How to get the WKWebView 's userAgent immediately?

You cannot synchronously evaluate JavaScript, that is true.

If you are overriding the full user agent string via

Code Block 
WKWebView.customUserAgent

then you don't need to fetch it, because you'll know what it is already.

If you change the user agent string on each navigation by modifying

Code Block 
WKWebPagePreferences.preferredContentMode

then you'd need to re-fetch it after each navigation.

If you override part of the user agent via

Code Block 
WKWebViewConfiguration.applicationNameForUserAgent

or you do not override it at all, you'd only need to fetch it once after creating the web view, as it wouldn't change after that.

WKWebView *webView = [WKWebView new];  NSString  *ua = [webView valueForKey:@"userAgent"];