Custom encoding strategy to swap CodingKeys during encoding

AnyKey.swift

struct AnyKey: CodingKey {
  var intValue: Int?
  var stringValue: String
  
  init(stringValue: String) {
    self.stringValue = stringValue
    self.intValue = nil
  }
  
  init?(intValue: Int) {
    self.stringValue = String(intValue)
    self.intValue = intValue
  }
}

let myCustomEncoder = JSONEncoder()
myCustomEncoder.keyEncodingStrategy = .custom { keys in 
    if keys.last!.stringValue == "foo" {
      return AnyKey(stringValue: "bar"
    } else {
        return keys.last!
    }                                         
}