Can you explain what AsyncSequence is and how it is used in Swift?

AsyncSequence in Swift is a protocol that allows working with asynchronous sequences of values. It is similar to a regular Sequence but is designed for asynchronously obtaining elements, for example, from a network or other sources where data arrives with a delay.

Using AsyncSequence enables writing code that iterates over elements with the for await construct, making asynchronous data streams convenient to read and process.

Example of using AsyncSequence:

struct AsyncCounter: AsyncSequence {
    typealias Element = Int
    let limit: Int

    struct AsyncIterator: AsyncIteratorProtocol {
        var current = 0
        let limit: Int

        mutating func next() async -> Int? {
            guard current < limit else { return nil }
            current += 1
            // Simulate asynchronous delay
            try? await Task.sleep(nanoseconds: 500_000_000) // 0.5 seconds
            return current
        }
    }

    func makeAsyncIterator() -> AsyncIterator {
        return AsyncIterator(current: 0, limit: limit)
    }
}

// Usage:
Task {
    for await number in AsyncCounter(limit: 5) {
        print(number)
    }
}

This approach is convenient for working with data streams that arrive with a delay, such as web sockets, timers, or asynchronous APIs.