Custom decoder configuration in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Correct syntax for custom decoder init

Which option correctly implements a custom initializer for decoding a Swift struct with a nested container?

iOS Swift

struct User: Decodable {
  let name: String
  let age: Int

  enum CodingKeys: String, CodingKey {
    case info
  }

  enum InfoKeys: String, CodingKey {
    case name
    case age
  }

  init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    let infoContainer = try container.nestedContainer(keyedBy: InfoKeys.self, forKey: .info)
    name = try infoContainer.decode(String.self, forKey: .name)
    age = try infoContainer.decode(Int.self, forKey: .age)
  }
}

A

init(from decoder: Decoder) throws {
  let container = try decoder.unkeyedContainer()
  name = try container.decode(String.self)
  age = try container.decode(Int.self)
}

B

init(from decoder: Decoder) throws {
  let container = try decoder.container(keyedBy: InfoKeys.self)
  name = try container.decode(String.self, forKey: .name)
  age = try container.decode(Int.self, forKey: .age)
}

C

init(from decoder: Decoder) throws {
  let container = try decoder.container(keyedBy: CodingKeys.self)
  name = try container.decode(String.self, forKey: .name)
  age = try container.decode(Int.self, forKey: .age)
}

D

init(from decoder: Decoder) throws {
  let container = try decoder.container(keyedBy: CodingKeys.self)
  let infoContainer = try container.nestedContainer(keyedBy: InfoKeys.self, forKey: .info)
  name = try infoContainer.decode(String.self, forKey: .name)
  age = try infoContainer.decode(Int.self, forKey: .age)
}

💡 Hint

Remember to access the nested container using the correct key and key type.

Option D correctly accesses the nested container 'info' using CodingKeys, then decodes 'name' and 'age' from InfoKeys. Other options either use wrong container types or keys.

❓ ui_behavior

intermediate

Effect of missing key in custom decoder

What happens if a required key is missing when decoding with a custom decoder initializer in Swift?

iOS Swift

struct Product: Decodable {
  let id: Int
  let name: String

  enum CodingKeys: String, CodingKey {
    case id
    case name
  }

  init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    id = try container.decode(Int.self, forKey: .id)
    name = try container.decode(String.self, forKey: .name)
  }
}

// JSON missing 'name' key
let jsonData = "{\"id\": 101}".data(using: .utf8)!

A The decoding succeeds and 'name' is set to an empty string.

B The decoding succeeds and 'name' is set to nil.

C The decoding throws a DecodingError.keyNotFound error at runtime.

D The decoding silently ignores the missing key and sets default values.

💡 Hint

Think about how Swift Decodable handles missing required keys by default.

When a required key is missing, decoding throws a DecodingError.keyNotFound error. It does not assign default or nil values automatically.

❓ lifecycle

advanced

Custom decoder with date decoding strategy

How can you configure a JSONDecoder to decode dates in a custom format when decoding a struct with a custom decoder initializer?

iOS Swift

struct Event: Decodable {
  let title: String
  let date: Date

  enum CodingKeys: String, CodingKey {
    case title
    case date
  }

  init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    title = try container.decode(String.self, forKey: .title)
    date = try container.decode(Date.self, forKey: .date)
  }
}

let decoder = JSONDecoder()
// Configure decoder here for custom date format

A

decoder.dateDecodingStrategy = .custom { decoder in
  let container = try decoder.singleValueContainer()
  let dateString = try container.decode(String.self)
  let formatter = DateFormatter()
  formatter.dateFormat = "yyyy-MM-dd HH:mm:ss"
  guard let date = formatter.date(from: dateString) else {
    throw DecodingError.dataCorruptedError(in: container, debugDescription: "Invalid date format")
  }
  return date
}

B decoder.dateDecodingStrategy = .formatted(DateFormatter())

C decoder.dateDecodingStrategy = .secondsSince1970

D decoder.dateDecodingStrategy = .iso8601

💡 Hint

Use a custom closure to parse the date string with your own format.

Option A shows how to use a custom dateDecodingStrategy with a closure that parses the date string using a DateFormatter with a specific format. Other options use predefined strategies or an empty formatter.

❓ navigation

advanced

Decoding nested JSON with optional nested container

Given a JSON where a nested object may be missing, how do you safely decode an optional nested container in a custom decoder initializer?

iOS Swift

struct Profile: Decodable {
  let username: String
  let address: Address?

  struct Address: Decodable {
    let city: String
    let zip: String
  }

  enum CodingKeys: String, CodingKey {
    case username
    case address
  }

  init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    username = try container.decode(String.self, forKey: .username)
    if container.contains(.address) {
      let addressContainer = try container.nestedContainer(keyedBy: AddressCodingKeys.self, forKey: .address)
      let city = try addressContainer.decode(String.self, forKey: .city)
      let zip = try addressContainer.decode(String.self, forKey: .zip)
      address = Address(city: city, zip: zip)
    } else {
      address = nil
    }
  }

  enum AddressCodingKeys: String, CodingKey {
    case city
    case zip
  }
}

A Decode address as optional directly with decodeIfPresent(Address.self, forKey: .address).

B Use try? container.nestedContainer(keyedBy: AddressCodingKeys.self, forKey: .address) and unwrap optional.

C Force unwrap nested container with try container.nestedContainer(keyedBy: AddressCodingKeys.self, forKey: .address) without checking.

D Use container.contains(.address) to check before decoding nested container.

💡 Hint

Swift Decodable supports decoding optional nested objects directly.

Option A is the simplest and safest way to decode an optional nested object by using decodeIfPresent. Options A and B work but are more verbose. Option A risks runtime error if key is missing.

🔧 Debug

expert

Identify the cause of decoding failure in custom decoder

Why does the following custom decoder initializer fail to decode the JSON correctly?

iOS Swift

struct Item: Decodable {
  let id: Int
  let tags: [String]

  enum CodingKeys: String, CodingKey {
    case id
    case tags
  }

  init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    id = try container.decode(Int.self, forKey: .id)
    let tagsContainer = try container.nestedUnkeyedContainer(forKey: .tags)
    tags = try tagsContainer.decode([String].self)
  }
}

// JSON example: {"id": 5, "tags": ["swift", "ios"]}

A nestedUnkeyedContainer returns an UnkeyedDecodingContainer which cannot decode an array directly.

B decode([String].self) is invalid inside a nestedUnkeyedContainer; should decode elements one by one.

C The code should use decodeIfPresent instead of decode for tags.

D The JSON format is invalid for decoding tags as an array.

💡 Hint

Unkeyed containers decode elements one at a time, not whole arrays.

nestedUnkeyedContainer returns a container to decode elements sequentially. You cannot decode an entire array from it directly. You must decode elements one by one or decode the array directly from the main container.