关键是要设置customUserAgent（>=iOS 9.0）,否则执行evaluateJavaScript:@"navigator.userAgent"获取不到webView的UA：
//修改userAgent
+ (void)addToWebViewUserAgent:(NSString *)addAgent
{
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
WKWebView *webView = [WKWebView new];
[webView evaluateJavaScript:@"navigator.userAgent" completionHandler:^(id _Nullable oldAgent, NSError * _Nullable error) {
if (![oldAgent isKindOfClass:[NSString class]]) {
// 为了避免没有获取到oldAgent，所以设置一个默认的userAgent
// Mozilla/5.0 (iPhone; CPU iPhone OS 12_2 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Mobile/15E148
oldAgent = [NSString stringWithFormat:@"Mozilla/5.0 (%@; CPU iPhone OS %@ like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Mobile/15E148", [[UIDevice currentDevice] model], [[[UIDevice currentDevice] systemVersion] stringByReplacingOccurrencesOfString:@"." withString:@"_"]];
}
//自定义user-agent
if (![oldAgent hasSuffix:addAgent]) {
NSString *newAgent = [oldAgent stringByAppendingFormat:@" DWD_HSQ/%@",addAgent];
[[NSUserDefaults standardUserDefaults] registerDefaults:@{@"UserAgent":newAgent}];
// 一定要设置customUserAgent，否则执行navigator.userAgent拿不到oldAgent
webView.customUserAgent = newAgent;
}
}];
});
}
