@interface ViewController () <NSURLSessionTaskDelegate>
@property (nonatomic, strong) NSURLSession *session;
@end

@implementation ViewController

- (NSURLSession *)session {
    if (_session == nil) {
        NSURLSessionConfiguration *config = [NSURLSessionConfiguration defaultSessionConfiguration];
        _session = [NSURLSession sessionWithConfiguration:config delegate:self delegateQueue:nil];
    }
    return _session;
}


- (void)viewDidLoad {
    [super viewDidLoad];
    //https://kyfw.12306.cn/otn/
    NSURL *url = [NSURL URLWithString:@"https://kyfw.12306.cn/otn/"];
    
    [[self.session dataTaskWithURL:url completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        NSString *html = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
        NSLog(@"%@",html);
    }] resume];
}

//代理的方法
//接受服务器的挑战，信任服务器
- (void)URLSession:(NSURLSession *)session didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition, NSURLCredential * _Nullable))completionHandler {
    if (challenge.protectionSpace.authenticationMethod == NSURLAuthenticationMethodServerTrust) {
        NSURLCredential *credential = [NSURLCredential credentialForTrust:challenge.protectionSpace.serverTrust];
        completionHandler(0,credential);
    }
}
@end