iOS 网络请求中容易忽视的特性（1）

昨天在浏览 github 时，发现了一个关于 iOS 请求重定向的特性
Alamofire#issues#788

下面是测试代码。代码的意图很简单，通过设置请求 header 头 Authorization 字段的方式获取数据。

通过打印相关的数据，我们会发现，发生302重定向时，系统默认会清除 Authorization 字段，并设置一些常见的字段头Accept Accept-Encoding Accept-Language。

测试代码

  
- (void)test302 {
    NSURLSession *session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration] delegate:self delegateQueue:[NSOperationQueue mainQueue]];

    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:@"http://weibo.com"]];
    [request setValue:@"test" forHTTPHeaderField:@"User-Agent"];
    [request setValue:@"sun" forHTTPHeaderField:@"Authorization"];

    NSURLSessionDataTask *task = [session dataTaskWithRequest:request];
    [task resume];
}

- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task willPerformHTTPRedirection:(NSHTTPURLResponse *)response newRequest:(NSURLRequest *)request completionHandler:(void (^)(NSURLRequest *_Nullable))completionHandler {
    NSLog(@"willPerformHTTPRedirection = %@", response);
    NSLog(@"task.originalRequest.allHTTPHeaderFields = %@", task.originalRequest.allHTTPHeaderFields);
    NSLog(@"newRequest.allHTTPHeaderFields = %@", request.allHTTPHeaderFields);
    completionHandler(request);
}

输出数据

 willPerformHTTPRedirection = <NSHTTPURLResponse: 0x7fd683e00820> { URL: http://weibo.com/ } { status code: 302, headers {
    "Cache-Control" = "no-cache, no-store";
    "Content-Type" = "text/html";
    "DPOOL_HEADER" = surtur175;
    Date = "Sat, 06 Aug 2016 03:21:10 GMT";
    Expires = "Mon, 26 Jul 1997 05:00:00 GMT";
    "LB_HEADER" = alice28;
    "Last-Modified" = "Sat, 06 Aug 2016 03:21:10 GMT";
    Location = "https://passport.weibo.com/visitor/visitor?entry=miniblog&a=enter&url=http%3A%2F%2Fweibo.com%2F&domain=.weibo.com&ua=php-sso_sdk_client-0.6.23&_rand=1470453670.9099";
    Pramga = "no-cache";
    "Proxy-Connection" = Close;
    Server = WeiBo;
    "Set-Cookie" = "TC-Ugrow-G0=5e22903358df63c5e3fd2c757419b456;Path=/";
    "Transfer-Encoding" = Identity;
} }
-------
task.originalRequest.allHTTPHeaderFields = {
    Authorization = sun;
    "User-Agent" = test;
}
-------
newRequest.allHTTPHeaderFields = {
    Accept = "*/*";
    "Accept-Encoding" = "gzip, deflate";
    "Accept-Language" = "en-us";
    "User-Agent" = test;
}
-------

造成影响

在 iOS 的常用库中，SDWebImage``Alamofire AFNetworking 均使用了系统的默认处理，并没有添加特殊处理。所以，如果有些资源是通过设置Authorization 字段的方式获取数据并可能发生重定向时，一定添加相应的处理代码，防止无法获取资源。