Swift 异步 let 带循环

使用 Swift 编程语言，我们可以很方便地进行异步编程。其中一个强大的特性就是使用 `let` 带循环，让我们能够同时处理多个异步任务。本文将介绍如何使用 `let` 带循环进行异步编程，并通过一个案例代码来演示其用法。

swift
import Foundation
let urls = [
    URL(string: "https://www.example.com")!,
    URL(string: "https://www.apple.com")!,
    URL(string: "https://www.google.com")!
]
let group = DispatchGroup()
let queue = DispatchQueue.global()
for url in urls {
    group.enter()
    
    let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
        if let data = data, let html = String(data: data, encoding: .utf8) {
            if let title = html.extractTitle() {
                print("Title of \(url.absoluteString): \(title)")
            } else {
                print("Failed to extract title from \(url.absoluteString)")
            }
        } else if let error = error {
            print("Failed to fetch data from \(url.absoluteString): \(error)")
        } else {
            print("Unknown error occurred while fetching data from \(url.absoluteString)")
        }
        
        group.leave()
    }
    
    queue.async {
        task.resume()
    }
}
group.notify(queue: .main) {
    print("All tasks completed!")
}
extension String {
    func extractTitle() -> String? {
        let pattern = ""
        guard let regex = try? NSRegularExpression(pattern: pattern, options: .caseInsensitive) else {
            return nil
        }
        
        let range = NSRange(location: 0, length: self.utf16.count)
        let matches = regex.matches(in: self, range: range)
        
        if let match = matches.first {
            return (self as NSString).substring(with: match.range(at: 1))
        } else {
            return nil
        }
    }
}