Codable protocol for JSON parsing in iOS Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a struct that can be encoded and decoded from JSON.

iOS Swift

struct User: [1] {
  var name: String
  var age: Int
}

Drag options to blanks, or click blank then click option'

AIdentifiable

BCodable

CEquatable

DHashable

Attempts:

3 left

💡 Hint

Common Mistakes

Using Equatable instead of Codable.

Forgetting to conform to any protocol.

The Codable protocol allows the struct to be encoded to and decoded from JSON easily.

2fill in blank

medium

Complete the code to decode JSON data into a User instance.

iOS Swift

let decoder = JSONDecoder()
let user = try decoder.[1](User.self, from: jsonData)

Drag options to blanks, or click blank then click option'

Aencode

Bparse

Cdecode

Dserialize

Attempts:

3 left

💡 Hint

Common Mistakes

Using encode instead of decode.

Trying to use parse or serialize which are not JSONDecoder methods.

The decode method of JSONDecoder converts JSON data into a Swift object.

3fill in blank

hard

Fix the error in the decoding code by completing the blank.

iOS Swift

do {
  let user = try JSONDecoder().decode([1], from: data)
  print(user.name)
} catch {
  print("Failed to decode")
}

Drag options to blanks, or click blank then click option'

AUser() -> User

BUser()

CUser.Type

DUser.self

Attempts:

3 left

💡 Hint

Common Mistakes

Passing an instance like User() instead of the type.

Using User.Type which is a metatype but not the correct syntax here.

The decode method requires the type itself, not an instance, so use User.self.

4fill in blank

hard

Fill both blanks to encode a User instance into JSON data.

iOS Swift

let encoder = JSONEncoder()
let jsonData = try encoder.[1]([2])

Drag options to blanks, or click blank then click option'

Aencode

Buser

Cdecode

DUser.self

Attempts:

3 left

💡 Hint

Common Mistakes

Using decode instead of encode.

Passing the type User.self instead of the instance user.

Use encode to convert the user instance into JSON data.

5fill in blank

hard

Fill all three blanks to create a dictionary from JSON data using Codable.

iOS Swift

struct Item: Codable {
  var id: Int
  var name: String
}

let items = try JSONDecoder().decode([[1]].self, from: [2])
let dict = Dictionary(uniqueKeysWithValues: items.map { ($0.[3], $0.name) })

Drag options to blanks, or click blank then click option'

AItem

BjsonData

Cid

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong type in decode.

Passing wrong variable names in map closure.

Decode an array of Item from jsonData. Then map each item using its id as the key.

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