Setting the user agent correctly on an iOS app's HTTP requests can be important, but it's a pain to get right. This lightweight solution creates an "offscreen" webView, loads a page, and grabs the user agent string. The result is cached so subsequent calls are instantaneous.

GetUserAgent.swift

import WebKit

class GetUserAgent: NSObject, WKNavigationDelegate {
    private static var cachedUserAgent: String?
    
    private var continuation: CheckedContinuation<String, Error>?
    private var webView: WKWebView?
    
    func get() async throws -> String {
        if let cached = Self.cachedUserAgent {
            return cached
        }
        
        return try await withCheckedThrowingContinuation { continuation in
            self.continuation = continuation
            
            let configuration = WKWebViewConfiguration()
            let webView = WKWebView(frame: .zero, configuration: configuration)
            self.webView = webView
            webView.navigationDelegate = self
            
            // Load a lightweight URL to fetch the user agent
            if let url = URL(string: "https://www.example.com") {
                let request = URLRequest(url: url)
                webView.load(request)
            } else {
                continuation.resume(throwing: NSError(domain: "GetUserAgent", code: -1, userInfo: [NSLocalizedDescriptionKey: "Invalid URL"]))
                self.continuation = nil
            }
        }
    }

    func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
        // Execute JavaScript to get the navigator.userAgent string
        webView.evaluateJavaScript("navigator.userAgent") { [weak self] result, error in
            guard let self = self else { return }
            
            if let error = error {
                // Resume with error if JavaScript evaluation fails
                self.continuation?.resume(throwing: error)
            } else if let userAgent = result as? String {
                // Resume with the retrieved user agent string
                Self.cachedUserAgent = userAgent
                self.continuation?.resume(returning: userAgent)
            } else {
                let unexpectedError = NSError(
                    domain: "GetUserAgent",
                    code: -2,
                    userInfo: [NSLocalizedDescriptionKey: "Unexpected result while fetching user agent"]
                )
                self.continuation?.resume(throwing: unexpectedError)
            }
            
            // Clean up
            self.continuation = nil
            self.webView = nil
        }
    }
}

Usage:

let userAgent = try await GetUserAgent().get()