firebase-ios-sdk/FirebaseMLModelDownloader/Tests/Unit/ModelDownloaderUnitTests.swift

// Copyright 2021 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.

// Skip keychain tests on Catalyst and macOS. Tests are skipped because they
// involve interactions with the keychain that require a provisioning profile.
// This is because MLModelDownloader depends on Installations.
// See go/firebase-macos-keychain-popups for more details.
#if !targetEnvironment(macCatalyst) && !os(macOS)

  @testable import FirebaseCore
  @testable import FirebaseInstallations
  @testable import FirebaseMLModelDownloader
  import XCTest
  #if SWIFT_PACKAGE
    internal import GoogleUtilities_UserDefaults
  #else
    internal import GoogleUtilities
  #endif // SWIFT_PACKAGE

  /// Mock options to configure default Firebase app.
  private enum MockOptions {
    static let appID = "1:123:ios:123abc"
    static let gcmSenderID = "mock-sender-id"
    static let projectID = "mock-project-id"
    static let apiKey = "ABcdEf-APIKeyWithValidFormat_0123456789"
  }

  // TODO: Create separate files for each class tested.
  final class ModelDownloaderUnitTests: XCTestCase {
    let fakeModelName = "fakeModelName"
    let fakeModelPath = "fakeModelPath"
    let fakeModelHash = "fakeModelHash"
    let fakeDownloadURL = URL(string: "www.fake-download-url.com")!
    let fakeFileURL = URL(string: "www.fake-model-file.com")!
    let fakeExpiryTime = "2021-01-20T04:20:10.220Z"
    let fakeModelSize = 20
    let fakeProjectID = "fakeProjectID"
    let fakeAPIKey = "fakeAPIKey"
    let fakeAppName = "fakeAppName"
    var fakeModelJSON: String {
      """
      {
        "downloadUri":"\(fakeDownloadURL)",
        "expireTime":"\(fakeExpiryTime)",
        "sizeBytes":"\(fakeModelSize)"
      }
      """
    }

    let successAuthTokenProvider =
      { (completion: @escaping (Result<String, DownloadError>) -> Void) in
        completion(.success("fakeFISToken"))
      }

    override class func setUp() {
      let options = FirebaseOptions(
        googleAppID: MockOptions.appID,
        gcmSenderID: MockOptions.gcmSenderID
      )
      options.apiKey = MockOptions.apiKey
      options.projectID = MockOptions.projectID
      FirebaseApp.configure(options: options)
      FirebaseConfiguration.shared.setLoggerLevel(.debug)
    }

    override func setUp() {
      do {
        try ModelFileManager.emptyModelsDirectory()
      } catch {
        XCTFail("Could not empty models directory.")
      }
    }

    override class func tearDown() {
      try? FileManager.default.removeItem(atPath: NSTemporaryDirectory())
      FirebaseApp.app()?.delete { _ in }
    }

    /// Test if the same downloader instance is returned for an app.
    func testModelDownloaderInit() {
      guard let testApp = FirebaseApp.app() else {
        XCTFail("Default app was not configured.")
        return
      }
      testApp.isDataCollectionDefaultEnabled = false

      let modelDownloader1 = ModelDownloader.modelDownloader(app: testApp)
      let modelDownloader2 = ModelDownloader.modelDownloader()
      XCTAssert(modelDownloader1 === modelDownloader2)
    }

    /// Test if app deletion removes associated downloader instance.
    func testAppDeletion() {
      guard let testApp = FirebaseApp.app() else {
        XCTFail("Default app was not configured.")
        return
      }
      testApp.isDataCollectionDefaultEnabled = false

      let modelDownloader1 = ModelDownloader.modelDownloader()
      testApp.delete { success in
        XCTAssertTrue(success)
      }
      ModelDownloaderUnitTests.setUp()
      let modelDownloader2 = ModelDownloader.modelDownloader()
      XCTAssert(modelDownloader1 !== modelDownloader2)
    }

    /// Test invalid model file deletion.
    func testDeleteInvalidModel() {
      guard let testApp = FirebaseApp.app() else {
        XCTFail("Default app was not configured.")
        return
      }
      testApp.isDataCollectionDefaultEnabled = false

      let modelDownloader = ModelDownloader.modelDownloader()
      modelDownloader.deleteDownloadedModel(name: fakeModelName) { result in
        switch result {
        case .success: XCTFail("Unexpected successful model deletion.")
        case let .failure(error):
          XCTAssertNotNil(error)
        }
      }
    }

    func testGetModelNameFromFilePath() {
      var fakeURL = URL(fileURLWithPath: "modelDirectory/@@appName@@\(fakeModelName).tflite")
      XCTAssertEqual(ModelFileManager.getModelNameFromFilePath(fakeURL), fakeModelName)
      fakeURL = URL(fileURLWithPath: "modelDirectory/--appName--\(fakeModelName).tflite")
      XCTAssertNil(ModelFileManager.getModelNameFromFilePath(fakeURL))
    }

    /// Test listing models.
    func testListModels() {
      guard let testApp = FirebaseApp.app() else {
        XCTFail("Default app was not configured.")
        return
      }
      testApp.isDataCollectionDefaultEnabled = false

      let testName = String(#function.dropLast(2))
      let modelDownloader = ModelDownloader.modelDownloaderWithDefaults(
        .createUnitTestInstance(testName: testName),
        app: testApp
      )

      let tempModelFileURL = tempModelFile()
      let completionExpectation = expectation(description: "Completion handler")

      let localModelInfo = fakeLocalModelInfo()
      localModelInfo.writeToDefaults(
        .getUnitTestInstance(testName: testName),
        appName: testApp.name
      )

      modelDownloader.listDownloadedModels { result in
        completionExpectation.fulfill()
        switch result {
        case let .success(models):
          /// Expected success since model info was written to user defaults.
          XCTAssertEqual(models.first?.path, tempModelFileURL.path)
        case let .failure(error):
          XCTFail("Error - \(error)")
        }
      }
      wait(for: [completionExpectation], timeout: 0.5)
      try? ModelFileManager.removeFile(at: tempModelFileURL)
    }

    /// Test invalid path in model directory.
    func testListModelsInvalidPath() {
      guard let testApp = FirebaseApp.app() else {
        XCTFail("Default app was not configured.")
        return
      }
      testApp.isDataCollectionDefaultEnabled = false

      let modelDownloader = ModelDownloader.modelDownloader()
      let invalidTempModelFileURL = tempModelFile(invalid: true)
      let completionExpectation = expectation(description: "Completion handler")

      modelDownloader.listDownloadedModels { result in
        completionExpectation.fulfill()
        switch result {
        case .success:
          XCTFail("Unexpected success of list models.")
        case let .failure(error):
          switch error {
          case let .internalError(description):
            XCTAssertTrue(description
              .contains("List models failed due to unexpected model file name"))
          default: XCTFail("Expected failure due to bad model name.")
          }
        }
      }
      wait(for: [completionExpectation], timeout: 0.5)
      try? ModelFileManager.removeFile(at: invalidTempModelFileURL)
    }

    /// Test listing models without local info in user defaults.
    func testListModelsNoLocalInfo() {
      guard let testApp = FirebaseApp.app() else {
        XCTFail("Default app was not configured.")
        return
      }
      testApp.isDataCollectionDefaultEnabled = false

      let testName = String(#function.dropLast(2))
      let tempModelFileURL = tempModelFile()
      let modelDownloader = ModelDownloader.modelDownloaderWithDefaults(
        .createUnitTestInstance(testName: testName),
        app: testApp
      )

      let completionExpectation = expectation(description: "Completion handler")

      modelDownloader.listDownloadedModels { result in
        completionExpectation.fulfill()
        switch result {
        case .success:
          XCTFail("Unexpected success of list models.")
        case let .failure(error):
          switch error {
          /// Expected failure since no model info was written to user defaults.
          case let .internalError(description):
            XCTAssertTrue(description.contains("List models failed due to no local model info"))
          default: XCTFail("Expected failure due to bad model name.")
          }
        }
      }
      wait(for: [completionExpectation], timeout: 0.5)
      try? ModelFileManager.removeFile(at: tempModelFileURL)
    }

    /// Test how URL conversion behaves if there are spaces in the path.
    func testURLConversion() {
      let fakeURLWithSpace = URL(string: "file:///fakeDir1/fake%20Dir2/fakeFile")!

      XCTAssertEqual(fakeURLWithSpace, URL(string: fakeURLWithSpace.absoluteString))
      XCTAssertEqual(fakeURLWithSpace, URL(fileURLWithPath: fakeURLWithSpace.path))

      /// Strings without spaces should work fine either way.
      let fakeURLWithoutSpace = URL(string: "fakeDir1/fakeDir2/fakeFile")!
      XCTAssertEqual(fakeURLWithoutSpace, URL(string: fakeURLWithoutSpace.absoluteString)!)
      XCTAssertEqual(fakeURLWithoutSpace, URL(string: fakeURLWithoutSpace.path)!)
    }

    /// Test on-device logging.
    func testDeviceLogging() {
      DeviceLogger.logEvent(level: .error,
                            message: "This is a test error logged to device.",
                            messageCode: .testError)
    }

    /// Compare proto serialization methods.
    func testTelemetryEncoding() {
      let fakeModel = CustomModel(
        name: fakeModelName,
        size: 10,
        path: fakeModelPath,
        hash: fakeModelHash
      )
      var modelOptions = ModelOptions()
      modelOptions.setModelOptions(modelName: fakeModel.name, modelHash: fakeModel.hash)

      guard let binaryData = try? modelOptions.serializedData(),
            let jsonData = try? modelOptions.jsonUTF8Data(),
            let binaryEvent = try? ModelOptions(serializedBytes: binaryData),
            let jsonEvent = try? ModelOptions(jsonUTF8Data: jsonData) else {
        XCTFail("Encoding error.")
        return
      }

      XCTAssertNotNil(binaryData)
      XCTAssertNotNil(jsonData)
      XCTAssertLessThan(binaryData.count, jsonData.count)
      XCTAssertEqual(binaryEvent, jsonEvent)
    }

    /// Get model info if server returns a new model info.
    func testGetModelInfoWith200() {
      let session = fakeModelInfoSessionWithURL(fakeDownloadURL, statusCode: 200)
      let modelInfoRetriever = fakeModelRetriever(fakeSession: session)
      let completionExpectation = expectation(description: "Completion handler")

      modelInfoRetriever.downloadModelInfo { result in
        completionExpectation.fulfill()
        switch result {
        case let .success(fakeModelInfoResult):
          switch fakeModelInfoResult {
          case let .modelInfo(remoteModelInfo):
            XCTAssertEqual(remoteModelInfo.name, self.fakeModelName)
            XCTAssertEqual(remoteModelInfo.downloadURL, self.fakeDownloadURL)
            XCTAssertEqual(remoteModelInfo.size, self.fakeModelSize)
            XCTAssertEqual(remoteModelInfo.modelHash, self.fakeModelHash)
          case .notModified: XCTFail("Expected new model info from server.")
          }
        case let .failure(error): XCTFail("Failed with error - \(error)")
        }
      }
      wait(for: [completionExpectation], timeout: 0.5)
    }

    /// Get model info if model info is not modified.
    func testGetModelInfoWith304() {
      let session = fakeModelInfoSessionWithURL(fakeDownloadURL, statusCode: 304)
      let localModelInfo = fakeLocalModelInfo(mismatch: false)
      let modelInfoRetriever = fakeModelRetriever(
        fakeSession: session,
        fakeLocalModelInfo: localModelInfo
      )
      let completionExpectation = expectation(description: "Completion handler")

      modelInfoRetriever.downloadModelInfo { result in
        completionExpectation.fulfill()
        switch result {
        case let .success(fakeModelInfoResult):
          switch fakeModelInfoResult {
          case .modelInfo: XCTFail("Unexpected new model info from server.")
          case .notModified: break
          }
        case let .failure(error): XCTFail("Failed with error - \(error)")
        }
      }
      wait(for: [completionExpectation], timeout: 0.5)
    }

    /// Get model info if model info is not modified but local model info is not set.
    func testGetModelInfoWith304MissingLocalInfo() {
      let session = fakeModelInfoSessionWithURL(fakeDownloadURL, statusCode: 304)
      let modelInfoRetriever = fakeModelRetriever(fakeSession: session)
      let completionExpectation = expectation(description: "Completion handler")

      modelInfoRetriever.downloadModelInfo { result in
        completionExpectation.fulfill()
        switch result {
        case let .success(fakeModelInfoResult):
          switch fakeModelInfoResult {
          case .modelInfo: XCTFail("Unexpected new model info from server.")
          case .notModified: XCTFail("Expected failure since local model info was not set.")
          }
        case let .failure(error):
          switch error {
          case let .internalError(description: errorMessage): XCTAssertEqual(errorMessage,
                                                                             "Model info was deleted unexpectedly.")
          default: XCTFail("Expected failure since local model info was not set.")
          }
        }
      }
      wait(for: [completionExpectation], timeout: 0.5)
    }

    /// Get model info if model info is not modified but local model info is not set.
    func testGetModelInfoWith304HashMismatch() {
      let session = fakeModelInfoSessionWithURL(fakeDownloadURL, statusCode: 304)
      let localModelInfo = fakeLocalModelInfo(mismatch: true)
      let modelInfoRetriever = fakeModelRetriever(
        fakeSession: session,
        fakeLocalModelInfo: localModelInfo
      )
      let completionExpectation = expectation(description: "Completion handler")

      modelInfoRetriever.downloadModelInfo { result in
        completionExpectation.fulfill()
        switch result {
        case let .success(fakeModelInfoResult):
          switch fakeModelInfoResult {
          case .modelInfo: XCTFail("Unexpected new model info from server.")
          case .notModified: XCTFail("Expected failure since model hash does not match.")
          }
        case let .failure(error):
          switch error {
          case let .internalError(description: errorMessage): XCTAssertEqual(errorMessage,
                                                                             "Unexpected model hash value.")
          default: XCTFail("Expected failure since local model info was not set.")
          }
        }
      }
      wait(for: [completionExpectation], timeout: 0.5)
    }

    /// Get model info if model name is invalid.
    func testGetModelInfoWith400() {
      let session = fakeModelInfoSessionWithURL(fakeDownloadURL, statusCode: 400)
      let localModelInfo = fakeLocalModelInfo(mismatch: false)
      let modelInfoRetriever = fakeModelRetriever(
        fakeSession: session,
        fakeLocalModelInfo: localModelInfo
      )
      let completionExpectation = expectation(description: "Completion handler")

      modelInfoRetriever.downloadModelInfo { result in
        completionExpectation.fulfill()
        switch result {
        case let .success(fakeModelInfoResult):
          switch fakeModelInfoResult {
          case .modelInfo: XCTFail("Unexpected new model info from server.")
          case .notModified: XCTFail("Expected failure since model name is invalid.")
          }
        case let .failure(error):
          XCTAssertEqual(error, .invalidArgument)
        }
      }
      wait(for: [completionExpectation], timeout: 0.5)
    }

    /// Get model info if model name does not exist.
    func testGetModelInfoWith404() {
      let session = fakeModelInfoSessionWithURL(fakeDownloadURL, statusCode: 404)
      let localModelInfo = fakeLocalModelInfo(mismatch: false)
      let modelInfoRetriever = fakeModelRetriever(
        fakeSession: session,
        fakeLocalModelInfo: localModelInfo
      )
      let completionExpectation = expectation(description: "Completion handler")

      modelInfoRetriever.downloadModelInfo { result in
        completionExpectation.fulfill()
        switch result {
        case let .success(fakeModelInfoResult):
          switch fakeModelInfoResult {
          case .modelInfo: XCTFail("Unexpected new model info from server.")
          case .notModified: XCTFail("Expected failure since model name is not available.")
          }
        case let .failure(error):
          XCTAssertEqual(error, .notFound)
        }
      }
      wait(for: [completionExpectation], timeout: 0.5)
    }

    /// Get model info if resource exhausted due to too many requests.
    func testGetModelInfoWith429() {
      let session = fakeModelInfoSessionWithURL(fakeDownloadURL, statusCode: 429)
      let localModelInfo = fakeLocalModelInfo(mismatch: false)
      let modelInfoRetriever = fakeModelRetriever(
        fakeSession: session,
        fakeLocalModelInfo: localModelInfo
      )
      let completionExpectation = expectation(description: "Completion handler")

      modelInfoRetriever.downloadModelInfo { result in
        completionExpectation.fulfill()
        switch result {
        case let .success(fakeModelInfoResult):
          switch fakeModelInfoResult {
          case .modelInfo: XCTFail("Unexpected new model info from server.")
          case .notModified: XCTFail("Expected failure since resource exhausted.")
          }
        case let .failure(error):
          XCTAssertEqual(error, .resourceExhausted)
        }
      }
      wait(for: [completionExpectation], timeout: 0.5)
    }

    /// Get model if server returns a model file.
    func testModelDownloadWith200() {
      let tempFileURL = tempFile()
      let remoteModelInfo = fakeModelInfo(expired: false, oversized: false)
      let fakeResponse = HTTPURLResponse(url: fakeFileURL,
                                         statusCode: 200,
                                         httpVersion: nil,
                                         headerFields: nil)!
      let downloader = MockModelFileDownloader()
      let fileDownloaderExpectation = expectation(description: "File downloader")
      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }
      let progressExpectation = expectation(description: "Progress handler")
      progressExpectation.expectedFulfillmentCount = 2

      let completionExpectation = expectation(description: "Completion handler")
      let taskProgressHandler: ModelDownloadTask.ProgressHandler = {
        progress in
        progressExpectation.fulfill()
        XCTAssertEqual(progress, 0.4)
      }
      let taskCompletion: ModelDownloadTask.Completion = { result in
        completionExpectation.fulfill()
        switch result {
        case let .success(model):
          self.checkModel(model: model)
          self.deleteFile(at: model.path)
        case let .failure(error):
          XCTFail("Error - \(error)")
        }
      }
      let testName = String(#function.dropLast(2))
      let modelDownloadTask = ModelDownloadTask(remoteModelInfo: remoteModelInfo,
                                                appName: "fakeAppName",
                                                defaults: .createUnitTestInstance(
                                                  testName: testName
                                                ),
                                                downloader: downloader,
                                                progressHandler: taskProgressHandler,
                                                completion: taskCompletion)

      modelDownloadTask.resume()
      wait(for: [fileDownloaderExpectation], timeout: 0.1)

      downloader.progressHandler?(100, 250)
      downloader.progressHandler?(100, 250)
      wait(for: [progressExpectation], timeout: 0.5)

      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponse,
                                                     fileURL: tempFileURL)))
      wait(for: [completionExpectation], timeout: 0.5)

      try? ModelFileManager.removeFile(at: tempFileURL)
    }

    /// Get model if server returns an error due to expired url.
    func testModelDownloadWith400Expired() {
      let tempFileURL = tempFile()
      let remoteModelInfo = fakeModelInfo(expired: true, oversized: false)
      let fakeResponse = HTTPURLResponse(url: fakeFileURL,
                                         statusCode: 400,
                                         httpVersion: nil,
                                         headerFields: nil)!
      let downloader = MockModelFileDownloader()
      let fileDownloaderExpectation = expectation(description: "File downloader")
      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }

      let completionExpectation = expectation(description: "Completion handler")
      let taskCompletion: ModelDownloadTask.Completion = { result in
        completionExpectation.fulfill()
        switch result {
        case .success: XCTFail("Unexpected successful model download.")
        case let .failure(error):
          XCTAssertEqual(error, .expiredDownloadURL)
        }
      }
      let testName = String(#function.dropLast(2))
      let modelDownloadTask = ModelDownloadTask(remoteModelInfo: remoteModelInfo,
                                                appName: "fakeAppName",
                                                defaults: .createUnitTestInstance(
                                                  testName: testName
                                                ),
                                                downloader: downloader,
                                                completion: taskCompletion)
      modelDownloadTask.resume()
      wait(for: [fileDownloaderExpectation], timeout: 0.1)

      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponse,
                                                     fileURL: tempFileURL)))
      wait(for: [completionExpectation], timeout: 0.5)

      try? ModelFileManager.removeFile(at: tempFileURL)
    }

    /// Get model if server returns an error due to invalid model name.
    func testModelDownloadWith400Invalid() {
      let tempFileURL = tempFile()
      let remoteModelInfo = fakeModelInfo(expired: false, oversized: false)
      let fakeResponse = HTTPURLResponse(url: fakeFileURL,
                                         statusCode: 400,
                                         httpVersion: nil,
                                         headerFields: nil)!
      let downloader = MockModelFileDownloader()
      let fileDownloaderExpectation = expectation(description: "File downloader")
      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }

      let completionExpectation = expectation(description: "Completion handler")
      let taskCompletion: ModelDownloadTask.Completion = { result in
        completionExpectation.fulfill()
        switch result {
        case .success: XCTFail("Unexpected successful model download.")
        case let .failure(error):
          XCTAssertEqual(error, .invalidArgument)
        }
      }
      let testName = String(#function.dropLast(2))
      let modelDownloadTask = ModelDownloadTask(remoteModelInfo: remoteModelInfo,
                                                appName: "fakeAppName",
                                                defaults: .createUnitTestInstance(
                                                  testName: testName
                                                ),
                                                downloader: downloader,
                                                completion: taskCompletion)
      modelDownloadTask.resume()
      wait(for: [fileDownloaderExpectation], timeout: 0.1)

      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponse,
                                                     fileURL: tempFileURL)))
      wait(for: [completionExpectation], timeout: 0.5)

      try? ModelFileManager.removeFile(at: tempFileURL)
    }

    /// Get model info if Firebase ML API is not enabled or permission denied.
    func testGetModelInfoWith403() {
      let errorMessage =
        """
        {
          "error":
            {
              "message":"API Disabled."
            }
        }
        """
      let session = fakeModelInfoSessionWithURL(
        fakeDownloadURL,
        statusCode: 403,
        customJSON: errorMessage
      )
      let localModelInfo = fakeLocalModelInfo(mismatch: false)
      let modelInfoRetriever = fakeModelRetriever(
        fakeSession: session,
        fakeLocalModelInfo: localModelInfo
      )
      let completionExpectation = expectation(description: "Completion handler")

      modelInfoRetriever.downloadModelInfo { result in
        completionExpectation.fulfill()
        switch result {
        case .success:
          XCTFail("Unexpected new model info from server.")
        case let .failure(error):
          XCTAssertEqual(error, .permissionDenied)
        }
      }
      wait(for: [completionExpectation], timeout: 0.5)
    }

    /// Get model if multiple duplicate requests are made.
    func testModelDownloadWithMergeRequests() {
      let tempFileURL = tempFile()
      let numberOfRequests = 5
      let remoteModelInfo = fakeModelInfo(expired: false, oversized: false)
      let fakeResponse = HTTPURLResponse(url: fakeFileURL,
                                         statusCode: 200,
                                         httpVersion: nil,
                                         headerFields: nil)!
      let downloader = MockModelFileDownloader()
      let fileDownloaderExpectation = expectation(description: "File downloader")

      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }

      let completionExpectation = expectation(description: "Completion handler")
      completionExpectation.expectedFulfillmentCount = numberOfRequests + 1

      let taskCompletion: ModelDownloadTask.Completion = { result in
        completionExpectation.fulfill()
        switch result {
        case let .success(model):
          self.checkModel(model: model)
        case let .failure(error):
          XCTFail("Error - \(error)")
        }
      }

      let testName = String(#function.dropLast(2))
      let modelDownloadTask = ModelDownloadTask(remoteModelInfo: remoteModelInfo,
                                                appName: fakeAppName,
                                                defaults: .createUnitTestInstance(
                                                  testName: testName
                                                ),
                                                downloader: downloader,
                                                completion: taskCompletion)

      for i in 1 ... numberOfRequests {
        let newTaskCompletion: ModelDownloadTask.Completion = { result in
          completionExpectation.fulfill()
          switch result {
          case let .success(model):
            self.checkModel(model: model)
            if i == numberOfRequests {
              self.deleteFile(at: model.path)
            }
          case let .failure(error):
            XCTFail("Error - \(error)")
          }
        }
        modelDownloadTask.resume()
        modelDownloadTask.merge(newCompletion: newTaskCompletion)
      }

      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponse,
                                                     fileURL: tempFileURL)))
      wait(for: [fileDownloaderExpectation], timeout: 1)
      wait(for: [completionExpectation], timeout: 2)

      try? ModelFileManager.removeFile(at: tempFileURL)
    }

    /// Get model if multiple duplicate requests are made (test at the API surface).
    func testGetModelWithMergeRequests() {
      guard let testApp = FirebaseApp.app() else {
        XCTFail("Default app was not configured.")
        return
      }
      testApp.isDataCollectionDefaultEnabled = false

      let modelDownloader = ModelDownloader.modelDownloader()
      let conditions = ModelDownloadConditions()
      let session = fakeModelInfoSessionWithURL(fakeDownloadURL, statusCode: 200)
      let modelInfoRetriever = fakeModelRetriever(fakeSession: session)
      let tempFileURL = tempFile()
      let numberOfRequests = 6
      let fakeResponse = HTTPURLResponse(url: fakeFileURL,
                                         statusCode: 200,
                                         httpVersion: nil,
                                         headerFields: nil)!
      let downloader = MockModelFileDownloader()
      let fileDownloaderExpectation = expectation(description: "File downloader")

      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }

      let completionExpectation = expectation(description: "Completion handler")
      completionExpectation.expectedFulfillmentCount = numberOfRequests

      for i in 1 ... numberOfRequests {
        modelDownloader.downloadInfoAndModel(modelName: "fakeModelName",
                                             modelInfoRetriever: modelInfoRetriever,
                                             downloader: downloader,
                                             conditions: conditions) { result in
          completionExpectation.fulfill()
          switch result {
          case let .success(model):
            self.checkModel(model: model)
            if i == numberOfRequests {
              self.deleteFile(at: model.path)
            }
          case let .failure(error):
            XCTFail("Error - \(error)")
          }
        }
      }
      wait(for: [fileDownloaderExpectation], timeout: 0.5)

      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponse,
                                                     fileURL: tempFileURL)))

      wait(for: [completionExpectation], timeout: 1)

      try? ModelFileManager.removeFile(at: tempFileURL)
    }

    /// Get model if server returns an error due to model not found.
    func testModelDownloadWith404() {
      let tempFileURL = tempFile()
      let remoteModelInfo = fakeModelInfo(expired: false, oversized: false)
      let fakeResponse = HTTPURLResponse(url: fakeFileURL,
                                         statusCode: 404,
                                         httpVersion: nil,
                                         headerFields: nil)!
      let downloader = MockModelFileDownloader()
      let fileDownloaderExpectation = expectation(description: "File downloader")
      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }

      let completionExpectation = expectation(description: "Completion handler")
      let taskCompletion: ModelDownloadTask.Completion = { result in
        completionExpectation.fulfill()
        switch result {
        case .success: XCTFail("Unexpected successful model download.")
        case let .failure(error):
          XCTAssertEqual(error, .notFound)
        }
      }
      let testName = String(#function.dropLast(2))
      let modelDownloadTask = ModelDownloadTask(remoteModelInfo: remoteModelInfo,
                                                appName: "fakeAppName",
                                                defaults: .createUnitTestInstance(
                                                  testName: testName
                                                ),
                                                downloader: downloader,
                                                completion: taskCompletion)
      modelDownloadTask.resume()
      wait(for: [fileDownloaderExpectation], timeout: 0.1)

      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponse,
                                                     fileURL: tempFileURL)))
      wait(for: [completionExpectation], timeout: 0.5)

      try? ModelFileManager.removeFile(at: tempFileURL)
    }

    /// Get model if server returns an error due to model not found.
    func testModelDownloadNetworkError() {
      let tempFileURL = tempFile()
      let remoteModelInfo = fakeModelInfo(expired: false, oversized: false)
      let downloader = MockModelFileDownloader()
      let fileDownloaderExpectation = expectation(description: "File downloader")
      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }

      let completionExpectation = expectation(description: "Completion handler")
      let taskCompletion: ModelDownloadTask.Completion = { result in
        completionExpectation.fulfill()
        switch result {
        case .success: XCTFail("Unexpected successful model download.")
        case let .failure(error):
          XCTAssertEqual(error, .failedPrecondition)
        }
      }
      let testName = String(#function.dropLast(2))
      let modelDownloadTask = ModelDownloadTask(remoteModelInfo: remoteModelInfo,
                                                appName: "fakeAppName",
                                                defaults: .createUnitTestInstance(
                                                  testName: testName
                                                ),
                                                downloader: downloader,
                                                completion: taskCompletion)
      modelDownloadTask.resume()
      wait(for: [fileDownloaderExpectation], timeout: 0.1)

      let offlineError = DownloadError.internalError(description: "Network appears to be offline.")
      downloader
        .completion?(.failure(FileDownloaderError.networkError(offlineError)))
      wait(for: [completionExpectation], timeout: 0.5)

      try? ModelFileManager.removeFile(at: tempFileURL)
    }

    /// Get model with a valid server response.
    func testGetModelSuccessful() {
      guard let testApp = FirebaseApp.app() else {
        XCTFail("Default app was not configured.")
        return
      }
      testApp.isDataCollectionDefaultEnabled = false

      let modelDownloader = ModelDownloader.modelDownloader()
      let conditions = ModelDownloadConditions()
      let session = fakeModelInfoSessionWithURL(fakeDownloadURL, statusCode: 200)
      let modelInfoRetriever = fakeModelRetriever(fakeSession: session)
      let downloader = MockModelFileDownloader()
      let tempFileURL = tempFile()

      let fakeResponseNotFound = HTTPURLResponse(url: fakeFileURL,
                                                 statusCode: 200,
                                                 httpVersion: nil,
                                                 headerFields: nil)!

      let fileDownloaderExpectation = expectation(description: "File downloader")
      let completionExpectation = expectation(description: "Completion handler")

      // Handles initial response.
      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }

      modelDownloader.downloadInfoAndModel(modelName: fakeModelName,
                                           modelInfoRetriever: modelInfoRetriever,
                                           downloader: downloader,
                                           conditions: conditions) { result in
        completionExpectation.fulfill()
        switch result {
        case let .success(model):
          self.checkModel(model: model)
          self.deleteFile(at: model.path)
        case let .failure(error):
          XCTFail("Error - \(error)")
        }
      }
      wait(for: [fileDownloaderExpectation], timeout: 0.5)

      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponseNotFound,
                                                     fileURL: tempFileURL)))
      wait(for: [completionExpectation], timeout: 0.5)

      try? ModelFileManager.removeFile(at: tempFileURL)
    }

    /// Get model if download url expired but retry was successful.
    func testGetModelSuccessfulRetry() {
      guard let testApp = FirebaseApp.app() else {
        XCTFail("Default app was not configured.")
        return
      }
      testApp.isDataCollectionDefaultEnabled = false

      let modelDownloader = ModelDownloader.modelDownloader()
      let conditions = ModelDownloadConditions()
      let session = fakeModelInfoSessionWithURL(fakeDownloadURL, statusCode: 200)
      let modelInfoRetriever = fakeModelRetriever(fakeSession: session)
      let downloader = MockModelFileDownloader()
      let tempFileURL = tempFile()

      let fakeResponseExpired = HTTPURLResponse(url: fakeFileURL,
                                                statusCode: 400,
                                                httpVersion: nil,
                                                headerFields: nil)!

      let fakeResponseSucceeded = HTTPURLResponse(url: fakeFileURL,
                                                  statusCode: 200,
                                                  httpVersion: nil,
                                                  headerFields: nil)!

      let fileDownloaderExpectation1 = expectation(description: "File downloader before retry")
      let completionExpectation = expectation(description: "Completion handler.")

      // Handles initial response.
      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation1.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }

      modelDownloader.downloadInfoAndModel(modelName: fakeModelName,
                                           modelInfoRetriever: modelInfoRetriever,
                                           downloader: downloader,
                                           conditions: conditions) { result in
        completionExpectation.fulfill()
        switch result {
        // Retry successful - model returned.
        case let .success(model):
          self.checkModel(model: model)
          self.deleteFile(at: model.path)
        case let .failure(error):
          XCTFail("Error - \(error)")
        }
      }
      wait(for: [fileDownloaderExpectation1], timeout: 0.5)

      // Handles response after one retry.
      let fileDownloaderExpectation2 = expectation(description: "File downloader after retry")
      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation2.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }
      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponseExpired,
                                                     fileURL: tempFileURL)))
      wait(for: [fileDownloaderExpectation2], timeout: 0.5)

      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponseSucceeded,
                                                     fileURL: tempFileURL)))
      wait(for: [completionExpectation], timeout: 0.5)

      try? ModelFileManager.removeFile(at: tempFileURL)
    }

    /// Get model if model doesn't exist.
    func testGetModelNotFound() {
      guard let testApp = FirebaseApp.app() else {
        XCTFail("Default app was not configured.")
        return
      }
      testApp.isDataCollectionDefaultEnabled = false

      let modelDownloader = ModelDownloader.modelDownloader()
      let conditions = ModelDownloadConditions()
      let session = fakeModelInfoSessionWithURL(fakeDownloadURL, statusCode: 200)
      let modelInfoRetriever = fakeModelRetriever(fakeSession: session)
      let downloader = MockModelFileDownloader()
      let tempFileURL = tempFile()

      let fakeResponseNotFound = HTTPURLResponse(url: fakeFileURL,
                                                 statusCode: 404,
                                                 httpVersion: nil,
                                                 headerFields: nil)!

      let fileDownloaderExpectation = expectation(description: "File downloader before retry")
      let completionExpectation = expectation(description: "Completion handler")

      // Handles initial response.
      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }

      modelDownloader.downloadInfoAndModel(modelName: fakeModelName,
                                           modelInfoRetriever: modelInfoRetriever,
                                           downloader: downloader,
                                           conditions: conditions) { result in
        completionExpectation.fulfill()
        switch result {
        // Retry failed due to another expired url - error returned.
        case .success: XCTFail("Unexpected successful model download.")
        case let .failure(error):
          switch error {
          case .notFound: break
          default: XCTFail("Expected failure due to model not found.")
          }
        }
      }
      wait(for: [fileDownloaderExpectation], timeout: 0.5)

      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponseNotFound,
                                                     fileURL: tempFileURL)))
      wait(for: [completionExpectation], timeout: 0.5)

      try? ModelFileManager.removeFile(at: tempFileURL)
    }

    /// Get model if invalid or insufficient permissions.
    func testGetModelUnauthorized() {
      guard let testApp = FirebaseApp.app() else {
        XCTFail("Default app was not configured.")
        return
      }
      testApp.isDataCollectionDefaultEnabled = false

      let modelDownloader = ModelDownloader.modelDownloader()
      let conditions = ModelDownloadConditions()
      let session = fakeModelInfoSessionWithURL(fakeDownloadURL, statusCode: 200)
      let modelInfoRetriever = fakeModelRetriever(fakeSession: session)
      let downloader = MockModelFileDownloader()
      let tempFileURL = tempFile()

      let fakeResponseNotFound = HTTPURLResponse(url: fakeFileURL,
                                                 statusCode: 403,
                                                 httpVersion: nil,
                                                 headerFields: nil)!

      let fileDownloaderExpectation = expectation(description: "File downloader")
      let completionExpectation = expectation(description: "Completion handler")

      // Handles initial response.
      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }

      modelDownloader.downloadInfoAndModel(modelName: fakeModelName,
                                           modelInfoRetriever: modelInfoRetriever,
                                           downloader: downloader,
                                           conditions: conditions) { result in
        completionExpectation.fulfill()
        switch result {
        case .success: XCTFail("Unexpected successful model download.")
        case let .failure(error):
          switch error {
          case .permissionDenied: break
          default: XCTFail("Expected failure due to bad permissions.")
          }
        }
      }
      wait(for: [fileDownloaderExpectation], timeout: 0.5)

      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponseNotFound,
                                                     fileURL: tempFileURL)))
      wait(for: [completionExpectation], timeout: 0.5)

      try? ModelFileManager.removeFile(at: tempFileURL)
    }

    /// Get model if download url expired and retry url also expired.
    func testGetModelFailedRetry() {
      guard let testApp = FirebaseApp.app() else {
        XCTFail("Default app was not configured.")
        return
      }
      testApp.isDataCollectionDefaultEnabled = false

      let modelDownloader = ModelDownloader.modelDownloader()
      let conditions = ModelDownloadConditions()
      let session = fakeModelInfoSessionWithURL(fakeDownloadURL, statusCode: 200)
      let modelInfoRetriever = fakeModelRetriever(fakeSession: session)
      let downloader = MockModelFileDownloader()
      let tempFileURL = tempFile()

      let fakeResponseExpired = HTTPURLResponse(url: fakeFileURL,
                                                statusCode: 400,
                                                httpVersion: nil,
                                                headerFields: nil)!

      let fileDownloaderExpectation1 = expectation(description: "File downloader before retry")
      let completionExpectation = expectation(description: "Completion handler")

      // Handles initial response.
      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation1.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }

      modelDownloader.downloadInfoAndModel(modelName: fakeModelName,
                                           modelInfoRetriever: modelInfoRetriever,
                                           downloader: downloader,
                                           conditions: conditions) { result in
        completionExpectation.fulfill()
        switch result {
        // Retry failed due to another expired url - error returned.
        case .success: XCTFail("Unexpected successful model download.")
        case let .failure(error):
          switch error {
          case let .internalError(description: errorMessage): XCTAssertEqual(errorMessage,
                                                                             "Unable to update expired model info.")
          default: XCTFail("Expected failure due to expired model info.")
          }
        }
      }
      wait(for: [fileDownloaderExpectation1], timeout: 0.5)

      let fileDownloaderExpectation2 = expectation(description: "File downloader after retry")
      // Handles response after one retry.
      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation2.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }
      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponseExpired,
                                                     fileURL: tempFileURL)))
      wait(for: [fileDownloaderExpectation2], timeout: 0.5)

      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponseExpired,
                                                     fileURL: tempFileURL)))
      wait(for: [completionExpectation], timeout: 0.5)

      try? ModelFileManager.removeFile(at: tempFileURL)
    }

    /// Get model if multiple retries all returned expired urls.
    func testGetModelMultipleFailedRetries() {
      guard let testApp = FirebaseApp.app() else {
        XCTFail("Default app was not configured.")
        return
      }
      testApp.isDataCollectionDefaultEnabled = false

      let modelDownloader = ModelDownloader.modelDownloader()
      modelDownloader.numberOfRetries = 2
      let conditions = ModelDownloadConditions()
      let session = fakeModelInfoSessionWithURL(fakeDownloadURL, statusCode: 200)
      let modelInfoRetriever = fakeModelRetriever(fakeSession: session)
      let downloader = MockModelFileDownloader()
      let tempFileURL = tempFile()

      let fakeResponseExpired = HTTPURLResponse(url: fakeFileURL,
                                                statusCode: 400,
                                                httpVersion: nil,
                                                headerFields: nil)!

      let fileDownloaderExpectation1 = expectation(description: "File downloader before retry")
      let completionExpectation = expectation(description: "Completion handler")

      // Handles initial response.
      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation1.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }

      modelDownloader.downloadInfoAndModel(modelName: fakeModelName,
                                           modelInfoRetriever: modelInfoRetriever,
                                           downloader: downloader,
                                           conditions: conditions) { result in
        completionExpectation.fulfill()
        switch result {
        // Two retries failed due to expired urls - error returned.
        case .success: XCTFail("Unexpected successful model download.")
        case let .failure(error):
          switch error {
          case let .internalError(description: errorMessage): XCTAssertEqual(errorMessage,
                                                                             "Unable to update expired model info.")
          default: XCTFail("Expected failure due to expired model info.")
          }
        }
      }
      wait(for: [fileDownloaderExpectation1], timeout: 0.5)

      let fileDownloaderExpectation2 = expectation(description: "file downloader after first retry")
      // Handles response after first retry.
      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation2.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }
      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponseExpired,
                                                     fileURL: tempFileURL)))
      wait(for: [fileDownloaderExpectation2], timeout: 0.5)

      let fileDownloaderExpectation3 =
        expectation(description: "File downloader after second retry")
      // Handles response after second retry.
      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation3.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }
      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponseExpired,
                                                     fileURL: tempFileURL)))
      wait(for: [fileDownloaderExpectation3], timeout: 0.5)

      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponseExpired,
                                                     fileURL: tempFileURL)))
      wait(for: [completionExpectation], timeout: 0.5)

      try? ModelFileManager.removeFile(at: tempFileURL)
    }

    /// Get model if a retry finally returns an unexpired download url.
    func testGetModelMultipleRetriesWithSuccess() {
      guard let testApp = FirebaseApp.app() else {
        XCTFail("Default app was not configured.")
        return
      }
      testApp.isDataCollectionDefaultEnabled = false

      let modelDownloader = ModelDownloader.modelDownloader()
      modelDownloader.numberOfRetries = 4
      let conditions = ModelDownloadConditions()
      let session = fakeModelInfoSessionWithURL(fakeDownloadURL, statusCode: 200)
      let modelInfoRetriever = fakeModelRetriever(fakeSession: session)
      let downloader = MockModelFileDownloader()
      let tempFileURL = tempFile()

      let fakeResponseExpired = HTTPURLResponse(url: fakeFileURL,
                                                statusCode: 400,
                                                httpVersion: nil,
                                                headerFields: nil)!

      let fakeResponseSucceeded = HTTPURLResponse(url: fakeFileURL,
                                                  statusCode: 200,
                                                  httpVersion: nil,
                                                  headerFields: nil)!

      let fileDownloaderExpectation1 = expectation(description: "File downloader before retry")
      let completionExpectation = expectation(description: "Completion handler")

      // Handles initial response.
      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation1.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }

      modelDownloader.downloadInfoAndModel(modelName: fakeModelName,
                                           modelInfoRetriever: modelInfoRetriever,
                                           downloader: downloader,
                                           conditions: conditions) { result in
        completionExpectation.fulfill()
        switch result {
        // One retry failed but the next retry succeeded - model returned.
        case let .success(model):
          self.checkModel(model: model)
          self.deleteFile(at: model.path)
        case let .failure(error):
          XCTFail("Error - \(error)")
        }
      }
      wait(for: [fileDownloaderExpectation1], timeout: 0.5)

      let fileDownloaderExpectation2 = expectation(description: "File downloader after first retry")
      // Handles response after first retry.
      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation2.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }
      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponseExpired,
                                                     fileURL: tempFileURL)))
      wait(for: [fileDownloaderExpectation2], timeout: 0.5)

      let fileDownloaderExpectation3 =
        expectation(description: "File downloader after second retry")
      // Handles response after second retry.
      downloader.downloadFileHandler = { url in
        fileDownloaderExpectation3.fulfill()
        XCTAssertEqual(url, self.fakeDownloadURL)
      }
      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponseExpired,
                                                     fileURL: tempFileURL)))
      wait(for: [fileDownloaderExpectation3], timeout: 0.5)

      downloader
        .completion?(.success(FileDownloaderResponse(urlResponse: fakeResponseSucceeded,
                                                     fileURL: tempFileURL)))
      wait(for: [completionExpectation], timeout: 0.5)

      try? ModelFileManager.removeFile(at: tempFileURL)
    }
  }

  /// Mock URL session for testing.
  class MockModelInfoRetrieverSession: ModelInfoRetrieverSession {
    var data: Data?
    var response: URLResponse?
    var error: Error?

    func getModelInfo(with request: URLRequest,
                      completion: @escaping (Data?, URLResponse?, Error?) -> Void) {
      completion(data, response, error)
    }
  }

  class MockModelFileDownloader: FileDownloader {
    var progressHandler: ProgressHandler?
    var completion: CompletionHandler?
    var downloadFileHandler: ((_ url: URL) -> Void)?

    func downloadFile(with url: URL, progressHandler: @escaping ProgressHandler,
                      completion: @escaping CompletionHandler) {
      self.progressHandler = progressHandler
      self.completion = completion
      downloadFileHandler?(url)
    }
  }

  extension GULUserDefaults {
    /// Returns a new instance of user defaults.
    static func createUnitTestInstance(testName: String) -> GULUserDefaults {
      let suiteName = "com.google.firebase.ml.test.\(testName)"
      let defaults = GULUserDefaults(suiteName: suiteName)
      return defaults
    }

    /// Returns the existing user defaults instance.
    static func getUnitTestInstance(testName: String) -> GULUserDefaults {
      let suiteName = "com.google.firebase.ml.test.\(testName)"
      return GULUserDefaults(suiteName: suiteName)
    }
  }

  // MARK: - Helpers

  extension ModelDownloaderUnitTests {
    func fakeModelInfoSessionWithURL(_ url: URL, statusCode: Int,
                                     customJSON: String? = nil) -> MockModelInfoRetrieverSession {
      let fakeSession = MockModelInfoRetrieverSession()
      let fakeJSON = customJSON ?? fakeModelJSON
      fakeSession.data = fakeJSON.data(using: .utf8)
      fakeSession.response = HTTPURLResponse(
        url: url,
        statusCode: statusCode,
        httpVersion: nil,
        headerFields: ["Etag": "fakeModelHash"]
      )
      fakeSession.error = nil
      return fakeSession
    }

    func fakeModelRetriever(fakeSession: MockModelInfoRetrieverSession,
                            fakeLocalModelInfo: LocalModelInfo? = nil) -> ModelInfoRetriever {
      // TODO: Replace with fake to check if it was used correctly by the download task.
      let modelInfoRetriever = ModelInfoRetriever(
        modelName: fakeModelName,
        projectID: fakeProjectID,
        apiKey: fakeAPIKey,
        appName: fakeAppName, authTokenProvider: successAuthTokenProvider,
        session: fakeSession, localModelInfo: fakeLocalModelInfo
      )
      return modelInfoRetriever
    }

    func checkModel(model: CustomModel) {
      XCTAssertEqual(model.name, fakeModelName)
      XCTAssertEqual(model.size, fakeModelSize)
      XCTAssertEqual(model.hash, fakeModelHash)
      let modelURL = URL(fileURLWithPath: model.path)
      XCTAssertTrue(ModelFileManager.isFileReachable(at: modelURL))
    }

    func tempFile(name: String = "fake-model-file.tmp") -> URL {
      let tempDirectoryURL = URL(fileURLWithPath: NSTemporaryDirectory(),
                                 isDirectory: true)
      let tempFileURL = tempDirectoryURL.appendingPathComponent(name)
      let tempData: Data = "fakeModelData".data(using: .utf8)!
      try? tempData.write(to: tempFileURL)
      return tempFileURL
    }

    func tempModelFile(invalid: Bool = false) -> URL {
      let modelDirectoryURL = ModelFileManager.modelsDirectory!
      let modelFileName = invalid ? "fbml_model_fake-model-file.tmp" :
        "fbml_model@@__FIRAPP_DEFAULT@@\(fakeModelName)"
      let tempFileURL = modelDirectoryURL.appendingPathComponent(modelFileName)
        .appendingPathExtension("tflite")
      let tempData: Data = "fakeModelData".data(using: .utf8)!
      try? tempData.write(to: tempFileURL)
      return tempFileURL
    }

    func deleteFile(at path: String) {
      let url = URL(fileURLWithPath: path)
      do {
        try ModelFileManager.removeFile(at: url)
      } catch {
        XCTFail("Failed to delete file at \(path).")
      }
    }

    func fakeLocalModelInfo(mismatch: Bool = false) -> LocalModelInfo {
      let hash = mismatch ? "wrongModelHash" : fakeModelHash
      return LocalModelInfo(
        name: fakeModelName,
        modelHash: hash,
        size: 20
      )
    }

    func fakeModelInfo(expired: Bool, oversized: Bool) -> RemoteModelInfo {
      let expiryTime = expired ? Date() : Date().addingTimeInterval(1000)
      let size = oversized ? Int(Int64.max) / 4 : fakeModelSize
      return RemoteModelInfo(name: fakeModelName,
                             downloadURL: fakeDownloadURL,
                             modelHash: fakeModelHash,
                             size: size,
                             urlExpiryTime: expiryTime)
    }
  }

#endif // !targetEnvironment(macCatalyst) && !os(macOS)