我们可以使用下面的代码片段获得userAgent，

import UIKit
import WebKit

class ViewController: UIViewController{
 
    var WKwebView: WKWebView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        getUserAgent()
    }
    
    func getUserAgent() {
        let webConfiguration = WKWebViewConfiguration()
        WKwebView = WKWebView(frame: .zero, configuration: webConfiguration)
        WKwebView.evaluateJavaScript(
            "navigator.userAgent", 
            completionHandler: { (result, error) in
                debugPrint(result as Any)
                debugPrint(error as Any)

                if let unwrappedUserAgent = result as? String {
                    print("userAgent: \(unwrappedUserAgent)")
                } else {
                    print("failed to get the user agent")
                }
            }
        )
    }    
}

输出：userAgent：Mozilla/5.0（iPhone; CPU iPhone OS 13_3 like Mac OS X）AppleWebKit/605.1.15（KHTML，like Gecko）移动的/15 E148

使用WebKit代替。

import WebKit

let webView = WKWebView()
webView.evaluateJavaScript("navigator.userAgent") { (result, error) in
    print(result as! String)
}

// Output 
Mozilla/5.0 (iPad; CPU OS 13_4_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Mobile/15E148

最好使用这段代码。

let userAgent = WKWebView().value(forKey: "userAgent")

Haiiro提供的同步功能更实用，这里是objective-c版本。

NSString *userAgent = [[[WKWebView alloc] init] valueForKey:@"userAgent"];