在和前端交互过程中，为了区分是我们APP内加载网页还是其他应用或浏览器加载的网页，我们需要使用在UserAgent后追加字符串来区分。下面介绍全局和局部修改UserAgent。

设置全局UserAgent

下面两种方法都可以修改全局UserAgent，因为修改的是系统配置，等于全局公用属性，所以不论使用UIWebView还是WKWebview，修改都是同一个属性。

方法一：WKWebView配置UserAgent

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    
    self.webView = [[WKWebView alloc] init];
    [self.webView evaluateJavaScript:@"navigator.userAgent" completionHandler:^(id result, NSError *error) {
        NSString *userAgent = result;
        NSString *newUserAgent = [userAgent stringByAppendingString:@" Appended Custom User Agent"];
        NSDictionary *dictionary = [NSDictionary dictionaryWithObjectsAndKeys:newUserAgent, @"UserAgent", nil];
        [[NSUserDefaults standardUserDefaults] registerDefaults:dictionary];
    }];
    
    return YES;
}

注意：WKWebView不能使用局部声明，需要放在类扩展中，否则无效

方法二：UIWebView配置UserAgent

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    
    UIWebView *webView = [[UIWebView alloc] init];
    NSString *userAgent = [webView stringByEvaluatingJavaScriptFromString:@"navigator.userAgent"];
    NSString *newUserAgent = [userAgent stringByAppendingString:@" Appended Custom User Agent"];
    NSDictionary *dictionary = [NSDictionary dictionaryWithObjectsAndKeys:newUserAgent, @"UserAgent", nil];
    [[NSUserDefaults standardUserDefaults] registerDefaults:dictionary];
    
    return YES;
}

• 方法二放在其他ViewController也是可以使用，webView初始化要放在设置UserAgent后才会生效。
• 需要注意的是在当前页面销毁后，设置的UserAgen就会失效，如果push或者present后面的页面，UserAgen依然有效。
• AppDelegate不会销毁以一直有效

设置局部UserAgent

UIWebView *tempWebView = [[UIWebView alloc] init];
NSString *userAgent = [tempWebView stringByEvaluatingJavaScriptFromString:@"navigator.userAgent"];
NSString *newUserAgent = [userAgent stringByAppendingString:@" Appended Custom User Agent"];
self.webView.customUserAgent = newUserAgent;
// 下面这句了打印userAgent，可以不要
[self.webView evaluateJavaScript:@"navigator.userAgent" completionHandler:^(id result, NSError *error) {
    NSLog(@"%@", result);
}];

总结：

• WKWebView需要等待回调才能设置UserAgent；
• UIWebView可以直接返回UserAgent效率更高；