//重写UserAgent
- (void)setUserAgent{
    UIWebView* webView = [[UIWebView alloc] initWithFrame:CGRectZero];
    NSString *oldAgent = [webView stringByEvaluatingJavaScriptFromString:@"navigator.userAgent"];
    if ([oldAgent rangeOfString:@"新userAgent的标志"].location == NSNotFound) {
      NSString *newAgent = [NSString stringWithFormat:@"新userAgent"];
      NSDictionary *dictionnary = [NSDictionary dictionaryWithObjectsAndKeys:newAgent, @"UserAgent", nil];
      [[NSUserDefaults standardUserDefaults] registerDefaults:dictionnary];
      [[NSUserDefaults standardUserDefaults] synchronize];
  }
}

在- (void)viewWillAppear:(BOOL)animated中调用。
新初始化一个UIWebView重写UserAgent，就能规避在第二次进入才能修改的问题。

新userAgent仿微信的写法是：

if ([oldAgent rangeOfString:@"NetType"].location == NSNotFound) {
    NSString *appInfoStr = [NSString stringWithFormat:@"%@/%@",[[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleExecutableKey] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleIdentifierKey] , [[NSBundle mainBundle] infoDictionary][@"CFBundleShortVersionString"] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleVersionKey]];
    NSString *netTypeStr = [NSString stringWithFormat:@"NetType/%@",HttpClient.netWork];
    NSString *LanguageStr = [NSString stringWithFormat:@"Language/%@", [[NSLocale currentLocale] objectForKey:NSLocaleLanguageCode]];
    NSString *newAgent = [NSString stringWithFormat:@"%@ %@ %@ %@", oldAgent,appInfoStr,netTypeStr,LanguageStr];
    NSDictionary *dictionnary = [NSDictionary dictionaryWithObjectsAndKeys:newAgent, @"UserAgent", nil];
    [[NSUserDefaults standardUserDefaults] registerDefaults:dictionnary];
    [[NSUserDefaults standardUserDefaults] synchronize];
}

其中netType的写法是：

#import <CoreTelephony/CTTelephonyNetworkInfo.h>

- (NSString *)netWork{
    NSString *netWork = @" ";
    NetworkReachabilityStatus status = self.networkReachablilityStatus;
    if (status == NetworkReachabilityStatusReachableViaWiFi)
    {
        netWork = @"wifi";
    }
    else if(status == NetworkReachabilityStatusReachableViaWWAN)
    {
        CTTelephonyNetworkInfo * info = [[CTTelephonyNetworkInfo alloc] init];
        NSString *currentRadioAccessTechnology = info.currentRadioAccessTechnology;
        
        if ([currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyLTE])
        {
            netWork = @"4g";
        }
        else if ([currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyEdge] ||
                 [currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyGPRS] ||
                 [currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyCDMA1x])
        {
            netWork = @"2g";
        }
        else if ([currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyHSDPA] ||
                 [currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyWCDMA] ||
                 [currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyHSUPA] ||
                 [currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyCDMAEVDORev0] ||
                 [currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyCDMAEVDORevA] ||
                 [currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyCDMAEVDORevB] ||
                 [currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyeHRPD])
        {
            netWork = @"3g";
        }
        else
        {
            netWork = @"Unknow";
        }
    }
    else
    {
        netWork = @"Unknow";
    }
    
    return netWork;
}