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.

Practice

1. What does the Codable protocol in Swift help you do?

easy

A. Convert Swift objects to and from JSON easily

B. Create user interface elements

C. Manage app navigation

D. Handle network connections

Solution

1. Step 1: Understand the purpose of Codable

   The Codable protocol is designed to simplify converting data between Swift objects and external formats like JSON.

2. Step 2: Identify the correct use case

   It is not related to UI, navigation, or networking directly, but specifically for encoding and decoding data.

3. Final Answer:

   Convert Swift objects to and from JSON easily -> Option A

4. Quick Check:

   Codable = Convert JSON [OK]

Hint: Codable means easy JSON conversion [OK]

Common Mistakes:

• Confusing Codable with UI or navigation tasks
• Thinking Codable manages network calls
• Assuming Codable is for database storage

2. Which of the following is the correct way to declare a Swift struct that can be encoded and decoded using Codable?

easy

A. struct User { var name: String }

B. struct User: Decodable { var name: String }

C. class User: Codable { var name: String }

D. struct User: Codable { var name: String }

Solution

1. Step 1: Check the struct declaration

   To use Codable, the struct must conform to the Codable protocol explicitly.

2. Step 2: Verify the syntax

   struct User: Codable { var name: String } correctly declares a struct conforming to Codable. struct User { var name: String } lacks Codable conformance. class User: Codable { var name: String } uses a class, which is valid but not asked here. struct User: Decodable { var name: String } only conforms to Decodable, missing Encodable.

3. Final Answer:

   struct User: Codable { var name: String } -> Option D

4. Quick Check:

   Struct + Codable = struct User: Codable { var name: String } [OK]

Hint: Use struct with Codable for JSON parsing [OK]

Common Mistakes:

• Forgetting to add Codable conformance
• Using class instead of struct when asked
• Using only Decodable without Encodable

3. Given this JSON string:

{"id": 10, "title": "Book"}

And this Swift struct:

struct Item: Codable { var id: Int; var title: String }

What will be the value of item.title after decoding?

medium

A. "id"

B. "Book"

C. 10

D. nil

Solution

1. Step 1: Match JSON keys to struct properties

   The JSON has keys "id" and "title" matching the struct's properties.

2. Step 2: Decode JSON to struct

   Decoding will assign the value "Book" to title and 10 to id.

3. Final Answer:

   "Book" -> Option B

4. Quick Check:

   Decoded title = "Book" [OK]

Hint: JSON keys match struct properties exactly [OK]

Common Mistakes:

• Confusing property names with values
• Expecting numeric value for title
• Assuming decoding fails and returns nil

4. What is wrong with this Swift code for decoding JSON?

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

medium

A. The JSONDecoder call is missing try/catch

B. The struct does not conform to Codable

C. The JSON string is not valid

D. The JSON data is nil

Solution

1. Step 1: Check JSON string validity

   The JSON string is valid and properly escaped.

2. Step 2: Verify Codable conformance

   The struct Person conforms to Codable correctly.

3. Step 3: Analyze JSONDecoder usage

   The decode call uses try but lacks a do-catch block or try? to handle errors, which is required.

4. Final Answer:

   The JSONDecoder call is missing try/catch -> Option A

5. Quick Check:

   Missing error handling for try [OK]

Hint: Always handle errors with try when decoding JSON [OK]

Common Mistakes:

• Ignoring error handling with try
• Assuming JSON string is invalid
• Forgetting Codable conformance

5. You have this JSON:

{"user": {"id": 1, "name": "Bob"}}

And these Swift structs:

struct User: Codable { var id: Int; var name: String }
struct Response: Codable { var user: User }

How do you decode the JSON into a Response object?

hard

A. Use JSONSerialization.jsonObject(with: jsonData)

B. Use JSONDecoder().decode(User.self, from: jsonData)

C. Use JSONDecoder().decode(Response.self, from: jsonData)

D. Use JSONEncoder().encode(Response.self, from: jsonData)

Solution

1. Step 1: Identify the top-level JSON structure

   The JSON has a top-level key "user" matching the Response struct's property.

2. Step 2: Choose correct decoding target

   To decode the entire JSON, decode to Response.self, which contains User.

3. Final Answer:

   Use JSONDecoder().decode(Response.self, from: jsonData) -> Option C

4. Quick Check:

   Decode top-level struct = Use JSONDecoder().decode(Response.self, from: jsonData) [OK]

Hint: Decode to the struct matching JSON root [OK]

Common Mistakes:

• Decoding to nested User instead of Response
• Using JSONEncoder instead of JSONDecoder
• Using JSONSerialization instead of Codable