block方式处理返回结果:

- (void)get{
    // 获得NSURLSession对象
    NSURLSession *session = [NSURLSession sharedSession];
    // 创建任务
    NSString *urlString = @"http://www.example.com:8080/login?username=solozyx&pwd=solozyx";
    NSURL *url = [NSURL URLWithString:urlString];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    NSURLSessionDataTask *task = [session dataTaskWithRequest:request
                                            completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
                                                if (error == nil) {
                                                    NSDictionary *dict = [NSJSONSerialization JSONObjectWithData:data
                                                                                                         options:NSJSONReadingMutableContainers
                                                                                                           error:nil];
                                                    NSLog(@"%@", dict); // {"success":"登录成功"}
                                                }
                                            }];
    // 启动任务
    [task resume];
}

- (void)get2{
    NSURLSession *session = [NSURLSession sharedSession];
    NSString *urlString = @"http://www.example.com:8080/login?username=solozyx&pwd=solozyx";
    NSURL *url = [NSURL URLWithString:urlString];
    NSURLSessionDataTask *task = [session dataTaskWithURL:url
                                        completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
                                            NSLog(@"%@", [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:nil]);
                                            // {"success":"登录成功"}
                                        }];
    [task resume];
}

- (void)post{
    NSURLSession *session = [NSURLSession sharedSession];

    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:@"http://www.example.com:8080/login"]];
    request.HTTPMethod = @"POST";
    request.HTTPBody = [@"username=solozyx&pwd=solozyx" dataUsingEncoding:NSUTF8StringEncoding];
    
    NSURLSessionDataTask *task = [session dataTaskWithRequest:request
                                            completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
                                                NSLog(@"%@", [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:nil]);
                                                // {"success":"登录成功"}
                                            }];
    [task resume];
}

- (void)download{
    NSURLSession *session = [NSURLSession sharedSession];
    NSString *urlString = @"http://www.example.com:8080/resources/videos/minion_01.mp4";
    NSURL *url = [NSURL URLWithString:urlString];
    NSURLSessionDownloadTask *task = [session downloadTaskWithURL:url
                                                completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {
                                                    if (error == nil) {
                                                        // 文件将来存放的真实路径
                                                        NSString *file = [[NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject] stringByAppendingPathComponent:response.suggestedFilename];
                                                        // 剪切location的临时文件到真实路径
                                                        NSFileManager *mgr = [NSFileManager defaultManager];
                                                        [mgr moveItemAtURL:location toURL:[NSURL fileURLWithPath:file] error:nil];
                                                        NSLog(@"%@",file);
                                                    }
                                                }];
    [task resume];
}

delegate方式处理返回结果:
<NSURLSessionDataDelegate>

#import "ViewController.h"

@interface ViewController () <NSURLSessionDataDelegate>

@end


@implementation ViewController

- (void)startDataDask {
    // 获得NSURLSession对象
    NSURLSession *session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]
                                                          delegate:self
                                                     delegateQueue:[[NSOperationQueue alloc] init]];
    // 创建任务
    NSString *urlString = @"http://www.example.com:8080/login?username=solozyx&pwd=solozyx";
    NSURL *url = [NSURL URLWithString:urlString];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    NSURLSessionDataTask *task = [session dataTaskWithRequest:request];
    // 启动任务
    [task resume];
}

#pragma mark - <NSURLSessionDataDelegate>
/**
 * 1.接收到服务器的响应
 */
- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
didReceiveResponse:(NSURLResponse *)response
 completionHandler:(void (^)(NSURLSessionResponseDisposition))completionHandler{
    NSLog(@"%s", __func__);
    // 允许处理服务器的响应，才会继续接收服务器返回的数据
    completionHandler(NSURLSessionResponseAllow);
}

/**
 * 2.接收到服务器的数据（可能会被调用多次）
 */
- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
    didReceiveData:(NSData *)data{
    NSLog(@"%s", __func__);
}

/**
 * 3.请求成功或者失败（如果失败，error有值）
 */
- (void)URLSession:(NSURLSession *)session
              task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error{
    NSLog(@"%s", __func__);
}

@end

// -[ViewController URLSession:dataTask:didReceiveResponse:completionHandler:]
// -[ViewController URLSession:dataTask:didReceiveData:]
// -[ViewController URLSession:task:didCompleteWithError:]