Welcome to the next step in your journey of mastering API interactions with Swift! 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 Swift, 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 Swift, URLSession and URLRequest make this process seamless. You can configure a URLRequest to use the POST method and set the appropriate headers for multipart/form-data. Swift's networking capabilities allow you to handle file uploads efficiently, focusing on the functionality of your application.

Now, let's delve into the process of uploading a file using Swift. Consider the following code example, which utilizes the "/notes" endpoint to upload a file named meeting_notes.txt.

Swift
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1import Foundation
2import FoundationNetworking
3
4// Base URL for the API
5let baseURL = URL(string: "http://localhost:8000/notes")!
6
7// Specify the file name to upload
8let fileName = "meeting_notes.txt"
9
10// Create a DispatchGroup
11let dispatchGroup = DispatchGroup()
12
13do {
14    // Get the file URL
15    let fileURL = URL(fileURLWithPath: fileName)
16    
17    // Read the file data
18    let fileData = try Data(contentsOf: fileURL)
19    
20    // Create a URLRequest
21    var request = URLRequest(url: baseURL)
22    request.httpMethod = "POST"
23    
24    // Set the content type to multipart/form-data
25    let boundary = UUID().uuidString
26    request.setValue("multipart/form-data; boundary=\(boundary)", forHTTPHeaderField: "Content-Type")
27    
28    // Create the multipart form data
29    var body = Data()
30    body.append("--\(boundary)\r\n".data(using: .utf8)!)
31    body.append("Content-Disposition: form-data; name=\"file\"; filename=\"\(fileName)\"\r\n".data(using: .utf8)!)
32    body.append("Content-Type: application/octet-stream\r\n\r\n".data(using: .utf8)!)
33    body.append(fileData)
34    body.append("\r\n--\(boundary)--\r\n".data(using: .utf8)!)
35    
36    request.httpBody = body
37    
38    // Enter the DispatchGroup
39    dispatchGroup.enter()
40    
41    // Create a URLSession data task
42    let task = URLSession.shared.dataTask(with: request) { data, response, error in
43        if let error = error {
44            print("Error occurred: \(error)")
45        } else if let httpResponse = response as? HTTPURLResponse, (200...299).contains(httpResponse.statusCode) {
46            print("File uploaded successfully: \(fileName)")
47        } else {
48            print("Failed to upload file.")
49        }
50        
51        // Leave the DispatchGroup
52        dispatchGroup.leave()
53    }
54    
55    // Start the task
56    task.resume()
57    
58    // Wait for the task to complete
59    dispatchGroup.wait()
60    
61} catch {
62    print("File not found or could not be read: \(fileName)")
63}

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

• The Data(contentsOf:) method reads the file data, which is essential for properly handling the file data.
• A URLRequest is configured to send a POST request to the API's /notes endpoint, attaching the file in the request.
• If the upload is successful, a success message is printed to the console.

In this example, requests are encapsulated in a do-catch block to gracefully address potential issues. An error is handled if the file specified for upload cannot be located or read. 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.

Swift
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1// Verify the file upload by retrieving the file content from the server
2// Since we did not specify `httpMethod` explicitly, the request is a GET by default.
3let verifyURL = baseURL.appendingPathComponent(fileName)
4
5let verifyTask = URLSession.shared.dataTask(with: verifyURL) { data, response, error in
6    if let error = error {
7        print("Error occurred: \(error)")
8        return
9    }
10    
11    if let httpResponse = response as? HTTPURLResponse, (200...299).contains(httpResponse.statusCode), let data = data {
12        if let fileContent = String(data: data, encoding: .utf8) {
13            print(fileContent)
14        }
15    } else {
16        print("Failed to verify file upload.")
17    }
18}
19
20// Start the verification task
21verifyTask.resume()

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 Swift's URLSession and URLRequest. We explored the steps to set up your environment, the importance of reading files correctly, and the method to send POST requests for file uploads. You also learned how robust error handling can lead to more reliable applications.

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!