Dio package for advanced HTTP in Flutter - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Dio package in Flutter?

Dio is a powerful HTTP client for Flutter that supports advanced features like interceptors, global configuration, FormData, request cancellation, file downloading, and timeout.

intermediate

How do you add headers globally using Dio?

You can add headers globally by setting them in Dio's options: dio.options.headers = {'Authorization': 'Bearer token'}; This applies headers to all requests.

intermediate

What is an interceptor in Dio?

An interceptor lets you run code before a request is sent, after a response is received, or when an error occurs. It helps modify requests or responses globally.

advanced

How can you cancel an ongoing HTTP request with Dio?

You create a CancelToken and pass it to the request. Calling cancelToken.cancel() stops the request before it completes.

intermediate

What is FormData in Dio and when is it used?

FormData is used to send data like files or key-value pairs in POST requests, especially for uploading files or submitting forms.

Which Dio feature allows you to modify requests before they are sent?

A CancelToken

B Timeout

C FormData

D Interceptor

Interceptors let you run code before requests or after responses, allowing modification.

How do you globally set a base URL for all Dio requests?

A dio.options.baseUrl = 'https://api.example.com';

B dio.setBaseUrl('https://api.example.com');

C dio.baseUrl = 'https://api.example.com';

D dio.config.baseUrl = 'https://api.example.com';

The baseUrl is set via dio.options.baseUrl property.

What does CancelToken do in Dio?

A Cancels an ongoing HTTP request

B Adds headers to requests

C Uploads files

D Handles JSON parsing

CancelToken allows you to cancel requests before they finish.

Which Dio class helps to upload files in a POST request?

A Interceptor

B CancelToken

C FormData

D Response

FormData is used to send files and form fields in POST requests.

How can you handle errors globally in Dio?

A Setting dio.options.errorHandler

B Using an interceptor's onError callback

C Using CancelToken

D By catching exceptions only in each request

Interceptors have an onError callback to handle errors globally.

Explain how to use interceptors in Dio to add an authorization token to every request.

Think about modifying request headers before sending.

Describe the steps to cancel a long-running HTTP request using Dio.

Focus on how to stop a request before it finishes.

Practice

1. What is the primary purpose of the Dio package in Flutter?

easy

A. To manage local databases

B. To create UI animations

C. To make HTTP requests easily with advanced features

D. To handle user input gestures

Solution

1. Step 1: Understand Dio's role

   Dio is designed as an HTTP client to simplify network calls in Flutter apps.

2. Step 2: Compare with other options

   Other options like UI animations or database management are unrelated to Dio's purpose.

3. Final Answer:

   To make HTTP requests easily with advanced features -> Option C

4. Quick Check:

   Dio = HTTP client [OK]

Hint: Dio is all about HTTP requests, not UI or storage [OK]

Common Mistakes:

• Confusing Dio with UI or database packages
• Thinking Dio handles gestures
• Assuming Dio is for local storage

2. Which of the following is the correct way to create a Dio instance with a base URL of 'https://api.example.com'?

easy

A. var dio = Dio(BaseOptions(baseUrl: 'https://api.example.com'));

B. var dio = Dio('https://api.example.com');

C. var dio = Dio().setBaseUrl('https://api.example.com');

D. var dio = Dio(baseUrl: 'https://api.example.com');

Solution

1. Step 1: Check Dio instance creation syntax

   Dio requires BaseOptions to set baseUrl during initialization.

2. Step 2: Validate each option

   var dio = Dio(BaseOptions(baseUrl: 'https://api.example.com')); uses BaseOptions correctly; others misuse constructor or methods.

3. Final Answer:

   var dio = Dio(BaseOptions(baseUrl: 'https://api.example.com')); -> Option A

4. Quick Check:

   BaseOptions sets baseUrl in Dio [OK]

Hint: Use BaseOptions to set baseUrl when creating Dio [OK]

Common Mistakes:

• Passing URL directly to Dio constructor
• Using nonexistent setBaseUrl method
• Setting baseUrl as a direct parameter

3. Given the code snippet:

var dio = Dio();
var response = await dio.get('https://jsonplaceholder.typicode.com/posts/1');
print(response.data['id']);

What will be printed?

medium

A. 1

B. null

C. An error message

D. The entire JSON response

Solution

1. Step 1: Understand the GET request

   The code fetches post with id 1 from the API.

2. Step 2: Access response data

   response.data['id'] accesses the 'id' field, which is 1.

3. Final Answer:

   1 -> Option A

4. Quick Check:

   response.data['id'] = 1 [OK]

Hint: Access response.data keys to get JSON fields [OK]

Common Mistakes:

• Expecting entire JSON instead of specific field
• Not awaiting the async call
• Using wrong key to access data

4. Identify the error in this Dio request code:

var dio = Dio();
var response = dio.get('https://api.example.com/data');
print(response.data);

medium

A. Dio instance not created properly

B. Incorrect URL format

C. response.data is not a valid property

D. Missing await keyword for async call

Solution

1. Step 1: Check async call usage

   dio.get returns a Future; it must be awaited to get response.

2. Step 2: Identify missing await

   Code lacks await, so response is a Future, not the actual response object.

3. Final Answer:

   Missing await keyword for async call -> Option D

4. Quick Check:

   Async calls need await [OK]

Hint: Always await async Dio calls to get response [OK]

Common Mistakes:

• Forgetting await on async requests
• Assuming dio.get is synchronous
• Trying to access data on Future object

5. How can you add a custom header 'Authorization: Bearer token123' to all requests made by a Dio instance?

hard

A. Add headers directly in each get/post call only

B. Set headers in BaseOptions when creating Dio instance

C. Use dio.addHeader('Authorization', 'Bearer token123') method

D. Modify the response interceptor to add headers

Solution

1. Step 1: Understand global headers setup

   Headers for all requests are set in BaseOptions.headers when creating Dio.

2. Step 2: Evaluate options

   Setting headers in BaseOptions applies them globally to all requests; adding them per get/post call is request-specific; there is no addHeader method on Dio; response interceptors handle responses, not request headers.

3. Final Answer:

   Set headers in BaseOptions when creating Dio instance -> Option B

4. Quick Check:

   Global headers = BaseOptions.headers [OK]

Hint: Use BaseOptions.headers for global request headers [OK]

Common Mistakes:

• Trying to add headers via nonexistent methods
• Setting headers only per request instead of globally
• Confusing response interceptors with request headers