iOS 修改webkit默认 UserAgent

有个项目需求，要区分打开H5是在本地APP还是在手机浏览器，前端伙伴说需要配合修改默认的 UserAgent,以便区分。

一、如何获取UserAgent

UIWebView方式：

UIWebView *webView = [[UIWebView alloc] initWithFrame:CGRectZero];
NSString *userAgent = [webView stringByEvaluatingJavaScriptFromString:@"navigator.userAgent"];
DLog(@"userAgent :%@", userAgent);

WKWebView方式：

// 注意这个方法是异步的
WKWebView *wkWebView = [[WKWebView alloc] initWithFrame:CGRectZero];
[wkWebView evaluateJavaScript:@"navigator.userAgent" completionHandler:^(id result, NSError *error) {
    DLog(@"userAgent :%@", result);
 }];

默认UserAgent输出：

Mozilla/5.0 (iPhone; CPU iPhone OS 8_4 like Mac OS X) AppleWebKit/600.1.4 (KHTML, like Gecko) Mobile/12H143

微信 iOS版的 ：UserAgent

mozilla/5.0 (iphone; cpu iphone os 5_1_1 like mac os x) applewebkit/534.46 (khtml, like gecko) mobile/9b206 micromessenger/5.0
其中micromessenger就是自定义的

二、如何修改UserAgent

方案一，修改全局UserAgent值（这里是在原有基础上拼接自定义的字符串）

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    UIWebView *webView = [[UIWebView alloc] initWithFrame:CGRectZero];
    NSString *userAgent = [webView stringByEvaluatingJavaScriptFromString:@"navigator.userAgent"];
    NSString *newUserAgent = [userAgent stringByAppendingString:@" native_iOS"];//自定义需要拼接的字符串
    NSDictionary *dictionary = [NSDictionary dictionaryWithObjectsAndKeys:newUserAgent, @"UserAgent", nil];
    [[NSUserDefaults standardUserDefaults] registerDefaults:dictionary];
}

方案二，自定义UserAgent值

WKWebView *wkWebView = [[WKWebView alloc] initWithFrame:self.view.bounds];
[self.view addSubview: wkWebView];
NSString *customUserAgent = @"native_iOS";
[[NSUserDefaults standardUserDefaults] registerDefaults:@{@"UserAgent":customUserAgent}];
NSURL *url = [NSURL URLWithString:self.strUrl];
NSURLRequest *request = [NSURLRequest requestWithURL:url
                                             cachePolicy:NSURLRequestUseProtocolCachePolicy
                                         timeoutInterval:10.f];
[self.wkWebView loadRequest:request];

方案三

 self.wkWebView = [[WKWebView alloc] initWithFrame:self.view.bounds];
 __weak typeof(self) weakSelf = self;
 [self.wkWebView evaluateJavaScript:@"navigator.userAgent" completionHandler:^(id result, NSError *error) {
        __strong typeof(weakSelf) strongSelf = weakSelf;
        NSString *userAgent = result;
        NSString *newUserAgent = [userAgent stringByAppendingString:@" native_iOS"];
        NSDictionary *dictionary = [NSDictionary dictionaryWithObjectsAndKeys:newUserAgent, @"UserAgent", nil];
        [[NSUserDefaults standardUserDefaults] registerDefaults:dictionary];
        // needs retain because `evaluateJavaScript:` is asynchronous
        strongSelf.wkWebView = [[WKWebView alloc] initWithFrame:strongSelf.view.bounds];
  }];
  [self.wkWebView loadRequest:request];

三、问题& 思考

在测试的时候，发现方案二、三第一次运行的时候，还是显示默认的值，第二次才会显示自定义的值，其中原因还不明，如有朋友解决麻烦告诉一下，谢谢。