AsyncIterator.swift

import UIKit
import _Concurrency
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true

// 1.
struct URLPollingSequence: AsyncSequence, AsyncIteratorProtocol {

    // 2.
    typealias AsyncIterator = URLPollingSequence
    typealias Element = Data?
    
    private let url: URL
    private let delay: Int
    public var isActive: Bool = true
    
    init(url: URL, delay: Int) {
        self.url = url
        self.delay = delay
    }
    
    mutating func next() async throws -> Element? {
        // 3.
        guard isActive else {
            return nil
        }
        
        try await Task.sleep(nanoseconds: UInt64(delay) * 1_000_000_000)
        // 4.
        let data = try await fetchData()
        return data
    }
    
    func makeAsyncIterator() -> URLPollingSequence {
        return self
    }
    
    private func fetchData() async throws -> Data {
        let (data, _) = try await URLSession.shared.data(from: url)
        return data
    }
}