NSURLConnection在iOS 9被弃用，NSURLSession是iOS7中新增接口，
NSURLSession，与NSURLConnection是并列的，且可以支持后台相关的网络操作的新特性，NSURLSession把NSURLConnection替换成NSURLSession, NSURLSessionConfiguration，NSURLSessionTask.

基础知识

NSURLSessionTask是一个抽象类，直接使用的三个子类NSURLSessionDataTask，NSURLSessionUploadTask和NSURLSessionDownloadTask.NSURLSessionDataTask可以执行后面两个子类的功能.
NSURLSessionDataTask-Get方法:

    NSURL *url = [NSURL URLWithString:@"http://www.jianshu.com/u/24da48b2ddb3"];

    NSMutableURLRequest *request =[NSMutableURLRequest requestWithURL:url];
    request.HTTPMethod = @"GET";

    NSURLSession *session = [NSURLSession sharedSession];
 
    NSURLSessionDataTask *dataTask = [session dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        NSLog(@"获取到数据");
    }];
    // 执行任务/继续执行
    [dataTask resume];
    //[dataTask suspend];//暂停

NSURLSessionDataTask-Post方法:

    NSURL *url = [NSURL URLWithString:@"http://www.jianshu.com/u/24da48b2ddb3"];
    
    NSMutableURLRequest *request =[NSMutableURLRequest requestWithURL:url];
    request.HTTPMethod = @"POST";
    
    NSString *params = @"userName=FlyElephant";//post参数
    request.HTTPBody = [params dataUsingEncoding:NSUTF8StringEncoding];
    
    NSURLSession *session = [NSURLSession sharedSession];
    
    NSURLSessionDataTask *dataTask = [session dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        NSLog(@"获取到数据");
    }];
    // 执行任务/继续执行
    [dataTask resume];
    //[dataTask suspend];//暂停```
NSURLSessionDownloadTask-下载文件:

   NSURLSession *session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]
                                                          delegate:self
                                                     delegateQueue:[[NSOperationQueue alloc] init]];
    
    NSURL *url = [NSURL URLWithString:@"http://www.jianshu.com/u/24da48b2ddb3"];
    
    NSMutableURLRequest *request =[NSMutableURLRequest requestWithURL:url];
    request.HTTPMethod = @"GET";
    
    NSURLSessionDownloadTask *dataTask = [session downloadTaskWithURL:url completionHandler:^(NSURL * _Nullable location, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        NSLog(@"FlyElephant-下载成功");
    }];
    // 执行任务/继续执行
    [dataTask resume];

第三种方式可以设置delegate,NSURLSessionDelegate定义如下:

@protocol NSURLSessionDelegate <NSObject>
@optional

/* The last message a session receives.  A session will only become
 * invalid because of a systemic error or when it has been
 * explicitly invalidated, in which case the error parameter will be nil.
 */
- (void)URLSession:(NSURLSession *)session didBecomeInvalidWithError:(nullable NSError *)error;

/* If implemented, when a connection level authentication challenge
 * has occurred, this delegate will be given the opportunity to
 * provide authentication credentials to the underlying
 * connection. Some types of authentication will apply to more than
 * one request on a given connection to a server (SSL Server Trust
 * challenges).  If this delegate message is not implemented, the 
 * behavior will be to use the default handling, which may involve user
 * interaction. 
 */
- (void)URLSession:(NSURLSession *)session didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge
                                             completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential * _Nullable credential))completionHandler;

/* If an application has received an
 * -application:handleEventsForBackgroundURLSession:completionHandler:
 * message, the session delegate will receive this message to indicate
 * that all messages previously enqueued for this session have been
 * delivered.  At this time it is safe to invoke the previously stored
 * completion handler, or to begin any internal updates that will
 * result in invoking the completion handler.
 */
- (void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session NS_AVAILABLE_IOS(7_0);

@end

有一点需要注意的是如果设置的NSURLSessionDataDelegate，需要在didReceiveResponse方法中加入以下代码:

completionHandler(NSURLSessionResponseAllow);//如果不设置, 将不会执行之后的代理方法

- (void)downloadData {
    NSURLSessionConfiguration *configura = [NSURLSessionConfiguration defaultSessionConfiguration];
    NSURLSession *session = [NSURLSession sessionWithConfiguration:configura delegate:self delegateQueue:nil];
    NSURL *url = [NSURL URLWithString:@"https://www.jianshu.com/u/24da48b2ddb3"];
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
    NSURLSessionDataTask *dataTask = [session dataTaskWithRequest:request];
    [dataTask resume];
    [session finishTasksAndInvalidate];
    
}

- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveResponse:(NSURLResponse *)response completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler
{
    completionHandler(NSURLSessionResponseAllow);//如果不设置, 将不会执行之后的代理方法
    NSLog(@"didReceiveResponse");
    
}

- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
    NSLog(@"result: %@", @"didCompleteWithError");
}

文件上传

POST每次都是以NSData的形式上传，如果我们POST传递的既需要传递参数，也需要上传文件，就没上面那么简单了，各位可以先想一下，实现如下:

- (void)originalUploadFile {
    NSDictionary *params = @{@"address"     : @"简书",
                             @"userName"    : @"FlyElephant"};
    
    NSString *path = [[NSBundle mainBundle] pathForResource:@"test" ofType:@"jpg"];
    NSString *boundary = [self generateBoundaryString];
    
    // 请求的Url
    NSURL *url = [NSURL URLWithString:@"http://www.jianshu.com/u/24da48b2ddb3"];
    
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url];
    [request setHTTPMethod:@"POST"];
    
    // 设置ContentType
    NSString *contentType = [NSString stringWithFormat:@"multipart/form-data; boundary=%@", boundary];
    [request setValue:contentType forHTTPHeaderField: @"Content-Type"];
    
    NSString *fieldName = @"CustomFile";
    NSData *httpBody = [self createBodyWithBoundary:boundary parameters:params paths:@[path] fieldName:fieldName];
    
    NSURLSessionTask *task = [[NSURLSession sharedSession] uploadTaskWithRequest:request fromData:httpBody completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
        if (error) {
            NSLog(@"error = %@", error);
            return;
        }
        
        NSString *result = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
        NSLog(@"FlyElephant-返回结果---result = %@", result);
    }];
    [task resume];
}

- (NSData *)createBodyWithBoundary:(NSString *)boundary
                        parameters:(NSDictionary *)parameters
                             paths:(NSArray *)paths
                         fieldName:(NSString *)fieldName {
    NSMutableData *httpBody = [NSMutableData data];
    
    // 文本参数
    
    [parameters enumerateKeysAndObjectsUsingBlock:^(NSString *parameterKey, NSString *parameterValue, BOOL *stop) {
        [httpBody appendData:[[NSString stringWithFormat:@"--%@\r\n", boundary] dataUsingEncoding:NSUTF8StringEncoding]];
        [httpBody appendData:[[NSString stringWithFormat:@"Content-Disposition: form-data; name=\"%@\"\r\n\r\n", parameterKey] dataUsingEncoding:NSUTF8StringEncoding]];
        [httpBody appendData:[[NSString stringWithFormat:@"%@\r\n", parameterValue] dataUsingEncoding:NSUTF8StringEncoding]];
    }];
    
    // 本地文件的NSData
    
    for (NSString *path in paths) {
        NSString *filename  = [path lastPathComponent];
        NSData   *data      = [NSData dataWithContentsOfFile:path];
        NSString *mimetype  = [self mimeTypeForPath:path];
        
        [httpBody appendData:[[NSString stringWithFormat:@"--%@\r\n", boundary] dataUsingEncoding:NSUTF8StringEncoding]];
        [httpBody appendData:[[NSString stringWithFormat:@"Content-Disposition: form-data; name=\"%@\"; filename=\"%@\"\r\n", fieldName, filename] dataUsingEncoding:NSUTF8StringEncoding]];
        [httpBody appendData:[[NSString stringWithFormat:@"Content-Type: %@\r\n\r\n", mimetype] dataUsingEncoding:NSUTF8StringEncoding]];
        [httpBody appendData:data];
        [httpBody appendData:[@"\r\n" dataUsingEncoding:NSUTF8StringEncoding]];
    }
    
    [httpBody appendData:[[NSString stringWithFormat:@"--%@--\r\n", boundary] dataUsingEncoding:NSUTF8StringEncoding]];
    
    return httpBody;
}

- (NSString *)mimeTypeForPath:(NSString *)path {
    
    CFStringRef extension = (__bridge CFStringRef)[path pathExtension];
    CFStringRef UTI = UTTypeCreatePreferredIdentifierForTag(kUTTagClassFilenameExtension, extension, NULL);

    NSString *mimetype = CFBridgingRelease(UTTypeCopyPreferredTagWithClass(UTI, kUTTagClassMIMEType));
    
    CFRelease(UTI);
    
    return mimetype;
}

- (NSString *)generateBoundaryString {
    return [NSString stringWithFormat:@"Boundary-%@", [[NSUUID UUID] UUIDString]];
}

实际开发中我们用到的AFNetWorking很容易就实现上面的代码:

    AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
    NSDictionary *parameters = @{@"userName": @"FlyElphant"};
    NSString *filePath = [[NSBundle mainBundle] pathForResource:@"test" ofType:@"jpg"];
    
    [manager POST:@"http://www.jianshu.com/u/24da48b2ddb3" parameters:parameters constructingBodyWithBlock:^(id<AFMultipartFormData> formData) {
        [formData appendPartWithFileURL:[NSURL fileURLWithPath:filePath] name:@"feature" error:nil];
    } success:^(AFHTTPRequestOperation *operation, id responseObject) {
        NSLog(@"FlyElephant: %@", responseObject);
    } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
        NSLog(@"FlyElephant: %@", error);
    }];