how do I download a file asynchronously

To download a file asynchronously in Swift, you can use the async and await keywords, which are part of Swift's concurrency model. Here's a basic example of how you might do this:

Define an async function to download the file:

import Foundation

func downloadFile(from url: URL) async throws -> Data {
    let (data, _) = try await URLSession.shared.data(from: url)
    return data
}

Call the async function:

Since async functions can only be called from other async functions or from a context that supports concurrency, you might call this function from within an async context, such as within a SwiftUI view or another async function.

import SwiftUI

struct ContentView: View {
    @State private var fileData: Data?

    var body: some View {
        VStack {
            if let fileData = fileData {
                Text("File downloaded: \(fileData.count) bytes")
            } else {
                Text("Downloading file...")
                    .onAppear {
                        Task {
                            do {
                                let url = URL(string: "https://example.com/file")!
                                fileData = try await downloadFile(from: url)
                            } catch {
                                print("Failed to download file: \(error)")
                            }
                        }
                    }
            }
        }
    }
}

Additional Context from WWDC Sessions

Concurrency in Swift: The async and await keywords are used to handle asynchronous operations in Swift. When an async function is called, the await keyword indicates that a suspension can occur on that line. This allows the CPU to perform other tasks while waiting for the asynchronous operation to complete. For more details, you can refer to the session A Swift Tour: Explore Swift’s features and design (18:50).
Handling Asynchronous Conditions in Testing: If you are testing asynchronous code, you can use withCheckedContinuation or withCheckedThrowingContinuation to convert completion handlers to async/await syntax. This is useful for ensuring that your tests can properly await the completion of asynchronous operations. For more information, see the session Go further with Swift Testing (24:26).

Relevant Sessions

If you need more specific details or examples, feel free to ask!