HTTP3是iOS15中一个全新特性^#，这个特性也需要你们后端服务支持，接下来讲一下如何在你的APP中使用这个能力

客户端

• 需要真机才能支持HTTP3。
• 开发者模式中的HTTP/3选项对结果有一些影响，但实测发现Xcode联调才有影响。
• 如果请求的protocol一直是h2，那么断开Xcode联调，改用console查看输出的protocol，应该会不一样。
• HTTP协议可能会是h3，但有些情况下会返回h2或是h1.1，这是竞速情况下的正常现象，通常情况会是先h2然后再显示h3。这种特性是由<alt-svc>实现^#。
• 另外charles不支持http3，所以连接代理无法发出http3请求，至于charles后续版本是否支持可以关注（Charles Version History）

服务端

NOTE: Servers that support HTTP3 should be based on Draft 29 or later.

使用姿势

1. Install iOS 15.0 beta on a test device (HTTP/3 is not supported in the simulator).
2. In Settings > Developer, under the Networking group, enable HTTP/3.
3. Using Xcode 13.0 beta, create a new project from the iOS > App template. Set the Interface popup to Storyboard and the Language popup to Swift.
4. try using the assumesHTTP3Capable property on the URLRequest being used to execute the dataTask in URLSession

import UIKit

class ViewController: UIViewController, URLSessionDataDelegate {

    private var session: URLSession!

    @IBAction
    private func testAction(_ sender: Any) {
        if self.session == nil {
            let config = URLSessionConfiguration.default
            config.requestCachePolicy = .reloadIgnoringLocalCacheData
            self.session = URLSession(configuration: config, delegate: self, delegateQueue: .main)
        }

        let urlStr = "<# Your H3 URL Here #>"
        let url = URL(string: urlStr)!
        var request = URLRequest(url: url, cachePolicy: .reloadIgnoringLocalCacheData, timeoutInterval: 60.0)
        request.assumesHTTP3Capable = true
        print("task will start, url: \\(url.absoluteString)")
        self.session.dataTask(with: request) { (data, response, error) in
            if let error = error as NSError? {
                print("task transport error \\(error.domain) / \\(error.code)")
                return
            }
            let response = response as! HTTPURLResponse
            let data = data!
            print("task finished with status \\(response.statusCode), bytes \\(data.count)")
        }.resume()
    }

    func urlSession(_ session: URLSession, task: URLSessionTask, didFinishCollecting metrics: URLSessionTaskMetrics) {
        let protocols = metrics.transactionMetrics.map { $0.networkProtocolName ?? "-" }
        print("protocols:", protocols)
    }
}

HTTP/3 test servers

参考

HTTP/3 in your App
Who is ready for HTTP/3?