Swift Async Algorithms: Powerful Tools for Processing Values Over Time

Swift's concurrency model with async/await revolutionized how we write asynchronous code. But what if you need more advanced tools for processing values over time? That's where Swift Async Algorithms comes in.

This open-source package extends Swift's AsyncSequence with a suite of powerful algorithms that handle complex scenarios like merging streams, debouncing user input, and chunking data over time.

What is Swift Async Algorithms?

Swift Async Algorithms is an official package from Apple that sits alongside other Swift packages like Swift Collections and Swift Algorithms. It provides advanced algorithms specifically designed for processing values asynchronously using AsyncSequence.

Quick recap on AsyncSequence: It's just like Sequence, but with two key differences:

• The next() function is async (can deliver values using Swift concurrency)
• It can handle failures using Swift's throw mechanism

If you know how to use Sequence, you already know how to use AsyncSequence.

Real-World Use Cases

Let me walk you through some practical examples from a messaging app that demonstrates the power of these algorithms.

1. Combining AsyncSequences with Zip

Problem: You're processing video messages and need to generate both a transcoded video and a preview thumbnail. Both operations run asynchronously, and you need to ensure they're paired correctly before uploading.

Solution: The zip algorithm.

// Zip concurrent operations together
for await (video, preview) in zip(transcodedVideos, videoPreviews) {
    // Upload the pair together
    await uploadToServer(video: video, preview: preview)
}

How it works:

• Iterates multiple AsyncSequences concurrently
• Produces tuples pairing elements from each sequence
• Waits for both sides to produce values before emitting a tuple
• No preference on which side completes first
• Rethrows errors if any sequence fails

2. Merging Multiple Streams

Problem: Your app supports multiple user accounts, each with its own stream of incoming messages. You need to handle all messages together as one unified sequence.

Solution: The merge algorithm.

let account1Messages = AsyncStream<Message> { /* ... */ }
let account2Messages = AsyncStream<Message> { /* ... */ }
let account3Messages = AsyncStream<Message> { /* ... */ }

// Merge all message streams into one
for await message in merge(account1Messages, account2Messages, account3Messages) {
    handleIncomingMessage(message)
}

How it works:

• Iterates multiple AsyncSequences concurrently
• Requires all sequences to have the same element type
• Emits elements as soon as any sequence produces them
• Continues until all sequences complete
• Cancels other iterations if any sequence throws an error

3. Debouncing User Input

Problem: Users type search queries rapidly. You don't want to fire off a server request for every keystroke—that would be wasteful and could overwhelm your backend.

Solution: The debounce algorithm.

let searchField = AsyncStream<String> { /* user input */ }

// Wait for a quiet period before searching
for await query in searchField.debounce(for: .milliseconds(300)) {
    await performSearch(query)
}

How it works:

• Waits for a "quiescence period" (quiet time) before emitting values
• Events can come in fast, but you only get values after typing stops
• By default uses ContinuousClock
• Perfect for search fields, form validation, and auto-save features

4. Chunking Data by Time

Problem: Your app sends messages to a server. Instead of sending individual messages, you want to batch them into groups every 500 milliseconds for efficiency.

Solution: The chunked algorithm with time-based chunking.

let messages = AsyncStream<Message> { /* outgoing messages */ }

// Group messages every 500ms
for await batch in messages.chunked(by: .milliseconds(500)) {
    await sendBatchToServer(batch)
}

How it works:

• Groups elements by time, count, or content
• Ensures efficient packet transmission
• Handles errors by rethrowing them
• Great for batching API calls or rate-limiting

Understanding Clocks in Swift

Swift 5.7 introduced three core time-related types: Clock, Instant, and Duration. These make working with time safe and consistent.

Two Common Clocks

ContinuousClock

• Measures time like a stopwatch
• Keeps running even when the device sleeps
• Use for measuring durations relative to humans (timeouts, delays)

SuspendingClock

• Suspends when the device sleeps
• Use for animations and UI timing
• Ensures animations don't jump ahead after device wakes

// Delay for 3 seconds using ContinuousClock
try await Task.sleep(for: .seconds(3), clock: .continuous)

// Measure execution time
let duration = await ContinuousClock().measure {
    await performExpensiveOperation()
}

Building Collections from AsyncSequences

Sometimes you need to convert an AsyncSequence back into a regular collection. The package provides initializers for this:

// Build an array from an AsyncSequence
let messages = try await Array(messageStream)

// Build a dictionary
let messageDict = try await Dictionary(uniqueKeysWithValues: messageStream)

// Build a set
let uniqueMessages = try await Set(messageStream)

Important: These only work with finite AsyncSequences. They'll await until the sequence completes.

More Algorithms Available

The Swift Async Algorithms package includes many more tools:

• Buffering: Control how values are buffered before consumption
• Joining: Combine sequences with delimiters
• Interspersing: Inject values intermittently between elements
• Rate limiting: Control the rate of value emission
• And more!

Getting Started

Add Swift Async Algorithms to your project via Swift Package Manager:

dependencies: [
    .package(
        url: "https://github.com/apple/swift-async-algorithms",
        from: "1.0.0"
    )
]

Then import it:

import AsyncAlgorithms

Key Takeaways

1. Zip when you need to pair concurrent operations together
2. Merge when combining multiple streams of the same type
3. Debounce for user input and rate-limiting
4. Chunked for batching data by time or count
5. Collection initializers for converting AsyncSequences to Arrays, Sets, or Dictionaries
6. Choose the right Clock for your use case (Continuous vs. Suspending)

Incremental Adoption

One of the best features of this package is that it supports incremental migration. You can keep your existing data structures (like Arrays) and gradually convert parts of your app to use Swift concurrency where it makes sense.

The Swift Async Algorithms package brings powerful, production-ready tools for handling asynchronous operations. Whether you're building a messaging app, processing real-time data, or managing user interactions, these algorithms will help you write cleaner, more efficient async code.

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Resources:

• Swift Async Algorithms on GitHub

Comments

[ArshTechPro]

Swift Async Algorithms is an official package from Apple that sits alongside other Swift packages like Swift Collections and Swift Algorithms. It provides advanced algorithms specifically designed for processing values asynchronously using AsyncSequence.