheckTokenGenerator.swift

@@ -0,0 +1,41 @@
+// Copyright 2026 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
+
+@objc(GACDeviceCheckTokenGenerator)
+public protocol AppCheckCoreDeviceCheckTokenGenerator: NSObjectProtocol {
+  @objc var isSupported: Bool { get }
+
+  @objc(generateTokenWithCompletionHandler:)
+  func generateToken(completionHandler: @escaping @Sendable (Data?, Error?) -> Void)
+}
+
+extension AppCheckCoreDeviceCheckTokenGenerator {
+  func generateTokenAsync() async throws -> Data {
+    try await withSafeCheckedThrowingContinuation { continuation in
+      self.generateToken { token, error in
+        if let error = error {
+          continuation.resume(throwing: error)
+        } else if let token = token {
+          continuation.resume(returning: token)
+        } else {
+          continuation
+            .resume(throwing: AppCheckCoreErrorUtil
+              .error(withFailureReason: "No token and no error."))
+        }
+      }
+    }
+  }
+}