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/API/AppCheckCoreDeviceCheckAPIService.swift

@@ -0,0 +1,63 @@
+// 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
+
+protocol AppCheckCoreDeviceCheckAPIServiceProtocol: NSObjectProtocol {
+  func appCheckToken(deviceToken: Data, limitedUse: Bool) async throws -> AppCheckCoreToken
+}
+
+class AppCheckCoreDeviceCheckAPIService: NSObject, AppCheckCoreDeviceCheckAPIServiceProtocol {
+  private let apiService: AppCheckCoreAPIServiceProtocol
+  private let resourceName: String
+
+  init(apiService: AppCheckCoreAPIServiceProtocol, resourceName: String) {
+    self.apiService = apiService
+    self.resourceName = resourceName
+    super.init()
+  }
+
+  func appCheckToken(deviceToken: Data, limitedUse: Bool) async throws -> AppCheckCoreToken {
+    guard !deviceToken.isEmpty else {
+      throw AppCheckCoreErrorUtil.error(withFailureReason: "DeviceCheck token must not be empty.")
+    }
+
+    let base64EncodedToken = deviceToken.base64EncodedString()
+    let payload: [String: Any] = [
+      "device_token": base64EncodedToken,
+      "limited_use": limitedUse,
+    ]
+
+    let payloadJSON: Data
+    do {
+      payloadJSON = try JSONSerialization.data(withJSONObject: payload)
+    } catch {
+      throw AppCheckCoreErrorUtil.jsonSerializationError(error)
+    }
+
+    let urlString = "\(apiService.baseURL)/\(resourceName):exchangeDeviceCheckToken"
+    guard let url = URL(string: urlString) else {
+      throw AppCheckCoreErrorUtil.error(withFailureReason: "Invalid URL.")
+    }
+
+    let response = try await apiService.sendRequest(
+      withURL: url,
+      httpMethod: "POST",
+      body: payloadJSON,
+      additionalHeaders: ["Content-Type": "application/json"]
+    )
+
+    return try apiService.appCheckToken(withAPIResponse: response)
+  }
+}