Encoding, Decoding and Serialization in Swift 4

Learn how to encode, decode and serialize data in Swift 4!

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  • Swift 4, iOS 11, Xcode 9

There are several scenarios where you’ll need to save data to a file or to send it over the network. In this tutorial, you’ll learn how to achieve these tasks by converting your instances to another representation, like a string or a stream of bytes. This process is known as encoding, also known as serialization.

The reverse process of turning the data into an instance is called decoding, or deserialization.

Imagine you have an instance you want to write to a file. The instance itself cannot be written as-is to the file, so you need to encode it into another representation, like as a stream of bytes:

Once the data is encoded and saved to a file, you can turn it back into an instance whenever you want by using a decoder:

Encodable and Decodable Protocols

The Encodable protocol is used by types that can be encoded to another representation. It declares a single method:

func encode(to: Encoder) throws

…which the compiler generates for you if all the stored properties of that type conform to Encodable as well. You’ll learn more about this later on in the tutorial.

The Decodable protocol is used by types that can be decoded. It declares just a single initializer:

init(from decoder: Decoder) throws

You will know when and how to implement these methods by the end of this tutorial.

What is Codable?

Codable is a protocol that a type can conform to, to declare that it can be encoded and decoded. It’s basically an alias for the Encodable and Decodable protocols.

typealias Codable = Encodable & Decodable

Automatic Encoding and Decoding

There are many types in Swift which are codable out of the box: Int, String, Date, Array and many other types from the Standard Library and the Foundation framework. If you want your type to be codable, the simplest way to do it is by conforming to Codable and making sure all its stored properties are also codable.

For example, let’s say you own a toy factory and you have this struct to store employee data:

struct Employee {
    var name: String
    var id: Int

All you need to do to be able to encode and decode this type is to conform to the Codable protocol, like so:

struct Employee: Codable {
    var name: String
    var id: Int

Wow, that was easy! You were able to do it because both name (String) and id (Int) are codable.

This works well when you’re only using types that are already Codable. But what if your type includes other custom types as properties? For example, looking at your Employee struct, assume that it also has a favoriteToy property:

struct Employee: Codable {
    var name: String
    var id: Int
    var favoriteToy: Toy

struct Toy: Codable {
    var name: String

By making sure Toy also conforms to Codable, you maintain the overall conformance to Codable for Employee as well.

All collections types, like Array and Dictionary are also codable if they contain codable types.

Encoding and Decoding Custom Types

There are several representations you can encode to or decode from, such as XML or a Property List. In this section, you’ll learn how to encode to and decode from JSON, by using Swift’s JSONEncoder and JSONDecoder classes.

JSON stands for JavaScript Object Notation, and is one of the most popular ways to serialize data. It’s easily readable by humans and easy for computers to parse and generate.

For example, if you were to encode an instance of type Employee to JSON, it might look something like this:

{ "name": "John Appleseed", "id": 7 }

You can easily understand how the Employee instance looked like before it was serialized into JSON.

JSONEncoder and JSONDecoder

Once you have a codable type, you can use JSONEncoder to convert your type to Data which can be either written to a file or sent over the network. Assume you have this employee instance:

let toy1 = Toy(name: "Teddy Bear");
let employee1 = Employee(name: "John Appleseed", id: 7, favoriteToy: toy1)

John’s birthday is coming up and you want to give him his favorite toy as a gift. You need to send over this data to the Gifts department. Before you can do that, you need to encode it like this:

let jsonEncoder = JSONEncoder()
let jsonData = try jsonEncoder.encode(employee1)

You’ll notice that you need to use try because encode(_:) might fail and throw an error.

If you try to print jsonData like this:


You’ll see that Xcode omits the data and only provides the number of bytes in jsonData. This is fine, because jsonData contains an unreadable representation of employee1. If you would like to create a readable version of this JSON as a string, you can use String’s initializer:

let jsonString = String(data: jsonData, encoding: .utf8)
// {"name":"John Appleseed","id":7,"favoriteToy":{"name":"Teddy Bear"}}

Now you can send jsonData or jsonString over to the gifts department using their special gift API.

If you want to decode the JSON data back into an instance, you need to use JSONDecoder:

let jsonDecoder = JSONDecoder()
let employee2 = try jsonDecoder.decode(Employee.self, from: jsonData)

Note that you need to tell the decoder what type to decode the JSON to, because the compiler can’t figure this out on its own.

Renaming Properties With CodingKeys

It turns out that the gifts department API requires that the employee ID appear as employeeId instead of id. Luckily, Swift provides a solution to this kind of problem.

CodingKey Protocol, CodingKeys Enum

The CodingKeys enum, which conforms to CodingKey protocol, lets you rename specific properties in case the serialized format doesn’t match the requirements of the API.

Add the nested enumeration CodingKeys like this:

struct Employee: Codable {
  var name: String
  var id: Int
  var favoriteToy: Toy

  enum CodingKeys: String, CodingKey {
    case id = "employeeId"
    case name
    case favoriteToy

There are several things to note here:

  1. CodingKeys is a nested enumeration in your type.
  2. It has to conform to CodingKey.
  3. You also need String as the raw type, since the keys are always strings.
  4. You have to include all properties in the enumeration, even if you don’t plan to rename them.
  5. By default, this enumeration is created by the compiler, but when you need to rename a key you need to implement it yourself.

Now if you print the JSON, you’ll see that the stored property id key has changed to employeeId:

{ "employeeId": 7, "name": "John Appleseed", "favoriteToy": {"name":"Teddy Bear"}}

Manual Encoding and Decoding

You try to send the data over to the gifts department, and again the data gets rejected. This time they claim that the information of the gift you want to send to the employee should not be inside a nested type, but rather as a property called gift. So the JSON should actually look like this:

{ "employeeId": 7, "name": "John Appleseed", "gift": "Teddy Bear" }

In this case you can’t use CodingKeys, since you need to alter the structure of the JSON and not just rename properties. You need to write your own encoding and decoding logic.

The encode Function

As mentioned earlier in the tutorial, Codable is actually just a typealias for the Encodable and Decodable protocols. You need to implement encode(to: Encoder) and describe how to encode each property.

It might sound complicated, but it’s pretty simple. First, update CodingKeys to use the key gift instead of favoriteToy:

enum CodingKeys: String, CodingKey {
  case id = "employeeId"
  case name
  case gift

Then, you need to remove Employee’s conformance to Codable and then add this extension:

extension Employee: Encodable {
  func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    try container.encode(name, forKey: .name)
    try container.encode(id, forKey: .id)
    try container.encode(favoriteToy.name, forKey: .gift)

First you get back the container of the encoder. This is a view into the storage of the encoder that you can access with keys. Note how you choose which properties to encode for which keys. Importantly, you flatten favoriteToy.name down to the .gift key.

If you stop now, you should get an error that says:

'Employee' does not conform to expected type 'Decodable'

This is because you removed the conformance to Codable and only added conformance to Encodable. For now you can comment out the code that decodes jsonString to employee2.

If you print jsonString once more, this is what you’ll get:

{"name":"John Appleseed","gift":"Teddy Bear","employeeId":7}


The decode Function

Once the data arrives at the gifts department, they need to be able to convert this JSON back to an instance in their system. For this they’ll need a decoder.

Add the following code to your playground to make Employee conform to Decodable (and thus also Codable):

extension Employee: Decodable {
  init(from decoder: Decoder) throws {
    let values = try decoder.container(keyedBy: CodingKeys.self)
    name = try values.decode(String.self, forKey: .name)
    id = try values.decode(Int.self, forKey: .id)
    let gift = try values.decode(String.self, forKey: .gift)
    favoriteToy = Toy(name: gift)

Here you’re pretty much doing the opposite of what you did in the encode method using the decoder’s keyed storage container.

Key Points

  • You need to encode (or serialize) an instance before you can save it to a file or send it over the web.
  • Your type should conform to the Codable protocol to make it codable. If all properties are codable, then the type is automatically codable as well.
  • JSON is the most common encoding in modern applications and web services, and you can use JSONEncoder and JSONDecoder to encode and decode your types to and from JSON.

Where to Go From Here?

You can download the final package from this tutorial here.

If you enjoyed what you learned in this tutorial, why not check out the complete Swift Apprentice book, available on our store?

Here’s a taste of what’s in the book:

  • Section I: Swift Basics: The first section of the book starts at the very beginning of the computing environment: first, how computers work, and then, how Swift’s playgrounds feature works. With those logistics out of the way, you’ll take a tour of the fundamentals of the Swift language and learn the basics of managing data, structuring your code, performing simple operations and calculations, working with types.
  • Section II: Collection Types: Stored data is a core component of any app, whether it’s a list of friends in your social networking app or a set of unlockable characters in your hit game. In this section, you’ll learn how to store collections of data in Swift.
  • Section III: Building Your Own Types: Swift comes with basic building blocks, but its real power is in the custom things you can build to model parts of your app. Swift has no idea about playable characters and monsters and power-ups, for example — these are things you need to build yourself! You’ll learn how to do that in this section.
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By the end of this book, you’ll have some great hands-on experience with all of the best features of the Swift 4 programming language.

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