iOS7以后发布了NSURLSession用来替换NSURLConnection，NSURLSession使用方式有以下两种：
1.block方式
（1）创建的步骤
获取单例会话对象
创建URL对象
隐含创建request
创建NSURLSessionDataTask

  // 1.获取会话对象
NSURLSession *session = [NSURLSession sharedSession];
//另外一种生成默认的GET请求的方法
NSURL *url = [NSURL URLWithString:@"http://localhost:8080/MJServer/video"];NSURLSessionDataTask *task = [session dataTaskWithURL:url completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {NSDictionary *dict = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableLeaves error:nil];NSLog(@"%@", dict);

(2)使用NSURLSessionDataTask
创建session
创建URL
创建URLRequest
创建NSURLSessionDataTask

// 2.创建的一URL地址
NSURL *url = [NSURL URLWithString:@"http://192.168.15.172:8080/MJServer/login"];
// 3.创建一个请求对象
NSMutableURLRequest * request = [NSMutableURLRequest requestWithURL:url];
request.HTTPMethod = @"POST";
request.HTTPBody = [@"username=123&pwd=123" dataUsingEncoding:NSUTF8StringEncoding];NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {NSDictionary *dict = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableLeaves error:nil];NSLog(@"%@", dict);
}];[task resume];

(3)使用NSURLSessionDownloadTask

// 1.得到session对象
NSURLSession *session = [NSURLSession sharedSession];
// 2.创建URL对象

NSURL *url = [NSURL URLWithString:@"http://localhost:8080/MJServer/resources/test.mp4"];NSURLSessionDownloadTask *task = [session downloadTaskWithURL:url completionHandler:^(NSURL * _Nullable location, NSURLResponse * _Nullable response, NSError * _Nullable error) {//NSString *caches = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];NSString *filePath = [caches stringByAppendingPathComponent:response.suggestedFilename];NSFileManager *fileManager = [NSFileManager defaultManager];[fileManager moveItemAtPath:location.path toPath:filePath error:nil];}];

2.代理方式

• (void)downLoadTask2 {
  NSURLSessionConfiguration *cfg = [NSURLSessionConfiguration defaultSessionConfiguration];
  // 使用配置对象获取会话对象
  NSURLSession *session = [NSURLSession sessionWithConfiguration:cfg delegate:self delegateQueue:[NSOperationQueue mainQueue]];

  // 创建一个URL
  NSURL *url = [NSURL URLWithString:@”http://localhost:8080/MJServer/resources/test.mp4“];
  NSURLSessionDownloadTask *task = [session downloadTaskWithURL:url];

  [task resume];

}
- (void)URLSession:(NSURLSession )session downloadTask:(NSURLSessionDownloadTask )downloadTask didFinishDownloadingToURL:(NSURL *)location {
NSString *caches = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];
NSString *filePath = [caches stringByAppendingPathComponent:downloadTask.response.suggestedFilename];
NSFileManager *fileManager = [NSFileManager defaultManager];
[fileManager moveItemAtPath:location.path toPath:filePath error:nil];
}

• (void)URLSession:(NSURLSession )session downloadTask:(NSURLSessionDownloadTask )downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite {
  double progress = (double)totalBytesWritten / totalBytesExpectedToWrite;
  NSLog(@”已经下载了：%f”, progress);

}