Viết Native Module iOS (Swift) — hello world step-by-step?

#import <Foundation/Foundation.h>
#import "RNCalculatorSpec.h"  // generated by Codegen

NS_ASSUME_NONNULL_BEGIN
@interface RNCalculator : NSObject <NativeCalculatorSpec>
@end
NS_ASSUME_NONNULL_END

import Foundation

@objc(RNCalculator)
class RNCalculator: NSObject, NativeCalculatorSpec {
  func add(_ a: NSNumber, b: NSNumber) -> NSNumber {
    return NSNumber(value: a.doubleValue + b.doubleValue)
  }

  func fetchUser(_ id: String, resolve: @escaping RCTPromiseResolveBlock,
                 reject: @escaping RCTPromiseRejectBlock) {
    resolve(["name": "Alice"])
  }

  static func moduleName() -> String { "Calculator" }
}

#import <React/RCTBridgeModule.h>

#import "RNCalculator.h"
#import "MyApp-Swift.h"

@implementation RNCalculator
RCT_EXPORT_MODULE(Calculator)
@end

Pod::Spec.new do |s|
  s.name = 'Calculator'
  s.source_files = "Calculator/**/*.{swift,h,m,mm}"
  s.dependency 'React-Core'
  install_modules_dependencies(s)
end