需要遵循NSURLConnectionDataDelegate協(xié)議
/**

• 當(dāng)前的長(zhǎng)度
  /
  @property (nonatomic ,assign) long long currentLenth;
  /*
• 總體的長(zhǎng)度
  /
  @property (nonatomic ,assign) long long totalLenth;
  /*
• 連接類
  /
  @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{
  //創(chuàng)建文件路徑
  NSString *path = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory,NSUserDomainMask,YES)lastObject];
  NSString *filePath = [path stringByAppendingPathComponent:@"123.rar"];
  // //文件管理器
  NSFileManager *manager = [NSFileManager defaultManager];
  // //用來(lái)創(chuàng)建一個(gè)空的文件
  [manager createFileAtPath:filePath contents:nil attributes:nil];
  // //創(chuàng)建文件句柄拓轻，用來(lái)給空文件寫(xiě)入數(shù)據(jù)
  self.writeHandle = [NSFileHandle fileHandleForWritingAtPath:filePath];
  }

• (IBAction)startOrPause:(id)sender {
  UIButton *Btn = sender;
  if (Btn.selected == YES) {
  //取消下載,停止此次下載站故，如果想繼續(xù)下載墅冷，就需要?jiǎng)?chuàng)建新的連接
  [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];
  //設(shè)置請(qǐng)求頭屹逛，從上次停止的位置開(kāi)始下載
  //假如是一個(gè)新的下載导狡，那self.currentLenth = 0,從0的位置開(kāi)始下載
  [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有數(shù)據(jù)，那么說(shuō)明他不是第一次下載
  if (self.currentLenth) {
  self.totalLenth = response.expectedContentLength + self.currentLenth;

    return;
  

  }
  //獲取到文件的長(zhǎng)度
  self.totalLenth = response.expectedContentLength;
  }

• (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data{
  //移動(dòng)到文件尾部
  [self.writeHandle seekToEndOfFile];
  //寫(xiě)入數(shù)據(jù)

  [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);
  //關(guān)閉文件句柄
  [self.writeHandle closeFile];
  self.writeHandle = nil;
  self.slider.value = 0;
  self.startOrPause.selected = NO;
  }

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

}