POST request with JSON body in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What happens when you send a POST request with JSON body in Swift?

You create a POST request in Swift with a JSON body. What is the expected behavior when the request is sent?

iOS Swift

let url = URL(string: "https://example.com/api")!
var request = URLRequest(url: url)
request.httpMethod = "POST"
request.setValue("application/json", forHTTPHeaderField: "Content-Type")
let json: [String: Any] = ["name": "John", "age": 30]
request.httpBody = try? JSONSerialization.data(withJSONObject: json)

let task = URLSession.shared.dataTask(with: request) { data, response, error in
  if let error = error {
    print("Error: \(error)")
  } else if let data = data {
    print("Response data received")
  }
}
task.resume()

A The server receives a POST request with a JSON body containing the name and age data.

B The server receives a GET request with URL parameters instead of a JSON body.

C The request fails because JSONSerialization cannot convert the dictionary to data.

D The request sends an empty body because httpBody is not set correctly.

💡 Hint

Check the httpMethod and httpBody properties of the URLRequest.

Setting httpMethod to "POST" and httpBody to JSON data sends a POST request with the JSON body to the server.

📝 Syntax

intermediate

Which code snippet correctly sets the JSON body for a POST request in Swift?

Choose the code snippet that correctly sets a JSON body for a POST request using URLRequest in Swift.

A request.httpBody = try? JSONSerialization.data(withJSONObject: ["key": "value"])

B request.httpBody = JSONSerialization.jsonObject(with: ["key": "value"])

C request.httpBody = Data(["key": "value"])

D request.httpBody = "{\"key\":\"value\"}"

💡 Hint

Look for the method that converts a dictionary to Data.

JSONSerialization.data(withJSONObject:) converts a dictionary to JSON Data. The other options are invalid or incorrect types.

❓ lifecycle

advanced

What happens if you forget to call resume() on the URLSession data task?

Consider this Swift code creating a POST request with JSON body but missing a call. What is the effect?

let task = URLSession.shared.dataTask(with: request) { data, response, error in
  // handle response
}

A The request is sent automatically without calling resume().

B The request is never sent because resume() was not called on the task.

C The app crashes with a runtime error.

D The request sends but the completion handler is never called.

💡 Hint

Check how URLSession tasks start their work.

URLSession data tasks start only after calling resume(). Without it, the task is created but never runs.

🔧 Debug

advanced

Why does this POST request with JSON body fail to send data?

Review this Swift code snippet. The POST request is sent but the server receives no JSON data. What is the likely cause?

var request = URLRequest(url: URL(string: "https://api.example.com")!)
request.httpMethod = "POST"
let json = ["user": "alice"]
request.httpBody = try? JSONSerialization.data(withJSONObject: json)
// Missing Content-Type header
let task = URLSession.shared.dataTask(with: request) { data, response, error in
  // handle response
}
task.resume()

A The URL is invalid, so the request fails silently.

B The httpMethod is not set to POST, so the body is ignored.

C JSONSerialization.data(withJSONObject:) throws an error and returns nil.

D The Content-Type header is missing, so the server does not recognize the body as JSON.

💡 Hint

Check the HTTP headers required for JSON data.

Without setting "Content-Type" to "application/json", the server may ignore or reject the JSON body.

🧠 Conceptual

expert

What is the correct way to handle JSON encoding errors in a POST request in Swift?

You want to send a POST request with a JSON body. How should you handle possible errors during JSON encoding?

A Use try? to convert JSON to Data and send the request only if data is not nil.

B Use try! to force JSON encoding and crash if it fails.

C Use a do-catch block to catch encoding errors and handle them gracefully before sending.

D Ignore errors and send an empty body if encoding fails.

💡 Hint

Think about safe error handling in Swift.

Using do-catch allows you to catch and handle JSON encoding errors properly before sending the request.

Practice

1. What is the main purpose of a POST request with a JSON body in iOS Swift networking?

easy

A. To send data to a server, usually to create or update resources

B. To retrieve data from a server without sending any data

C. To delete data from a server

D. To open a WebSocket connection

Solution

1. Step 1: Understand HTTP methods

   POST requests are used to send data to a server, often to create or update something.

2. Step 2: Recognize JSON role

   JSON is a format to structure the data sent in the body of the POST request.

3. Final Answer:

   To send data to a server, usually to create or update resources -> Option A

4. Quick Check:

   POST sends data = A [OK]

Hint: POST means send data to server, not just get [OK]

Common Mistakes:

• Confusing POST with GET method
• Thinking POST is for deleting data
• Ignoring the role of JSON in the body

2. Which Swift code snippet correctly sets the HTTP method to POST and adds a JSON Content-Type header?

easy

A. request.httpMethod = "POST"
request.setValue("application/json", forHTTPHeaderField: "Content-Type")

B. request.method = "POST"
request.addHeader("Content-Type", value: "application/json")

C. request.httpMethod = POST
request.setHeader("Content-Type", "application/json")

D. request.method = POST
request.setValue("json", forHTTPHeaderField: "Content-Type")

Solution

1. Step 1: Set HTTP method correctly

   The property to set HTTP method in URLRequest is httpMethod and it expects a String like "POST".

2. Step 2: Add JSON Content-Type header properly

   Use setValue(_:forHTTPHeaderField:) with "application/json" for the "Content-Type" header.

3. Final Answer:

   request.httpMethod = "POST"\nrequest.setValue("application/json", forHTTPHeaderField: "Content-Type") -> Option A

4. Quick Check:

   Correct method and header syntax = A [OK]

Hint: Use request.httpMethod and setValue for headers [OK]

Common Mistakes:

• Using wrong property names like method or setHeader
• Forgetting quotes around "POST"
• Using incorrect header field names or values

3. Given the Swift code below, what will be the printed output?

let url = URL(string: "https://example.com/api")!
var request = URLRequest(url: url)
request.httpMethod = "POST"
request.setValue("application/json", forHTTPHeaderField: "Content-Type")
let jsonBody = ["name": "Alice", "age": 30] as [String : Any]
request.httpBody = try? JSONSerialization.data(withJSONObject: jsonBody)
print(String(data: request.httpBody!, encoding: .utf8)!)

medium

A. ["name": "Alice", "age": 30]

B. {"name":"Alice","age":30}

C. Optional({"name":"Alice","age":30})

D. nil

Solution

1. Step 1: Serialize dictionary to JSON data

   JSONSerialization.data converts the dictionary to JSON data bytes representing {"name":"Alice","age":30}.

2. Step 2: Convert data to UTF-8 string

   Printing the data as UTF-8 string shows the JSON string without brackets or Optional wrapper.

3. Final Answer:

   {"name":"Alice","age":30} -> Option B

4. Quick Check:

   JSONSerialization outputs JSON string = B [OK]

Hint: JSONSerialization outputs JSON string, not Swift dictionary syntax [OK]

Common Mistakes:

• Expecting Swift dictionary syntax instead of JSON string
• Printing Optional wrapper
• Forgetting to unwrap data or encoding

4. Identify the error in this Swift code for sending a POST request with JSON body:

var request = URLRequest(url: URL(string: "https://api.example.com")!)
request.httpMethod = "POST"
let json = ["key": "value"]
request.httpBody = try? JSONSerialization.jsonObject(with: json)

medium

A. httpMethod should be lowercase "post"

B. Missing setting Content-Type header to application/json

C. Using JSONSerialization.jsonObject instead of data(withJSONObject:)

D. URLRequest initialization is incorrect

Solution

1. Step 1: Check JSON serialization method

   jsonObject(with:) parses JSON data to object; to create JSON data from object, use data(withJSONObject:).

2. Step 2: Confirm other parts

   Content-Type header missing is an issue but not the main error causing failure here.

3. Final Answer:

   Using JSONSerialization.jsonObject instead of data(withJSONObject:) -> Option C

4. Quick Check:

   Wrong JSON method used = D [OK]

Hint: Use data(withJSONObject:) to create JSON data, not jsonObject(with:) [OK]

Common Mistakes:

• Confusing jsonObject(with:) with data(withJSONObject:)
• Forgetting Content-Type header
• Incorrect URLRequest initialization

5. You want to send a POST request with a JSON body containing user info, but only include the "email" field if it is not empty. Which Swift code snippet correctly creates the JSON body for this conditional inclusion?

hard

A. var jsonBody = ["name": "Bob", "email": email]
if !email.isEmpty {
jsonBody.removeValue(forKey: "email")
}
request.httpBody = try? JSONSerialization.data(withJSONObject: jsonBody)

B. let jsonBody = ["name": "Bob", "email": email.isEmpty ? nil : email]
request.httpBody = try? JSONSerialization.data(withJSONObject: jsonBody)

C. let jsonBody = ["name": "Bob"]
if email != "" {
jsonBody["email"] = email
}
request.httpBody = try? JSONSerialization.data(withJSONObject: jsonBody)

D. var jsonBody = ["name": "Bob"]
if !email.isEmpty {
jsonBody["email"] = email
}
request.httpBody = try? JSONSerialization.data(withJSONObject: jsonBody)

Solution

1. Step 1: Understand conditional dictionary update

   var jsonBody = ["name": "Bob"] if !email.isEmpty { jsonBody["email"] = email } request.httpBody = try? JSONSerialization.data(withJSONObject: jsonBody) creates a mutable dictionary and adds "email" only if not empty, which is correct.

2. Step 2: Check other options for errors

   let jsonBody = ["name": "Bob", "email": email.isEmpty ? nil : email] request.httpBody = try? JSONSerialization.data(withJSONObject: jsonBody) uses nil in dictionary literal which is invalid; var jsonBody = ["name": "Bob", "email": email] if !email.isEmpty { jsonBody.removeValue(forKey: "email") } request.httpBody = try? JSONSerialization.data(withJSONObject: jsonBody) uses inverted condition, removing email when it is not empty, incorrect; let jsonBody = ["name": "Bob"] if email != "" { jsonBody["email"] = email } request.httpBody = try? JSONSerialization.data(withJSONObject: jsonBody) tries to mutate immutable dictionary.

3. Final Answer:

   var jsonBody = ["name": "Bob"]\nif !email.isEmpty {\n jsonBody["email"] = email\n}\nrequest.httpBody = try? JSONSerialization.data(withJSONObject: jsonBody) -> Option D

4. Quick Check:

   Mutable dict + conditional add = C [OK]

Hint: Use var dictionary and add keys conditionally [OK]

Common Mistakes:

• Using let dictionary and trying to mutate it
• Including nil values in dictionary literal
• Adding then removing keys instead of conditional add