最近遇到一個問題，就是透過 NSURLSession 的 NSURLSessionDataTask 送 post 到第二臺 server ，某種特定的架構下（例如連proxy 或 tunnel)，會讓第二臺 server 完全連不上。

What are the differences between the two session objects created in these two different ways:

NSURLSession *session = [NSURLSession sharedSession];

and

NSURLSessionConfiguration *configuration 
 
= [NSURLSessionConfiguration defaultSessionConfiguration];
NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration];

As you’re using them, they’re functionally very similar. But using sharedSession doesn’t give you the ability to customize the NSURLSessionConfiguration (e.g. tweak the cache, custom headers, etc.) nor use the delegate-based rendition of NSURLSession

說明：一般的情況下使用 sharedSession 應該就ok, 問題是出問題之後，請改用 NSURLSessionConfiguration.

session with basic set of properties that control various policies on a sessionwide basis. These properties are set on a session at the time of its creation and cannot be changed later. If you need to change these policy properties, create a new session with a modified session configuration.

Returns a shared singleton session object.

Please review Apple Documents first as these are the source of very information.

所以，每次都去 create new session 可以避免某些問題！

使用 proxy 的範例如下：

It turns out, the dictionary keys you want are the Stream variants, they are the ones that resolve down to “HTTPProxy” and such:

NSString* proxyHost = @"myProxyHost.com";
NSNumber* proxyPort = [NSNumber numberWithInt: 12345];

// Create an NSURLSessionConfiguration that uses the proxy
NSDictionary *proxyDict = @{
    @"HTTPEnable"  : [NSNumber numberWithInt:1],
    (NSString *)kCFStreamPropertyHTTPProxyHost  : proxyHost,
    (NSString *)kCFStreamPropertyHTTPProxyPort  : proxyPort,

    @"HTTPSEnable" : [NSNumber numberWithInt:1],
    (NSString *)kCFStreamPropertyHTTPSProxyHost : proxyHost,
    (NSString *)kCFStreamPropertyHTTPSProxyPort : proxyPort,
};

NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration ephemeralSessionConfiguration];
configuration.connectionProxyDictionary = proxyDict;

// Create a NSURLSession with our proxy aware configuration
NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:[NSOperationQueue mainQueue]];

// Form the request
NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:@"https://www.google.com?2"]];

// Dispatch the request on our custom configured session
NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:
                              ^(NSData *data, NSURLResponse *response, NSError *error) {
                                  NSLog(@"NSURLSession got the response [%@]", response);
                                  NSLog(@"NSURLSession got the data [%@]", data);
                              }];

NSLog(@"Lets fire up the task!");
[task resume];

如果直接使用上面的 code 可能會遇到這一個 warning:

Sending 'ViewController *const __strong' to parameter of incompatible type 'id<NSURLSessionDelegate> _Nullable'

解法：

Confirm <NSURLSessionDelegate> in your header file and it will solve the issue i think,

  @interface ViewController : UIViewController <NSURLSessionDataDelegate>

It is because you are setting delegate to self in NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil]; So it’s requires to confirm NSURLSessionDataDelegate protocol.