Revise Swift API style

PiperOrigin-RevId: 308739030
Change-Id: I63afcf15bc7f1d7f0a1071eef6ffe082f6433591

tensorflow/lite/experimental/swift/Sources/CoreMLDelegate.swift

@@ -22,9 +22,11 @@ public final class CoreMLDelegate: Delegate {
   public let options: Options
 
   // Conformance to the `Delegate` protocol.
-  public private(set) var cDelegate: CDelegate?
+  public private(set) var cDelegate: CDelegate
 
-  /// Creates a new instance configured with the given `options`.
+  /// Creates a new instance configured with the given `options`. Returns `nil` if the underlying
+  /// Core ML delegate could not be created because `Options.enabledDevices` was set to
+  /// `neuralEngine` but the device does not have the Neural Engine.
   ///
   /// - Parameters:
   ///   - options: Configurations for the delegate. The default is a new instance of
@@ -33,12 +35,10 @@ public final class CoreMLDelegate: Delegate {
     self.options = options
     var delegateOptions = TfLiteCoreMlDelegateOptions()
     delegateOptions.enabled_devices = options.enabledDevices.cEnabledDevices
-    delegateOptions.max_delegated_partitions = options.maxDelegatedPartitions
-    delegateOptions.min_nodes_per_partition = options.minNodesPerPartition
-    cDelegate = TfLiteCoreMlDelegateCreate(&delegateOptions)
-    if cDelegate == nil {
-      return nil
-    }
+    delegateOptions.max_delegated_partitions = Int32(options.maxDelegatedPartitions)
+    delegateOptions.min_nodes_per_partition = Int32(options.minNodesPerPartition)
+    guard let delegate = TfLiteCoreMlDelegateCreate(&delegateOptions) else { return nil }
+    cDelegate = delegate
   }
 
   deinit {
@@ -46,42 +46,41 @@ public final class CoreMLDelegate: Delegate {
   }
 }
 
-
 extension CoreMLDelegate {
+  /// A type indicating which devices the Core ML delegate should be enabled for.
+  public enum EnabledDevices: Equatable, Hashable {
+    /// Enables the delegate for devices with Neural Engine only.
+    case neuralEngine
+    /// Enables the delegate for all devices.
+    case all
+
+    /// The C `TfLiteCoreMlDelegateEnabledDevices` for the current `EnabledDevices`.
+    var cEnabledDevices: TfLiteCoreMlDelegateEnabledDevices {
+      switch self {
+      case .neuralEngine:
+        return TfLiteCoreMlDelegateDevicesWithNeuralEngine
+      case .all:
+        return TfLiteCoreMlDelegateAllDevices
+      }
+    }
+  }
+
   /// Options for configuring the `CoreMLDelegate`.
   // TODO(b/143931022): Add preferred device support.
   public struct Options: Equatable, Hashable {
-    /// A type determines Core ML delegate initialization on devices without Neural Engine. The
-    /// default is .devicesWithNeuralEngine, where the delegate will not be created for
-    /// devices that does not have Neural Engine.
-    public var enabledDevices: CoreMLDelegateEnabledDevices = .devicesWithNeuralEngine
-    /// Maximum number of Core ML delegates created.  Each graph corresponds to one delegated node
-    /// subset in the TFLite model. Set this to 0 to delegate all possible partitions.
-    public var maxDelegatedPartitions: Int32 = 0;
-
-    // Minimum number of nodes per partition delegated with
-    // Core ML delegate. Defaults to 2.
-    public var  minNodesPerPartition: Int32 = 2;
+    /// A type indicating which devices the Core ML delegate should be enabled for. The default
+    /// value is `.neuralEngine` indicating that the delegate is enabled for Neural Engine devices
+    /// only.
+    public var enabledDevices: EnabledDevices = .neuralEngine
+    /// The maximum number of Core ML delegate partitions created. Each graph corresponds to one
+    /// delegated node subset in the TFLite model. The default value is `0` indicating that all
+    /// possible partitions are delegated.
+    public var maxDelegatedPartitions = 0
+    /// The minimum number of nodes per partition to be delegated by the Core ML delegate. The
+    /// default value is `2`.
+    public var minNodesPerPartition = 2
 
     /// Creates a new instance with the default values.
     public init() {}
   }
 }
-
-/// A type determines Core ML delegate initialization on devices without Neural Engine.
-public enum CoreMLDelegateEnabledDevices: Equatable, Hashable {
-  /// Creates the delegate only for devices with Neural Engine.
-  case devicesWithNeuralEngine
-  /// Creates the delegate even when Neural Engine is not available.
-  case allDevices
-
-  /// The C `TfLiteCoreMlDelegateEnabledDevices` for the current `CoreMLDelegateEnabledDevices`.
-  var cEnabledDevices: TfLiteCoreMlDelegateEnabledDevices {
-    switch self {
-    case .devicesWithNeuralEngine:
-      return TfLiteCoreMlDelegateDevicesWithNeuralEngine
-    case .allDevices:
-      return TfLiteCoreMlDelegateAllDevices
-    }
-  }
-}

tensorflow/lite/experimental/swift/Sources/Delegate.swift

@@ -20,5 +20,5 @@ public protocol Delegate: class {
   typealias CDelegate = UnsafeMutablePointer<TfLiteDelegate>
 
   /// The delegate that performs model computations.
-  var cDelegate: CDelegate? { get }
+  var cDelegate: CDelegate { get }
 }

tensorflow/lite/experimental/swift/Sources/MetalDelegate.swift

@@ -23,7 +23,7 @@ public final class MetalDelegate: Delegate {
   public let options: Options
 
   // Conformance to the `Delegate` protocol.
-  public private(set) var cDelegate: CDelegate?
+  public private(set) var cDelegate: CDelegate
 
   /// Creates a new instance configured with the given `options`.
   ///