在iOS中下载大型文件，需要使用NSURLConnection 的代理方法：

• (void)touchesBegan:(NSSet)touches withEvent:(UIEvent *)event {
  NSURL *url = [NSURL URLWithString:@”http://d.3987.com/fengj_141112/007.jpg“];
  NSURLRequest *request = [NSURLRequest requestWithURL:url];

  [NSURLConnection connectionWithRequest:request delegate:self];
  }

• (void)connection:(NSURLConnection )connection didFailWithError:(NSError )error {
  NSLog(@”didFailWithError:%@”, [NSThread currentThread]);

}

• (void)connection:(NSURLConnection )connection didReceiveResponse:(NSURLResponse )response {
  NSLog(@”didReceiveResponse:%@”, [NSThread currentThread]);

  NSString *caches = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];
  NSString *filePath = [caches stringByAppendingPathComponent:@”videos.zip”];
  NSFileManager * fileManager = [NSFileManager defaultManager];
  [fileManager createFileAtPath:filePath contents:nil attributes:nil];

  self.writeHandle = [NSFileHandle fileHandleForWritingAtPath:filePath];

  self.fileLength = response.expectedContentLength;
  }

• (void)connection:(NSURLConnection )connection didReceiveData:(NSData )data {
  NSLog(@”didReceiveData:%@”, [NSThread currentThread]);
  [self.writeHandle seekToEndOfFile];
  [self.writeHandle writeData:data];

  self.currentLength += data.length;

  NSLog(@”下载进度： %f”, (double)self.currentLength / self.fileLength);

}

• (void)connectionDidFinishLoading:(NSURLConnection *)connection {
  NSLog(@”connectionDidFinishLoading:%@”, [NSThread currentThread]);
  self.currentLength = 0;
  self.fileLength = 0;
  [self.writeHandle closeFile];
  self.writeHandle = nil;
  }

总体思路：
（1）首先创建一个空的文件：
NSString *caches = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];
NSString *filePath = [caches stringByAppendingPathComponent:@”videos.zip”];
NSFileManager * fileManager = [NSFileManager defaultManager];
[fileManager createFileAtPath:filePath contents:nil attributes:nil];
（2）其次获取操作这个文件的句柄
self.writeHandle = [NSFileHandle fileHandleForWritingAtPath:filePath];
（3）当数据到来后，需要将其写入到文件中去
[self.writeHandle seekToEndOfFile];
[self.writeHandle writeData:data];
（4）最后在文件下载完毕后，需要关闭文件

[self.writeHandle closeFile];