iOS开发中，前端H5想获知H5页面是在App内部还是在其他浏览器被打开，则需要客户端配合修改默认的UserAgent, 以便区分。在iOS8之前一直用的都是UIWebView，但是在iOS9出了一个WKWebView，对比起来前者UIWebview就具有严重的内存问题，UIWebView会被慢慢淘汰。

WKWebView修改UserAgent代码如下：

- (void)configWebViewUserAgent {
  WKWebView *webView = [[WKWebView alloc] initWithFrame:CGRectZero];

  // 获取当前UserAgent, 并对其进行修改
  [webView evaluateJavaScript:@"navigator.userAgent" completionHandler:^(id userAgent, 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_4_6 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
      NSString *newAgent = oldAgent;
      if ([oldAgent rangeOfString:@"iOS_CFZQ_LEZHUAN"].location == NSNotFound) {
          newAgent = [NSString stringWithFormat:@"%@; iOS_CFZQ_LEZHUAN_%@",oldAgent,[[NSBundle mainBundle] objectForInfoDictionaryKey:@"CFBundleShortVersionString"]];
       }
            
       // 一定要设置customUserAgent，否则执行navigator.userAgent拿不到oldAgent
       webView.customUserAgent = newAgent;
       NSDictionary *dictionnary = [[NSDictionary alloc] initWithObjectsAndKeys:newAgent, @"UserAgent", nil];
       [[NSUserDefaults standardUserDefaults] registerDefaults:dictionnary];
       [[NSUserDefaults standardUserDefaults] synchronize];
     }];
  }];
}

切记：一定要设置customUserAgent，这行代码一定要写上。