Welcome to the next step in your journey of mastering API interactions with Dart! In our previous lesson, you learned how to download files efficiently using an API, enhancing your skills in file management. Today, we will take a look at the reverse process: uploading files to an API. This capability is crucial for creating applications that need to store or share files, such as documents, images, or any other type of data with an external server.

Understanding file uploads will further expand your ability to interact with APIs, equipping you to build more robust and feature-complete applications. By the end of this lesson, you will learn how to send a file to a server using Dart, ensuring that you can manage uploads confidently and efficiently.

To upload files via HTTP, the POST method is commonly used, as it's designed for submitting data to a server, including files. The key to sending files is using multipart/form-data, a format that allows both text and binary data to be sent together, organized into separate parts. This format ensures the server can properly handle the uploaded file along with any additional data.

In Dart, the http package makes this process seamless. When using http, you can easily create a MultipartRequest to handle the multipart/form-data format. This allows you to focus on the functionality of your application without worrying about the underlying details of the request format.

When working with file uploads, we need to use an HTTP client to manage the connection to the server. The client is responsible for establishing the connection, sending the request, and receiving the response.

For regular HTTP requests, the http package automatically manages resources. However, when using MultipartRequest, the client needs to be explicitly closed to release resources. Without closing the client, your application might experience resource leaks, especially in long-running applications or when handling large files.

Let's walk through the complete process of uploading a file using Dart's http package, focusing on proper resource management.

We begin by creating an HTTP client that will manage our connection:

Dart
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1// Create an HTTP client
2final client = http.Client();

This client is the foundation of our HTTP operations. It manages the connection to the server and ensures resources are properly allocated. For file uploads, we'll need to explicitly close this client when we're done.

Next, we need to read the file we want to upload:

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1// Open the file read as bytes
2final file = File("file.txt");
3final fileBytes = await file.readAsBytes();

Reading the file as bytes ensures we can send the raw binary data to the server. This preserves the file's integrity during the upload process, which is essential for all file types, especially non-text files like images or documents.

With our file ready, we create a multipart request specifically designed for file uploads:

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1// Create a multipart request
2final request = http.MultipartRequest(
3  'POST', 
4  Uri.parse("http://example.com/your/endpoint")
5);

The MultipartRequest handles the complex formatting required for multipart/form-data requests. It structures the request body to include both the file data and any additional form fields you might need to send. Note that the URL used here is just an example; you should replace it with the actual endpoint URL you intend to use for your file uploads.

Now we prepare the file for inclusion in the request:

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1// Create a multipart file from the bytes
2final multipartFile = http.MultipartFile.fromBytes(
3  'file', 
4  fileBytes, 
5  filename: "file.txt"
6);
7
8// Add the file to the request
9request.files.add(multipartFile);

The MultipartFile represents our file in the format needed for the request. We specify the field name ('file'), which the server will use to identify the uploaded file, the file content (fileBytes), and the original filename. Adding this to the request's files collection ensures it will be included in the request body.

Finally, we send the request and ensure we properly release resources. The client.send(request) method establishes a connection and transmits the file data. By wrapping this in a try-finally block, we ensure that regardless of whether the upload succeeds or fails, the client is closed and resources are released.

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1try {
2  // Send the request using the client
3  final response = await client.send(request);
4  
5  // Process the response...
6} finally {
7  // Close the client to release resources
8  client.close();
9}

This is particularly important with MultipartRequest because it can hold onto significant memory and connection resources, especially when uploading large files. Failing to close the client can lead to resource leaks that degrade application performance over time.

By following these steps and properly managing the HTTP client throughout the file upload process, you can ensure that your applications handle file uploads efficiently and reliably, without unnecessary resource consumption.

Now, let's delve into uploading a file to our API using Dart. Consider the following code, which utilizes the POST method on the "/notes" endpoint to upload a file named meeting_notes.txt.

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1import 'dart:io';
2import 'dart:convert';
3import 'package:http/http.dart' as http;
4
5void main() async {
6  // Base URL for the API
7  final String baseUrl = "http://localhost:8000";
8
9  // Specify the file name to upload
10  final String fileName = "meeting_notes.txt";
11
12  // Create an HTTP client
13  final client = http.Client();
14
15  try {
16    // Open the file and read it as bytes
17    final file = File(fileName);
18    final fileBytes = await file.readAsBytes();
19
20    // Create a multipart request
21    final request = http.MultipartRequest(
22      'POST',
23      Uri.parse('$baseUrl/notes'),
24    );
25
26    // Create a multipart file from the bytes
27    final multipartFile = http.MultipartFile.fromBytes(
28      'file',
29      fileBytes,
30      filename: fileName,
31    );
32
33    // Add the file to the request
34    request.files.add(multipartFile);
35
36    // Send the request using the client
37    final response = await client.send(request);
38
39    // Check for bad responses (4xx and 5xx status codes)
40    if (response.statusCode >= 400) {
41      final responseBody = await response.stream.bytesToString();
42      String errorMessage =
43          jsonDecode(responseBody)['error'] ?? 'Unknown error';
44      throw http.ClientException(errorMessage);
45    }
46
47    // Print success message if file is uploaded successfully
48    print("File uploaded successfully: $fileName");
49  } on http.ClientException catch (httpErr) {
50    print(httpErr.message);
51  } catch (err) {
52    print("Other error occurred: $err");
53  } finally {
54    // Close the client to release resources
55    client.close();
56  }
57}

This code example demonstrates how to properly upload a file to an API:

• We create an HTTP client that will be used to send the request and must be closed when done.
• The File class is used to open the file and read it as bytes, which is essential for properly handling the file data.
• We create a MultipartRequest for a POST request to the API's /notes endpoint.
• We create a MultipartFile from the file bytes and add it to the request.
• We send the request using the client and check for any errors in the response.
• If the upload is successful, a success message is printed to the console.
• Finally, we close the client to release resources, ensuring this happens even if an error occurs.

In this example, requests are encapsulated in a try-catch block to gracefully address potential issues. By managing these exceptions, you can ensure your application behaves predictably, even if something goes wrong during file upload.

Once a file is uploaded, it's important to verify it to ensure that the file is stored correctly on the server. You can achieve this by sending a GET request to the corresponding endpoint and checking the content of the uploaded file.

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1try {
2    // Send a GET request to retrieve the file content from the server
3    final response = await http.get(Uri.parse('$baseUrl/notes/$fileName'));
4
5    // Check for bad responses (4xx and 5xx status codes)
6    if (response.statusCode >= 400) {
7      String errorMessage = jsonDecode(response.body)['error'] ?? 'Unknown error';
8      throw http.ClientException(errorMessage);
9    }
10    
11    // Print the content of the file
12    print(response.body);
13} on http.ClientException catch (httpErr) {
14    print(httpErr.message);
15} catch (err) {
16    print("Other error occurred: $err");
17}

In this code, we retrieve the content of the file from the server and print it out. This allows us to confirm that the file has been uploaded and stored successfully.

Plain text
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1Meeting Notes
2
3Date: 2023-10-18
4Time: 3:00 PM
5Location: Conference Room A
6
7Attendees:
8- Alice Johnson
9- Bob Smith
10- Charlie Brown
11...

This output confirms that the file "meeting_notes.txt" is present on the server and its contents are intact, with details such as the date, time, location, and attendees of a meeting.

In this lesson, you built upon your previous knowledge of file downloads and learned to upload files to a server using Dart's http package. We explored the steps to set up your environment, the importance of opening files in binary mode, and the method to send POST requests for file uploads. You also learned how robust error handling can lead to more reliable applications, and the importance of properly managing resources by closing the HTTP client when done with multipart requests.

Now, it's time to get hands-on with the practical exercises following this lesson. Use these exercises as an opportunity to reinforce your understanding and experiment with different file types and sizes. This will not only enhance your skills but also prepare you for advanced API interactions in future lessons. Happy coding, and keep up the excellent work!