iOS7以后發布了NSURLSession用來替換NSURLConnection，NSURLSession使用方式有以下兩種：
1.block方式
（1）創建的步驟
獲取單例會話對象
創建URL對象
隱含創建request
創建NSURLSessionDataTask

  // 1.獲取會話對象
NSURLSession *session = [NSURLSession sharedSession];
//另外一種生成默認的GET請求的方法
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
創建session
創建URL
創建URLRequest
創建NSURLSessionDataTask

// 2.創建的一URL地址
NSURL *url = [NSURL URLWithString:@"http://192.168.15.172:8080/MJServer/login"];
// 3.創建一個請求對象
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對象
NSURLSession *session = [NSURLSession sharedSession];
// 2.創建URL對象

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];
  // 使用配置對象獲取會話對象
  NSURLSession *session = [NSURLSession sessionWithConfiguration:cfg delegate:self delegateQueue:[NSOperationQueue mainQueue]];

  // 創建一個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(@”已經下載了：%f”, progress);

}