Welcome back! Today's focus will be on downloading files from an API using Swift. Understanding how to retrieve files efficiently not only enhances your technical skills but also broadens your application's capabilities. In this lesson, we'll explore a practical scenario using our To-Do list API, which, in addition to managing tasks, supports handling text files such as notes. These notes can be downloaded or uploaded through the /notes endpoint, allowing functionality for storing supplementary information. For example, users might keep notes about a meeting or important reminders. By understanding how to interact with this endpoint, you can effectively manage notes within your application. By the end of this lesson, you'll know how to request a file from an API, save it locally, and verify its contents.

Let's dive into downloading files with precision and confidence!

GET requests are fundamental for retrieving files from an API. When you send a GET request using URLSession, your client communicates with the server at a specified URL, asking it to provide the file. The server responds with the file data, if available and permissible, along with an HTTP status code (like 200 OK).

Here's a basic example of downloading a file named welcome.txt from our API at http://localhost:8000/notes. This approach downloads the entire file at once, which is manageable for smaller files.

Swift
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1import Foundation
2import FoundationNetworking
3
4// Base URL for the API
5let baseURL = "http://localhost:8000"
6
7// Specify the note name to download
8let noteName = "welcome.txt"
9
10// Create a DispatchGroup
11let dispatchGroup = DispatchGroup()
12
13// Create the URL for the request
14if let url = URL(string: "\(baseURL)/notes/\(noteName)") {
15    dispatchGroup.enter() // Enter the group
16    let task = URLSession.shared.dataTask(with: url) { data, response, error in
17        if let error = error {
18            print("Error occurred: \(error)")
19            dispatchGroup.leave() // Leave the group
20            return
21        }
22        
23        if let httpResponse = response as? HTTPURLResponse, httpResponse.statusCode == 200 {
24            if let data = data {
25                // Save the file locally
26                let fileURL = FileManager.default.temporaryDirectory.appendingPathComponent("downloaded_\(noteName)")
27                do {
28                    try data.write(to: fileURL)
29                    print("File downloaded successfully to \(fileURL.path)")
30                } catch {
31                    print("File error occurred: \(error)")
32                }
33            }
34        } else {
35            print("Failed to download file. HTTP Status Code: \((response as? HTTPURLResponse)?.statusCode ?? 0)")
36        }
37        dispatchGroup.leave() // Leave the group
38    }
39    task.resume()
40}
41
42// Wait for the download to complete
43dispatchGroup.wait()
44print("Download task completed.")

In this code example, we use a GET request to download a file in its entirety. Here's a breakdown of the key components:

• URLSession.dataTask: This method is used to perform a data task, which retrieves the entire file data at once. It's suitable for smaller files.
• DispatchGroup: This is used to manage the asynchronous nature of the task, ensuring that the program waits for the download to complete before proceeding.
• Error Handling: The code checks for errors in the response and handles them appropriately.
• File Writing: Once the data is received, it's written to a local file using data.write(to:).

This method works well for small files but can strain memory for larger files.

When dealing with large files, downloading them all at once can be inefficient and strain memory. To address this, you can use URLSession to handle data in chunks, thus optimizing memory usage and maintaining efficiency.

Below is a detailed code example demonstrating how to download the same file using streaming:

Swift
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1import Foundation
2import FoundationNetworking
3
4// Base URL for the API
5let baseURL = "http://localhost:8000"
6
7// Specify the note name to download
8let noteName = "welcome.txt"
9
10// Create a DispatchGroup
11let dispatchGroup = DispatchGroup()
12
13// Create the URL for the request
14if let url = URL(string: "\(baseURL)/notes/\(noteName)") {
15    dispatchGroup.enter() // Enter the group
16    let task = URLSession.shared.downloadTask(with: url) { location, response, error in
17        if let error = error {
18            print("Error occurred: \(error)")
19            dispatchGroup.leave() // Leave the group
20            return
21        }
22        
23        if let httpResponse = response as? HTTPURLResponse, httpResponse.statusCode == 200 {
24            if let location = location {
25                // Define the filename for storing the downloaded content
26                let fileURL = FileManager.default.temporaryDirectory.appendingPathComponent("downloaded_\(noteName)")
27                do {
28                    try FileManager.default.moveItem(at: location, to: fileURL)
29                    print("File downloaded successfully to \(fileURL.path)")
30                } catch {
31                    print("File error occurred: \(error)")
32                }
33            }
34        } else {
35            print("Failed to download file. HTTP Status Code: \((response as? HTTPURLResponse)?.statusCode ?? 0)")
36        }
37        dispatchGroup.leave() // Leave the group
38    }
39    task.resume()
40}
41
42// Wait for the download to complete
43dispatchGroup.wait()
44print("Download task completed.")

The second code example demonstrates downloading a file using streaming, which is more efficient for larger files. Key differences include:

• URLSession.downloadTask: This method is used to perform a download task, which streams the file data to a temporary location on disk. This approach is memory-efficient and ideal for large files.
• Temporary File Handling: The file is initially stored in a temporary location, and then moved to a desired location using FileManager.default.moveItem(at:to:).
• Memory Efficiency: By streaming the file, the application avoids loading the entire file into memory, reducing memory usage and improving performance for large files.

Both methods utilize DispatchGroup to manage asynchronous tasks and ensure the program waits for the download to complete. The choice between these methods depends on the file size and memory constraints of your application.

Once you've downloaded a file, it's imperative to verify its contents to ensure a successful transfer. In our example, after downloading, you can open the file and print its content to confirm data integrity:

Swift
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1import Foundation
2import FoundationNetworking
3
4// Specify the note name to verify
5let noteName = "welcome.txt"
6
7// Define the file URL
8let fileURL = FileManager.default.temporaryDirectory.appendingPathComponent("downloaded_\(noteName)")
9
10// Create a DispatchGroup
11let dispatchGroup = DispatchGroup()
12
13dispatchGroup.enter() // Enter the group
14// Open and read the downloaded file to verify its content
15do {
16    let content = try String(contentsOf: fileURL, encoding: .utf8)
17    print(content)
18} catch {
19    print("File error occurred: \(error)")
20}
21dispatchGroup.leave() // Leave the group
22
23// Wait for the verification to complete
24dispatchGroup.wait()
25print("Verification task completed.")

If everything is functioning correctly, you should see an output similar to:

Plain text
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1Welcome to Your Notes! 📝
2
3This is a sample note that comes with the application.

This step is essential for data verification. The familiar error-handling techniques come into play once more, using Swift's error-handling constructs to gracefully address any issues during the download and verification process.

In this lesson, you explored two methods for downloading files from an API: a straightforward approach for smaller files and a more efficient streaming method for larger files. You've practiced verifying file integrity by reading its contents post-download and reinforced your knowledge of error management. As you proceed to the practice exercises, you'll have the opportunity to apply these skills, reinforcing your ability to manage API interactions effectively. Keep experimenting with different files and settings, as this will further enhance your understanding and proficiency. Exciting topics await, such as file uploads and handling paginated responses. Your journey in mastering API interactions continues!