Support multiple Codable data format versions and easily migrate even the oldest saved data to the latest version

CodableVersionsTests.swift

//
//  How to make Codable versioning tolerable
//
//  Watch the video: https://youtu.be/BOiFrlii13
// 
//  Created by Andrew Eades on 25/05/2021.
//
//  Run as a test in XCode

import XCTest

typealias Person = Person_V3

struct Person_V1: Codable, Equatable {
  var name = ""
  
  init(name: String) {
    self.name = name
  }
}

struct Person_V2: Codable, Equatable {
  var firstName = ""
  var lastName = ""
  
  init(firstName: String, lastName: String) {
    self.firstName = firstName
    self.lastName = lastName
  }
  
  init(previousVersion: Person_V1) {
    let names = previousVersion.name.components(separatedBy: .whitespaces)
    self.init(firstName: names.first!, lastName: names.last!)
  }
}

struct Person_V3: Codable, Equatable {
  var firstName = ""
  var lastName = ""
  var age: Int?
  
  init(firstName: String, lastName: String, age: Int?) {
    self.firstName = firstName
    self.lastName = lastName
    self.age = age
  }
  
  init(previousVersion: Person_V2) {
    self.init(firstName: previousVersion.firstName, lastName: previousVersion.lastName, age: nil)
  }
}

struct VersionSerialiser {
  var v1_0_0: Person_V1? = nil
  var v2_0_0: Person_V2? = nil
  var v3_0_0: Person_V3? = nil
}

extension VersionSerialiser: Codable {

  init(from decoder: Decoder) {
    do {
      let container = try decoder.container(keyedBy: CodingKeys.self)
      if let person = try? container.decode(Person_V3.self, forKey: .v3_0_0) {
        self.v3_0_0 = person
      } else if let previousVersion = try? container.decode(Person_V2.self, forKey: .v2_0_0)  {
        self.v3_0_0 = Person_V3(previousVersion: previousVersion)
      } else if let previousVersion = try? container.decode(Person_V1.self, forKey: .v1_0_0) {
        self.v3_0_0 = Person_V3(previousVersion: Person_V2(previousVersion: previousVersion))
      } else {
        XCTFail("Unknown version")
      }
    } catch {
      XCTFail("Unknown version")
    }
  }
}

class Tests_macOS: XCTestCase {
  var personV1: VersionSerialiser?
  var dataV1: Data?
  var jsonV1: String?
  
  override func setUp() {
    personV1 = VersionSerialiser(v1_0_0: Person_V1(name: "Andrew Eades"))
    dataV1 = try! JSONEncoder().encode(personV1)
    jsonV1 = String(data: dataV1!, encoding: .utf8)
  }
  
  func testMigration() {
    let versions = try! JSONDecoder().decode(VersionSerialiser.self, from: dataV1!)
    let person = versions.v3_0_0!
    XCTAssertEqual(person, Person_V3(firstName: "Andrew", lastName: "Eades", age: nil))
  }
  
  func testEncoding() {
    let v1 = try! JSONEncoder().encode(
      Person_V1(name: "Andrew Eades")
    )
    
    XCTAssertEqual(String(data: v1, encoding: .utf8)!,
      #"{"name":"Andrew Eades"}"#)
    
    let v2 = try! JSONEncoder().encode(
      Person_V2(firstName: "Andrew", lastName: "Eades"))
    
    XCTAssertEqual(String(data: v2, encoding: .utf8)!,
      #"{"firstName":"Andrew","lastName":"Eades"}"#)
    
  }
}

The video is here.