Built-in AsyncSequences You Should Know

You don't always need to build custom AsyncSequences. Swift provides several built-in ones that cover common use cases.

Follow along with the code: iOS-Practice on GitHub

URL.lines

Read a file or URL line by line, asynchronously:

let url = URL(string: "https://example.com/data.txt")!

for try await line in url.lines {
    print(line)
}

Perfect for processing large files without loading everything into memory.

FileHandle.bytes

Stream bytes from a file:

let handle = FileHandle(forReadingAtPath: "/path/to/file")!

for try await byte in handle.bytes {
    process(byte)
}

NotificationCenter.notifications

Subscribe to notifications as an async sequence:

let notifications = NotificationCenter.default.notifications(
    named: UIApplication.didBecomeActiveNotification
)

for await notification in notifications {
    print("App became active: \(notification)")
}

This replaces the old addObserver pattern with clean async iteration.

URLSession.bytes

Stream download data as it arrives:

let (bytes, response) = try await URLSession.shared.bytes(from: url)

for try await byte in bytes {
    // Process each byte as it downloads
}

Useful for progress tracking or processing large downloads.

AsyncStream Wrappers

Many APIs now offer async sequence versions:

// SwiftUI
AsyncStream(unfolding: { await nextFrame() })

// Combine bridge
publisher.values  // AsyncSequence from any Publisher

Combining with Other Async Code

These integrate naturally with structured concurrency:

func processFile(at url: URL) async throws {
    try await withThrowingTaskGroup(of: Void.self) { group in
        for try await line in url.lines {
            group.addTask {
                await process(line)
            }
        }
    }
}

When to Use Each

Interview Tip

Knowing these built-in sequences shows practical experience. When asked about handling streaming data, mention URL.lines for text or URLSession.bytes for network data—it demonstrates you know the standard library.