Codable protocol for JSON parsing in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Correct Codable struct syntax

Which option correctly defines a Swift struct that conforms to Codable for parsing JSON with keys name (String) and age (Int)?

A

class Person: Codable {
  var name: String
  var age: Int
}

B

struct Person {
  var name: String
  var age: Int
  func encode() {}
}

C

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D

struct Person: Codable {
  var name: String
  var age: Int
}

💡 Hint

Remember, Codable works with structs and classes, but types must match JSON keys exactly.

Option D correctly defines a struct with matching property types and conforms to Codable. Option D lacks Codable conformance. Option D uses a class which is valid but less common for simple models. Option D has wrong type for age (String instead of Int).

❓ ui_behavior

intermediate

Decoding JSON with Codable

Given this JSON string: {"name":"Alice","age":30}, which option correctly decodes it into a Person struct using JSONDecoder?

iOS Swift

struct Person: Codable {
  var name: String
  var age: Int
}

let jsonString = "{\"name\":\"Alice\",\"age\":30}"

A

let data = jsonString.data(using: .utf8)!
let person = try JSONDecoder().decode(Person.self, from: data)

B let person = try JSONDecoder().decode(Person.self, from: jsonString)

C

let data = jsonString.data(using: .ascii)!
let person = try JSONDecoder().decode(Person.self, from: data)

D

let data = jsonString.data(using: .utf8)!
let person = JSONDecoder().decode(Person.self, from: data)

💡 Hint

You must convert the JSON string to Data using UTF-8 encoding before decoding.

Option A correctly converts the string to Data with UTF-8 and uses try to decode. Option A tries to decode from String directly, which is invalid. Option A uses ASCII encoding which may fail for some characters. Option A misses try keyword causing compile error.

❓ lifecycle

advanced

Handling missing JSON keys during decoding

If a JSON response sometimes omits the age key, how should you define the Person struct to decode successfully without errors?

A

struct Person: Codable {
  var name: String
  var age: Int = 0
}

B

struct Person: Codable {
  var name: String
  var age: String?
}

C

struct Person: Codable {
  var name: String
  var age: Int?
}

D

struct Person: Codable {
  var name: String
  var age: Int
}

💡 Hint

Make the property optional to handle missing keys gracefully.

Option C uses an optional Int for age, so decoding succeeds even if age is missing. Option C requires age always present. Option C sets a default but decoding will fail if key is missing unless you implement custom init. Option C uses wrong type for age.

❓ navigation

advanced

Custom key mapping with Codable

Given JSON {"full_name":"Bob Smith","age":25}, how do you map full_name to a Swift property name in a Codable struct?

A

struct Person: Codable {
  var full_name: String
  var age: Int
}

B

struct Person: Codable {
  var name: String
  var age: Int
  enum CodingKeys: String, CodingKey {
    case name = "full_name"
    case age
  }
}

C

struct Person: Codable {
  var name: String
  var age: Int
  init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    name = try container.decode(String.self, forKey: .full_name)
    age = try container.decode(Int.self, forKey: .age)
  }
  enum CodingKeys: String, CodingKey {
    case full_name
    case age
  }
}

D

struct Person: Codable {
  var name: String
  var age: Int
  enum CodingKeys: String, CodingKey {
    case name
    case age
  }
}

💡 Hint

Use CodingKeys enum to map JSON keys to different property names.

Option B correctly maps JSON key "full_name" to property "name" using CodingKeys. Option B uses property name matching JSON key but does not rename. Option B tries manual decoding but uses wrong key in CodingKeys for name property. Option B does not map keys so decoding fails.

🔧 Debug

expert

Identify decoding error cause

This code throws a decoding error. What is the cause? struct Person: Codable { var name: String var age: Int } let json = "{\"name\":\"Eve\"}" // age key missing let data = json.data(using: .utf8)! let person = try JSONDecoder().decode(Person.self, from: data)

iOS Swift

struct Person: Codable {
  var name: String
  var age: Int
}

let json = "{\"name\":\"Eve\"}"
let data = json.data(using: .utf8)!
let person = try JSONDecoder().decode(Person.self, from: data)

A The JSON is missing the required 'age' key, causing a decoding failure.

B The JSON string is not valid UTF-8 encoded data.

C The 'name' property is not optional but missing in JSON.

D The struct must be a class to decode properly.

💡 Hint

Check if all non-optional properties have matching keys in JSON.

The decoding fails because 'age' is a non-optional Int property but the JSON does not include it. This mismatch causes a decoding error. The JSON is valid UTF-8 and contains 'name'. The struct can be a struct or class.