iOS 使用wkWebView 获取userAgent

  __block WKWebView *webView = [[WKWebView alloc] initWithframe:CGRectZero];
    [webView evaluateJavascript:@"navigator.userAgent" completionHandler:^(NSString *result, NSError * error) {
        webView = nil; /// 延迟webview释放，否则无法获取result
        if (!error && result && [result isKindOfClass:[NSString class]]) {
		NSString *userAgent = result;
        }
    }];