需要遵循NSURLConnectionDataDelegate协议
/**

• 当前的长度
  /
  @property (nonatomic ,assign) long long currentLenth;
  /*
• 总体的长度
  /
  @property (nonatomic ,assign) long long totalLenth;
  /*
• 连接类
  /
  @property (nonatomic ,strong) NSURLConnection connection;
  /
• 文件句柄
  */
  @property (nonatomic, strong) NSFileHandle *writeHandle;
  @property (weak, nonatomic) IBOutlet UISlider *slider;
  @property (weak, nonatomic) IBOutlet UIButton *startOrPause;

• （void）viewDidLoad{
  [super viewDidLoad];
  NSString *path = [NSSearchPathForDirectorInDomains (NSCacheDirectory,NSUserDomainMask,YES)lastObject];
  NSString *filePath = [path stringByAppendingPathComponent:@"123.rar"]
  NSDictionary *dic = [NSFileManager defaultManager] attributesOfItemAtPath:filePath error:nil];
  self.slider.value = 0;
  }

• (void)createFilePath{
  //创建文件路径
  NSString *path = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory,NSUserDomainMask,YES)lastObject];
  NSString *filePath = [path stringByAppendingPathComponent:@"123.rar"];
  // //文件管理器
  NSFileManager *manager = [NSFileManager defaultManager];
  // //用来创建一个空的文件
  [manager createFileAtPath:filePath contents:nil attributes:nil];
  // //创建文件句柄，用来给空文件写入数据
  self.writeHandle = [NSFileHandle fileHandleForWritingAtPath:filePath];
  }

• (IBAction)startOrPause:(id)sender {
  UIButton *Btn = sender;
  if (Btn.selected == YES) {
  //取消下载,停止此次下载，如果想继续下载，就需要创建新的连接
  [self.connection cancel];
  [self.writeHandle closeFile];
  self.writeHandle = nil;
  _slider.value = 0;
  }else{

  NSURL *url = [NSURL URLWithString:@"http://gdown.baidu.com/data/wisegame/49b4918a76c8eba0/xunlei_10560.apk"];
  NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
  NSString *range = [NSString stringWithFormat:@"bytes=%lld-",self.currentLenth];
  //设置请求头，从上次停止的位置开始下载
  //假如是一个新的下载，那self.currentLenth = 0,从0的位置开始下载
  [request setValue:range forHTTPHeaderField:@"range"];
  self.connection = [NSURLConnection connectionWithRequest:request delegate:self];
  }

  Btn.selected = !Btn.selected;
  }

pragma mark - delegate

• (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response{
  [self createFilePath];
  //如果self.currentLenth有数据，那么说明他不是第一次下载
  if (self.currentLenth) {
  self.totalLenth = response.expectedContentLength + self.currentLenth;

    return;
  

  }
  //获取到文件的长度
  self.totalLenth = response.expectedContentLength;
  }

• (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data{
  //移动到文件尾部
  [self.writeHandle seekToEndOfFile];
  //写入数据

  [self.writeHandle writeData:data];
  self.currentLenth += data.length;
  NSLog(@"%f",(float)self.currentLenth/(float)self.totalLenth);
  float sliderValue = (float)self.currentLenth/(float)self.totalLenth;
  [_slider setValue:sliderValue animated:YES];
  }

• (void)connectionDidFinishLoading:(NSURLConnection *)connection{

  self.currentLenth = 0;
  self.totalLenth = 0;
  NSLog(@"%lld---%lld",self.currentLenth,self.totalLenth);
  //关闭文件句柄
  [self.writeHandle closeFile];
  self.writeHandle = nil;
  self.slider.value = 0;
  self.startOrPause.selected = NO;
  }

• (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error{

}