Welcome to this lesson on sending data with POST requests. As you continue your exploration of interacting with RESTful APIs, you'll learn how to send data to a server using the POST method. POST requests are essential when you want to create new resources or submit data, such as filling out a web form or adding a new entry to a database. Unlike GET requests, which you have already encountered, POST requests do not solely retrieve information; they actually transmit data to an API.

Understanding these differences is crucial as you expand your skill set in HTTP methods. Let’s dive deeper into utilizing POST requests to comprehend how they stand apart from GET requests.

Before diving into POST requests, let's briefly compare them to GET requests:

• GET Requests:

  • Purpose: Retrieve data from a server.
  • Data Location: Data is sent in the URL as path or query parameters, which exposes them and makes them less secure for transmitting sensitive information, such as passwords.
  • Success Status: Expect a 200 status code for successful data retrieval.

• POST Requests:

  • Purpose: Send data to a server to create or update a resource, such as submitting a form, uploading a file, or adding a new item to a database.
  • Data Location: Data is sent in the request body, making it more suitable for handling sensitive data securely.
  • Success Status: Expect a 201 status code for successful resource creation.

These differences clarify when to use each method. POST requests, in particular, require careful handling of the request body.

For POST requests, the request body is crucial as it holds the data you want to send to the server. This data is usually structured in formats like JSON, XML, or form data, with JSON being a common choice due to its readability and compatibility.

Here’s an example of a JSON request body:

JSON
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1{
2  "title": "Learn Dart http package",
3  "description": "Complete a course on Dart API calls.",
4  "done": false
5}

This represents a new todo item, including a title, completion status, and description. Noticeably, we are not sending an id, as this is typically managed by the server upon resource creation.

With a solid understanding of GET vs. POST and the role of the request body, you’re ready to explore how to utilize Dart's http package to handle POST operations.

This lesson will continue to use Dart's http package, with a specific focus on how it facilitates POST requests. Ensure you have the base URL for your API endpoint, which in our scenario is http://localhost:8000. That said, here’s a quick reminder of what you need to get started:

Dart
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1import 'dart:convert';
2import 'package:http/http.dart' as http;
3
4void main() async {
5  // Base URL for the API
6  final String baseUrl = "http://localhost:8000";
7
8  // Rest of the code...
9}

This sets up the foundation for us to explore POST requests further. Now, let's walk through an example of how to craft a POST request to add a new todo item to our API.

First, we need to prepare the data we wish to send. Here, we'll be adding a new todo item with a specific title, a completion status, and a description. The data is structured as a map in Dart:

Dart
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1// New todo data
2final Map<String, dynamic> newTodo = {
3  "title": "Learn Dart http package",
4  "description": "Complete a course on Dart API calls.",
5  "done": false
6};

This map represents the data structure that will be converted to JSON format and sent in the request body.

In Dart, you can use the http.post() method to send the request body. You'll pass the data to the body parameter of this method, which requires the data to be encoded as JSON.

When sending a request, it's important to include headers, which are like labels that provide additional information about the request. One key header is the Content-Type, which tells the server what kind of data you're sending. In this case, we set the Content-Type to "application/json" to indicate that the data is in JSON format. This helps the server understand how to read and process the data correctly.

Here's how you send the POST request:

Dart
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1// Send POST request
2final response = await http.post(
3  Uri.parse('$baseUrl/todos'),
4  headers: {"Content-Type": "application/json"},
5  body: jsonEncode(newTodo),
6);

In this code, body: jsonEncode(newTodo) sends the map as JSON data in the request body. The Content-Type header is crucial because it ensures the server knows the format of the data being sent, allowing it to process the request accurately.

Interpreting the response from a POST request is an integral part of the process. After sending the request, the server provides a response indicating whether the operation was successful. A 201 status code signifies successful resource creation. Typically, POST requests return the newly created resource in the response body. This allows the client to immediately access and utilize details of the new resource, such as server-generated fields like IDs.

Dart
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1// Check if the request was successful
2if (response.statusCode == 201) {
3  print("New todo added successfully!");
4  // The response contains the details of the newly created resource
5  print(response.body);
6} else {
7  // Handle potential errors
8  print("Failed to add a new todo");
9  print("Status Code: ${response.statusCode}");
10  print("Error Details: ${response.body}");
11}

The code assesses the server's response: a 201 status code confirms successful creation, allowing you to print and use the details of the new todo, typically included in the server's response. Otherwise, the code handles potential errors by outputting relevant error information.

For example, if the operation is successful, the output will be:

Plain text
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1New todo added successfully!
2{"description": "Complete a course on Dart API calls.", "done": false, "id": 5, "title": "Learn Dart http package"}

A POST request may fail if required fields are missing. For example, omitting a "title" might lead to an error response:

Dart
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1// New todo data with missing title
2final Map<String, dynamic> newTodo = {
3  // "title": "Learn Dart http package",  // Title is missing
4  "done": false,
5  "description": "Complete a course on Dart API calls."
6};
7
8// Send POST request
9final response = await http.post(
10  Uri.parse('$baseUrl/todos'),
11  headers: {"Content-Type": "application/json"},
12  body: jsonEncode(newTodo),
13);
14
15// Check if the request was successful
16if (response.statusCode == 201) {
17  print("New todo added successfully!");
18  print(response.body);
19} else {
20  // Handle potential errors
21  print("Failed to add a new todo");
22  print("Status Code: ${response.statusCode}");
23  print("Error Details: ${response.body}");
24}

Running this code would produce:

Plain text
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1Failed to add a new todo
2Status Code: 400
3Error Details: {"error": "Invalid request. 'title' is required."}

The 400 status code signifies a bad request, which is often due to missing or incorrect data in the request body. In this case, the error message specifies that the "title" field is required, making it clear what needs to be corrected for the POST request to succeed. This feedback allows you to quickly identify and amend the missing or erroneous part of the data before resending the request.

In this lesson, you have learned how to send data to an API using POST requests. We explored how POST differs from GET in terms of its function of creating new resources. With Dart's http package, you are equipped to craft and send POST requests, handle responses, and manage errors to ensure robust API interactions.

As you proceed to the practice exercises, you will have the opportunity to apply everything you have learned here. Practice creating POST requests to reinforce your understanding and discover firsthand the nuances of sending data to an API. Keep up the excellent work, as each lesson brings you closer to mastering API interactions with Dart!