[swift] Implements union strings (#6589)

swift/Sources/FlatBuffers/String+extension.swift

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2021 Google Inc. All rights reserved.
+ *
+ * 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
+
+extension String: FlatbuffersInitializable {
+
+  /// Initailizes a string from a Flatbuffers ByteBuffer
+  /// - Parameters:
+  ///   - bb: ByteBuffer containing the readable string
+  ///   - o: Current position
+  public init(_ bb: ByteBuffer, o: Int32) {
+    let count = bb.read(def: Int32.self, position: Int(o))
+    self = bb.readString(
+      at: Int32(MemoryLayout<Int32>.size) + o,
+      count: count) ?? ""
+  }
+}
+
+extension String: ObjectAPIPacker {
+
+  public static func pack(_ builder: inout FlatBufferBuilder, obj: inout String?) -> Offset {
+    guard var obj = obj else { return Offset() }
+    return pack(&builder, obj: &obj)
+  }
+
+  public static func pack(_ builder: inout FlatBufferBuilder, obj: inout String) -> Offset {
+    builder.create(string: obj)
+  }
+
+  public mutating func unpack() -> String {
+    self
+  }
+
+}
+
+extension String: NativeObject {
+
+  public func serialize<T: ObjectAPIPacker>(type: T.Type) -> ByteBuffer where T.T == Self {
+    fatalError("serialize should never be called from string directly")
+  }
+
+  public func serialize<T: ObjectAPIPacker>(builder: inout FlatBufferBuilder, type: T.Type) -> ByteBuffer where T.T == Self {
+    fatalError("serialize should never be called from string directly")
+  }
+}